Recording head and recording apparatus provided with the same

Abstract: In a recording head for use in a recording apparatus for effecting recording on a recording medium, having a plurality of heat generating elements (11) and electrodes (13) for applying energy to the heat generating elements, when the effective width of the heat generating elements (11) is Wr and the width of the electrodes connected to the heat generating elements is We, 1/10 ≦ We/Wr ≦ 1/2, and when the length, in the direction of an electric current, of the portion of the heat generating elements in which the current distribution becomes roughest and the portion of the heat generating elements in which the current distribution becomes finest is L, the condition that 1/4 ≦ L/Wr ≦ 3/2 is satisfied.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a recording head capable of accomplishing half tone expression in effecting recording on a recording medium and a recording apparatus provided with such recording head. The term "recording apparatus" covers a printer, a facsimile apparatus, a copying apparatus, a word processor, an electronic typewriter, etc. Related Background Art A recording apparatus such as a printer or a facsimile apparatus is such that a dot pattern is formed on a recording sheet (a recording medium such as recording paper or a plastic sheet) while a plurality of dot forming elements provided on a recording head are selectively driven on the basis of recording information (image signals). As the types of such a recording apparatus, there are the serial type in which recording is effected while a recording head is moved widthwisely of a sheet, the line print type in which recording is effected collectively over a predetermined length in the direction of line, and the page print type in which recording is effected collectively for one page. Also, the recording systems include the thermal system, the ink jet system, the wire dot system, etc. Of these, the thermal system can be classified into the heat transfer system in which ink is transferred to plain paper by the use of an ink sheet and the thermosensitive system in which thermosensitive paper is heated by a thermal head to cause color forming. In these recording systems, a half tone recording method for expressing the density difference has heretofore been adopted during color recording or image recording using a plurality of colors such as cyan, magenta, yellow and black. In such conventional half tone recording, a method based on the principle of binary recording is generally adopted. As a technique for such gradation expression, there has been adopted a technique of expressing half tone falsely by an area gradation method such as the dither method in which with a plurality of dots as a unit, half tone is expressed by the rate of ON-OFF (two values) of the dots in the unit. However, when the above-described area gradation method is adopted, the number of dots necessary for one picture element increases in order to express many gradations. This poses the problem that the resolution of image is reduced. To obtain an image of a resolution of the order of 6 picture elements/mm, for example, at 64 gradations, by this area gradation method, the resolution of the recording head need be of the order of 48 dots/mm. To realize this in a thermal printer, a thermal head of 48 dots/mm becomes necessary. However, the manufacture of a thermal head of such high density is difficult in the present-day technique. Even if such a thermal head could be manufactured, the number of elements will become huge and therefore, a large-scale driving circuit will become necessary for the driving of the thermal head, and this is not realistic. That is, in two-value recording, there is a limit in obtaining a gradation record of high image quality, and it has been desired that a multivalue gradation record expressing the size of a dot in multiple gradations by some method or other be put into practical use. Here, in the recording by heat melting transfer using a thermal head heretofore generally utilized, the density variation corresponding to a variation in applied energy is as shown in Figure 9A of the accompanying drawings. That is, the rate of variation in the recording density to the applied energy E A is great, and the scattering width S W of the recording density D R is also great as indicated by the length of the vertical line in the graph. Therefore, it has been difficult to turn out intermediate density. In the prior-art thermal head shown in Figure 9B of the accompanying drawings, a heat generating element 101 and electrodes 102 and 104 are of such a shape that they are of the same width and therefore, the distribution of an electric current flowing to the heat generating element 101 becomes uniform. The reference numeral 103 indicates the direction in which the electric current flows. Therefore, the temperature distribution of the heat generating element 101 is of such a degree that the temperature becomes somewhat high in the central portion of the heat generating element 101 wherein the amount of radiation is relatively small. Accordingly, even if the time of application of a pulse voltage applied to the signal electrode 104 is varied to thereby vary the temperature distribution of the heat generating element 101 as indicated by 111A and 112A (the time of application is 111A < 112A) in Figure 9B, whether the temperature of the heat generating element 101 becomes higher than the melting point Tm of heat transfer ink is very subtle depending on the position thereof. Accordingly, even if the same applied energy is applied to the heat generating element 101 as shown in Figure 9A, the recording density will differ depending on a slight difference in the position of the heat generating element. Therefore, the heat transfer recording method using the prior-art thermal head could virtually accomplish only two-value or binary recording, and an improvement in the reproducibility of gradation has been desired. So, the applicant has proposed a thermal head capable of multivalue recording in Japanese Laid-Open Patent Application No. 63-54261 (filed in Japan on August 26, 1986 and laid open on March 8, 1988). According to this, the width of an electrode at the junction between the electrode and a heat generating element is made less than the effective recording width of the heat generating element. Thereby, in a heat generating member, particularly that portion of the heat generating member which is near the junction with the electrode is caused to generate heat concentratedly, whereby said portion can be endowed with a heat generation distribution. However, even with such a construction, there has been the undesirable possibility of sufficient gradation being not obtained when recording is effected on recording paper whose surface is not smooth, and a thermal head and a recording apparatus which are more excellent in gradation have been desired. Furthermore, it is known from PATENT ABSTRACTS OF JAPAN, vol. 12, no. 424 (M-761)(3271), 10/11/88, JP-A-63 158 270 and JP-A-63 258 271 that for enabling multi-gradation representation, the total width of one or a plurality of electrodes joined to a heating element is shorter than the effective recording width of the heating element. The heating width of the connecting electrode(s) of the heating element is made narrower than the width of the heating element. Consequently, almost all current flows within a width corresponding to the width of the electrode(s) of the heating element for a current level lower than a predetermined level, and the temperature only at a portion corresponding to the width of the electrode(s) increases particularly thus exhibiting a high temperature difference with respect to other portions. When a voltage is increased gradually in order to vary the applied energy, ink is fused under an area having higher temperature than the melting point of ink in an ink sheet and transferred to a recording sheet. Consequently, the high temperature area of dot to be transferred varies. SUMMARY OF THE INVENTION It is an object of the present invention to provide a recording head capable of obtaining clearcut records and a recording apparatus provided with such recording head. It is another object of the present invention to provide a recording head capable of obtaining records of good quality and a recording apparatus provided with such recording head. It is still another object of the present invention to provide a recording head capable of half tone recording and a recording apparatus provided with such recording head. It is yet still another object of the present invention to provide an inexpensive recording head in which the ratio of the width of an electrode to the width of a heat generating element and the ratio of the length of the heat generating element to the width of the heat generating element are made proper, whereby the latter is capable of multivalue recording and can easily realize recording at multiple gradations, and a heat recording apparatus provided with such recording head. It is a further object of the present invention to provide a thermal head which is formed so that there is created a difference in the density of an electric current flowing through a heat generating element and energy applied thereto is varied, whereby the distribution of generated heat can be easily changed. It is still a further object of the present invention to provide a heat transfer recording apparatus which is made excellent in the reproducibility of gradation by changing the heat generating area of the thermal head thereof correspondingly to the degree of gradation of image data to thereby change the transfer area. These objects will be achieved with a recording head in accordance with claim 1. Further advantageous features concerning preferred embodiments of such a recording head are stated in claims 2 to 10. A heat transfer recording apparatus meeting the above corresponding object is claimed in claim 11. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows the shapes of the heat generating element and the electrode portion of a thermal head according to a first embodiment of the present invention. Figure 2 shows the shape of the cross-section E - E₁ of Figure 1. Figure 3A shows the distribution of an electric current flowing through the thermal head according to the first embodiment. Figure 3B shows the temperature distribution relative to the widthwise position at the cross-section X - X₁ of Figure 3A. Figure 3C shows the heat transfer area corresponding to applied energy in the thermal head according to the first embodiment. Figure 4 shows the relation of recording density to applied energy in the thermal head according to the first embodiment. Figure 5 shows the recording density when the width of the electrodes is varied in the thermal head according to the first embodiment. Figure 6 shows the recording density when the length of the heat generating element is changed in the thermal head according to the first embodiment. Figures 7A - 7C show the shape of a thermal head according to a second embodiment, Figure 7A being a fragmentary enlarged view showing the shapes of the heat generating element and the electrodes of the thermal head, Figure 7B showing the distribution of an electric current flowing through the heat generating element, and Figure 7C showing the transfer area relative to applied energy. Figures 8A - 8C show the shape of a thermal head according to a third embodiment, Figure 8A being a fragmentary enlarged view showing the shapes of the heat generating elements and the electrodes of the thermal head, Figure 8B showing the distribution of an electric current flowing through the heat generating element, and Figure 8C showing the transfer area relative to applied energy. Figure 8D shows still another embodiment of the present invention. Figures 9A and 9B illustrate a thermal head according to the prior art. Figure 10 is a block diagram schematically showing the construction of a heat transfer recording apparatus using a thermal head according to an embodiment of the present invention. Figure 11 is a block diagram schematically showing the construction of a head driving pulse control circuit. Figure 12 shows the construction of the thermal head. Figure 13 shows the timing of signals applied to the thermal head. Figure 14 is a flow chart showing the recording process in a heat transfer recording apparatus according to an embodiment of the present invention. Figures 15A - 15C show the shape of a thermal head according to another embodiment of the present invention. Figure 16 shows the form of use of the thermal head of Figure 15. Figure 17A shows an electric current flowing through the thermal head according to the embodiment of Figure 15A. Figure 17B shows the temperature distribution relative to the widthwise position in the cross-section Z - Z₁ of Figure 17A. Figure 17C shows the heat transfer area of the thermal head according to the embodiment of Figure 15A. Figure 18 shows the relation of recording density to applied energy in the thermal head according to said embodiment. Figure 19 shows the shape of a thermal head according to still another embodiment. Figures 20A - 20C show the heat transfer area relative to applied energy in the thermal head according to the embodiment of Figure 19. Figure 21A shows the shape of a thermal head according to a further embodiment. Figure 21B shows the distribution of an electric current in the thermal head shown in Figure 21A. Figures 22A - 22C show the heat transfer area relative to applied energy in a thermal head according to still a further embodiment. Figures 23A - 23C show the shape of a thermal head according to one more embodiment. Figure 24A shows an electric current flowing through the thermal head according to the embodiment of Figure 23A. Figure 24B shows the temperature distribution relative to the widthwise position in the cross-section Z of Figure 24A. Figure 24C shows the heat transfer area of the thermal head according to the embodiment of Figure 23A. Figure 25 shows the relation of recording density to applied energy in the thermal head according to said embodiment. Figures 26A -26C show the shapes of a thermal head according to a ninth embodiment. Figures 27A - 27C show the heat generation distribution corresponding to applied energy to the thermal head according to the ninth embodiment. Figure 28 is an enlarged front view showing the shape of the heat generating element and the electrode portion of a thermal head according to a tenth embodiment. Figure 29 shows the cross-section E - E₁ of the heat generating element of Figure 28. Figure 30A shows the distribution of an electric current flowing through the thermal head according to the tenth embodiment. Figure 30B shows the temperature distribution relative to the widthwise position at the cross-section X - X₁ of Figure 30A. Figure 30C shows the heat transfer area corresponding to applied energy to the thermal head according to the tenth embodiment. Figure 31 shows the relation of recording density to applied energy in the thermal head according to said embodiment. Figure 32 is an enlarged front view showing the shapes of the heat generating element and the electrodes of a thermal head according to an eleventh embodiment. Figure 33 is an enlarged front view showing the shapes of the heat generating element and the electrodes of a thermal head according to a twelfth embodiment. Figure 34A shows the distribution of an electric current in the heat generating element of the thermal head according to the twelfth embodiment. Figure 34B shows the temperature distribution in the portion Y - Y₁ of Figure 34A. Figure 34C shows the transfer area relative to the temperature distribution. Figures 35 and 36 are enlarged front views showing the shapes of the heat generating elements and the electrodes of thermal heads according to thirteenth and fourteenth embodiments. Figure 37 shows the heat generation distribution in the heat generating element of Figure 36. Figure 38 is an enlarged front view showing the shapes of the heat generating element and the electrode portion of a thermal head according to still another embodiment. Figure 39 shows the cross-section E - E₁ of Figure 38. Figure 40A shows the distribution of an electric current flowing through a thermal head according to the fifteenth embodiment according to Figure 38. Figure 40B shows the temperature distribution relative to the widthwise position at the cross-section X - X₁ of Figure 40A. Figure 40C shows the heat generation temperature distribution at the cross-section Y - Y₁ of Figure 40A. Figure 41 shows the heat transfer area corresponding to applied energy to the thermal head according to the fifteenth embodiment. Figure 42 shows the relation of recording density to applied energy in the thermal head according to said embodiment. Figure 43 is an enlarged front view showing the shapes of the heat generating element and the electrodes of a thermal head according to a sixteenth embodiment. Figures 44A and 44B show the heat generation temperature distribution in the heat generating element in the sixteenth embodiment. Figure 45 is an enlarged front view showing the shapes of the heat generating element and the electrodes of a thermal head according to a seventeenth embodiment. Figure 46A is an enlarged front view showing the shapes of the heat generating element and the electrode portion of a thermal head according to a eighteenth embodiment. Figure 46B shows the cross-section E - E' of the heat generating element of Figure 46A. Figure 46C shows the cross-section F - F' of the heat generating element of Figure 46A. Figure 47A shows the distribution of an electric current flowing through the heat generating element in the eighteenth embodiment. Figure 47B shows the heat generation distribution in the cross-section G - G₁ of Figure 47A. Figure 47C shows the transfer area corresponding to applied energy to the heat generating element in the eighteenth embodiment. Figure 48 shows the relation of recording density to applied energy in the thermal head according to said embodiment. Figures 49A - 49C show the shapes of the heat generating element and the electrodes in a nineteenth embodiment. Figures 50A - 50C show the shapes of the heat generating element and the electrodes in a twentieth embodiment. Figure 51A shows the distribution of an electric current flowing through the heat generating element in the twentieth embodiment. Figure 51B shows the heat generation distribution in the cross-section G - G₁ of Figure 51A. Figure 51C shows the transfer area corresponding to applied energy to the heat generating element in the twentieth embodiment. Figures 52A - 52C show the shapes of the heat generating element and the electrodes in a twenty-first embodiment. Figures 53A - 53C show the shapes of the heat generating element and the electrodes in a twenty-second embodiment. Figures 54A - 54C show the transfer area when the applied energy by the heat generating elements in the twenty-first and twenty-second embodiments is changed. DESCRIPTION OF THE PREFERRED EMBODIMENTS The following embodiments will be described with respect to a thermal recording system using a thermal head, whereas the present invention is not restricted thereto, but is also applicable to a recording system which effects image recording by the use of heat, such as the ink jet recording system in which ink liquid is discharged by the use of heat to thereby effect image recording. [Description of Thermal Head (Figs. 1 - 4)] Figure 1 is an enlarged front view showing the shapes of the heat generating element 11 and the electrodes 12 and 13 of a thermal head 10 according to a first embodiment, and Figure 2 is a cross-sectional view showing the shape of the cross-section E - E₁ of Figure 1. In Figure 1, the reference numeral 11 designates a heat generating element of the thermal head 10, and this thermal head 10 is comprised of a plurality of such heat generating elements 11 (in this embodiment, each heat generating element has a width Wr of 150 µm and a length 2Lr = 160 µm) arranged in a row at a resolution of 6 dots/mm, and these heat generating elements are selectively electrically energized correspondingly to recording data, whereby they generate heat to effect image recording. The reference numeral 12 denotes a common electrode (having a width We of 40 µm) for supplying electric power to each heat generating element, and the reference numeral 13 designates a signal electrode (having a width We of 40 µm) provided correspondingly to each heat generating element, and the voltage level thereof is changed correspondingly to the recording data, whereby the control of the electrical energization of the heat generating elements 11 is executed. The heat generating elements 11 and the electrodes 12 and 13 of the thermal head 10 according to the present embodiment will now be described with reference to Figure 2. A glaze layer 16 as a heat accumulating portion is formed on an alumina substrate 17. Further, on the glaze layer 16, a resistance layer 15 and films of electrode layers 12 and 13 are formed as by vacuum evaporation or sputtering. A portion of the electrode layers and resistance layer is cut away as by photolithography or photoetching to thereby form the electrodes 12, 13 and heat generating elements 11, and further thereon, a wear resisting layer 14 is formed as by sputtering. Here, the portion called the heat generating element 11 refers to that portion of the resistance layer 15 which is laid free between the electrode layers 12 and 13. As is apparent from Figure 1, the electrode width We (40 µm) of portions 12a and 13a in which the heat generating element 11 and the electrodes 12 and 13 are connected together is smaller than the width Wr (150 µm) of the heat generating element 11. Figure 3A shows the flow of an electric current when a pulse voltage is applied to the heat generating element 11 of the thermal head 10 shown in Figure 1 through the electrodes 12 and 13 which are narrower than the width of the heat generating element 11. In Figure 3A, the reference numeral 20 designates an electric current flowing through the heat generating element 11, and the density of the electric current flowing through the heat generating element 11 is great near the junctions 12a and 13a between the narrow electrodes 12, 13 and the heat generating element 11. Figure 3B shows the temperature distribution near the electrode 12 in the heat generating element 11 (at the cross-section X - X₁ of Figure 3A). The temperature of the vicinity of the electrode 12 (or the electrode 13) (the vicinity of the junctions 12a and 13a) is particularly high in the heat generating element 11. As the time of application of the pulse signal is gradually lengthened to change the applied energy to the heat generating element 11, the widthwise temperature distribution at the cross-section X - X₁ of the heat generating element 11 varies as indicated by curves 42 → 43 → 44 in Figure 3B. In Figure 3B, Tm indicates the melting point of the ink of an ink sheet, and in a temperature area higher than this, the ink of the ink sheet is melted and transferred to a recording sheet. The pulse width of the applied pulse signal is 42 < 43 < 44. Accordingly, the area of the dot of the transferred ink becomes wider as indicated by 42A, 43A and 44A in Figure 3C, correspondingly to the variations in the temperature distributions 42, 43 and 44 of Figure 3B. In Figures 3B and 3C, the applied energy has been varied in three stages to vary the transfer area to three kinds, but if the applied energy is varied in further intermediate stages, the variation in the transfer area can be made finer and continuous gradation will become possible. That is, multivalue information can be recorded by a heat generating element 11. Further, as shown in Figure 3B, the temperature gradient near the electrodes of the heat generating element 11 in the present embodiment is sufficiently great as compared with the prior-art heat generating element 101 shown in Figure 9, and the temperature of the heat generating element 11 is of a value sufficiently greater than the melting point Tm of the ink. Therefore, the fluctuation of the recording density relative to a minute variation in the applied energy can be suppressed, and the fluctuation of the recording density D R relative to the applied energy assumes a gradient (a variation rate) as shown in the graph of Figure 4. Also, the scattering width S W of the recording density relative to each applied energy is as shown by the length of a vertical line in the graph, and is sufficiently small as compared with the case shown in Figure 9A. The reason why the thermal head according to the present embodiment is thus excellent in continuous gradation and also excellent in durability will hereinafter be described. (A) During the Recording of Small Dots When small dots are to be transferred for recording, the thermal head is driven with the applied energy to the heat generating element 11 of the thermal head made small. At this time, the current density in that portion of the heat generating element which is near the electrodes becomes high, and only that portion generates sufficient heat such that a small dot of ink is transferred. The energy applied to the heat generating element 11 at this time is small and therefore, the temperature of the heat generating element 11 does not rise so much and thus, there is no problem. (B) During the Recording of Medium Dots When dots of a medium size are to be transferred for recording, a somewhat higher energy is applied to the heat generating element 11 of the thermal head as compared with the recording of the aforedescribed small dots. Thereby, as in the aforedescribed case, the temperature of the portions near the junctions 12a and 13a between the electrodes 12, 13 and the heat generating element 11 first rises. Since the applied energy further rises, that portion of the heat generating element 11 which corresponds to the width of the electrodes generates heat and an elongate ink dot is transferred. This is because the electric current flowing through the heat generating element 11 is liable to flow over the shortest distance between the individual electrode 13 and the common electrode 12 and therefore the current density in that portion of the heat generating element 11 which corresponds to the width of the electrodes 12 and 13 becomes higher than in the other portions. If seems that there is no problem in the durability of the heat generating element 11 in this case because the applied energy is relatively small. (C) During the Recording of Large Dots When large dots are to be transferred for recording, a still higher energy is applied to the heat generating element 11 of the thermal head. Thereby, as in the aforedescribed case, the temperature of the portions near the junctions 12a and 13a between the electrodes 12, 13 and the heat generating element 11 and that portion of the heat generating element 11 which corresponds to the width of the electrodes 12 and 13 first rises, but since higher energy is applied to the heat generating element, that portion of the heat generating element which corresponds to the width of the electrodes becomes higher in temperature. Generally, the resistance value of the heat generating element 11 becomes greater as the temperature thereof rises and therefore, the resistance value of that portion of the heat generating element 11 which corresponds to the width of the electrodes increases due to the temperature rises and it becomes difficult for the electric current to flow through that portion. As a result, the electric current begins to flow through the outside of that portion of the heat generating element 11 which corresponds to the width of the electrodes, and the current density in that outside portion becomes higher and that portion generates heat. In this manner, a dot of a size corresponding to the size of the heat generating element 11 is transferred. Considering the durability of the heat generating element 11 at this time, the energy applied to the heat generating element 11 is great and thus the temperature thereof also becomes high. However, the resistance of that portion of the heat generating element 11 which becomes particularly high in temperature becomes great and it becomes difficult for the electric current to flow through that portion. Thereby, the electric current is dispersed to the outside of that portion of the heat generating element 11 which corresponds to the width of the electrodes. Thus, the peak temperature of the portion which becomes particularly high in temperature can be suppressed and the aforementioned outside portion can be caused to generate heat efficiently. Therefore, the deterioration of the heat generating element 11 during the formation of large dots is little, and this thermal head can be said to be a thermal head which is excellent in durability and heat efficiency. Accordingly, by controlling, for example, the time for which the pulse signal is applied to the thermal head, or the voltage value, i.e., the applied energy, the picture element density at which recording is effected by a heat generating element can be changed with good accuracy. Therefore, it becomes possible to express half tone and there can be provided a thermal head in which the irregularity of the density in half tone recording can be made sufficiently small and stable and which is sufficiently excellent in durability. Here, it has been found that the ratio of the width We of the electrodes to the width Wr of the heat generating element 11 and the ratio of the length (2Lr) of the heat generating element 11 to the width Wr of the heat generating element 11 greatly affect the multivalue gradation of the thermal head and the durability of the heat generating element 11. So, the relation between the applied energy and the recording density when use is made of a thermal head in which the width Wr of the heat generating element is fixed at 150 µm and the length (2Lr) of the heat generating element 11 is fixed at 160 µm and the width We of the electrodes 12 and 13 is changed from 10 µm to 120 µm has been found by the actual measurement The durability of the thermal head relative to the shape ratio (We/Wr) of the thermal head is shown in Table 1 below, and the relation of the recording density to the applied energy in each head is shown in Figure 5. Nos. of thermal heads Width (We) of electrodes We/Wr Durability of head (1) 10 µm 1/15 x (2) 15 µm 1/10 o (3) 40 µm 4/15 o (4) 75 µm 1/2 o (5) 120 µm 4/5 o Here, the durability of the thermal head is evaluated as "o" when for the pulse application 1 x 10⁷ (the printing rate 12.5 %), the rate of variation in the resistance value of the heat generating element 11 is equal to or less than ± 15 %. As is apparent from Table 1 above and Figure 5, the gradation and durability of the thermal heads Nos. (2), (3) and (4) in which We/Wr is within the range of 1/10 to 1/2 are very excellent, but in the head No. (1) wherein We/Wr is smaller than 1/10, the width of the electrodes is too small and therefore recording cannot be effected at a high density and moreover, the current density in the heat generating element becomes too high and therefore the durability of this heads is bad. Also, in the head No. (5) wherein We/Wr is greater than 1/2, the difference between the width of the electrodes and the width of the heat generating element is too small and therefore, a clear difference does not occur to the density of the electric current flowing through the heat generating element. Thus, no difference occurs to the heat generation distribution in the heat generating element, and as in the prior-art head, the irregularity of the density at the intermediate density becomes great and virtually only two-value recording can be effected. Next, the recording density relative to the applied energy has been actually measured by the use of a thermal head in which the width (Wr) of the heat generating element 11 is fixed at 150 µm and the width (We) of the electrodes 12 and 13 is fixed at 40 µm and the length (2Lr) of the heat generating element 11 is changed from 50 µm to 600 µm. The durability of the head for the shape (Lr/Wr) of the head which is the result of the measurement is shown in Table 2 below, and the recording density relative to each applied energy is shown in Figure 6. Here, Lr is the length between a point at which the exposed width of the electric current in the heat generating element 11 becomes greatest and a point at which the exposed width of the electric current becomes smallest. That is, in the thermal head of Figure 1, the distance from the center of the heat generating element 11 to the electrodes is indicated by Lr. Nos. of thermal heads Length (2Lr) of heat generating element Lr/Wr Durability of head (6) 50 µm 1/6 x (7) 75 µm 1/4 o (8) 150 µm 1/2 o (9) 450 µm 3/2 o (10) 600 µm 2 o As is apparent from Table 2 above and Figure 6, the gradation and durability of the thermal heads Nos. (7), (8) and (9) in which Lr/Wr is within the range of 1/4 - 3/2 are very excellent. However, in the head No. (6) wherein Lr/Wr is smaller than 1/4, the ratio of the length of the heat generating element 11 to the width of the heat generating element 11 is too small and therefore, going round of the electric current to the surroundings does not take place. Therefore, recording at a high density is difficult to accomplish and thus, the applied energy becomes higher and the durability of the head is reduced. Also, in the head No. (10) wherein Lr/Wr is greater than 3/2, the length of the heat generating element 11 is too great and therefore, the rate of the portion of the heat generating element 11 in which a difference in the current density occurs becomes smaller, and thus, as in the prior-art head, the irregularity at the intermediate density becomes greater and virtually only two-value recording can be effected. [Description of Thermal Head of Other Construction (Fig. 7)] Figure 7A is an enlarged front view showing the shapes of a heat generating element 11a and electrodes 12b and 13b in a thermal head 21 according to a second embodiment, Figure 7B shows the flow of an electric current in the heat generating element 11a, and Figure 7C shows the transfer area of ink when applied energy is varied. In this thermal head 21, the width of the common electrode 12b is smaller than the width of the heat generating element 11a, and the width of the individual electrode 13b is substantially equal to the width of the heat generating element 11a. Therefore, as shown in Figure 7B, the current density becomes higher in that portion of the heat generating element 11a which is near the common electrode 12b and the heat generation temperature thereof becomes higher. Figure 7C shows the transfer area of ink when this thermal head 21 is electrically energized. The transfer area of ink becomes gradually wider as indicated by 22, 23 and 24 as the applied energy is made greater, and when the greatest energy is applied, the transfer area extends substantially over the whole area of the heat generating element 11a. In this manner, the energy applied to the heat generating element 11a is adjusted to change the transfer area transferred to an ink sheet, whereby the expression of half tone becomes possible. Again in this head, the ratio of the width of the electrode 12b to the width of the heat generating element 11a and the ratio of the length of the heat generating element 11a to the width of the heat generating element 11a affect the multivalue gradation and durability. So, the relation between the applied energy and the recording density has been actually measured by the use of a thermal head in which the width (Wr) of the heat generating element 11a is fixed at 150 µm and the length (Lr) of the heat generating element 11a is fixed at 100 µm and the width (We) of the common electrode 13b is changed from 10 µm to 120 µm (the width of the individual electrode 13b is 150 µm). The result of the durability and the gradation reproducibility of the head for the shape (We/Wr) of the head at this time is shown in Table 3 below. Nos. of heads Width (We) of common electrode We/Wr Durability Harmonization (11) 10 µm 1/15 x o (12) 15 µm 1/10 o o (13) 40 µm 4/15 o o (14) 75 µm 1/2 o o (15) 120 µm 4/5 o x Here, the durability and gradation of the thermal head are excellent for We/Wr within the range of 1/10 - 1/2, for the same reason as that set forth in the first embodiment. Next, the recording density relative to the applied energy when in the thermal head 21 of Figure 7, the width (Wr) of the heat generating element 11a is fixed at 150 µm and the width of the electrode 12b is fixed at 40 µm and the length Lr of the heat generating element 11a is changed from 25 µm to 300 µm has been actually measured. Here, Lr is defined by the length between a point at which the exposed width of the electric current flowing through the heat generating element 11a becomes greatest (the vicinity of the individual electrode 13b) and a portion in which the exposed width of the electric current is narrowest (the vicinity of the common electrode 12b) and therefore, here, Lr is the length of the heat generating element 11a. The durability and the gradation reproducibility of the head relative to the shape (Lr/Wr) of the thermal head 21 at this time are shown in Table 4 below. Nos. of heads Length (Lr) of heat generating element Lr/Wr Durability Harmonization (16) 25 µm 1/6 x x (17) 37.5 µm 1/4 o o (18) 75 µm 1/2 o o (19) 225 µm 3/2 o o (20) 300 µm 2 o x The durability and gradation of this thermal head 21 are excellent when Lr/Wr is within the range of 1/4 - 3/2, as in the case of the previously described first embodiment. [Description of a Third Embodiment (Fig. 8)] Figures 8A - 8C illustrate a thermal head 27 according to a third embodiment. Figure 8A is an enlarged front view showing the shapes of the heat generating elements 11b and 11c and the electrodes 12c and 13c of the thermal head 27, Figure 8B shows the flows of an electric current in the heat generating elements 11b and 11c, and Figure 8C shows a variation in the ink transfer area when the energy applied to the heat generating elements 11b and 11c is varied. In this thermal head 27, the heat generating element is divided into two portions as indicated by 11b and 11c. These two heat generating elements 11b and 11c have substantially the same width as that of the electrodes 12c and 13c, and are connected together by the central conductor portion 13d which is narrower than the width of the common electrode 12c and the individual electrode 13c. The heat generating elements of this thermal head 27 are of a construction in which the heat generating element 11a in the second embodiment is connected in a pair with the conductor portion 13d interposed therebetween and therefore, the manner in which the electric current flows through the heat generating elements 11b and 11c and the variation in the transfer area of ink are basically similar to those in the case of the second embodiment, and are shown in Figures 8B and 8C, respectively. Here, the relation between the ratio of the width of the electrodes 12c and 13c to the width of the heat generating elements 11b and 11c and the ratio of the length of the heat generating elements 11b and 11c to the width of the heat generating elements 11b and 11c is also substantially similar to that in the second embodiment. That is, when the heat generating elements and the electrodes are of shapes which are in the relations that 1/10 ≦ We/Wr ≦ 1/2 1/4 ≦ Lr/Wr ≦ 3/2, the durability and gradation of the thermal head 27 become excellent. Further, the embodiment shown in Figure 8D is a modification of the embodiment shown in Figure 1, and also satisfies the aforementioned relations. That is, in this embodiment, a convex common electrode 12d and a signal electrode 13d are each connected to one of the ends of a heat generating element 11d, respectively. The common electrode 12d and the signal electrode 13d partly overlap the heat generating element 11d at their junctions with the heat generating element 11d. By the heat generating element 11d and the electrodes 12d, 13d being thus connected together with parts thereof overlapping each other, it becomes easier to manufacture the thermal head. An example of each size in the present embodiment will be shown below. We₁ = 50 µm We₂ = Wr = 147 µm 2Lr = 190 µm 2L = 150 µm Lo = 340 µm In the present embodiment, 2L is the distance between the electrodes 12d and 13d, but in some cases, it may be the length (2Lr) of the heat generating element. Again in the present embodiment, the aforementioned relations are satisfied and also, good half tone recording can be obtained. As described above , the durability and gradation of the thermal head are greatly related to the values of We/Wr and Lr/Wr. The reason for this is not clear, but the following reason is roughly conceivable. If the width of the electrodes is about one half of the width of the heat generating element, no difference occurs in the distribution of the density of the electric current flowing through the heat generating element, and the transfer area cannot be changed correspondingly to the applied energy and gradation cannot be reproduced. However, if conversely, the width of the electrodes is too small relative to the width of the heat generating element, however great energy is applied, the electric current flowing through the heat generating element will not expand to the marginal portion of the heat generating element and the transfer area will not expand correspondingly to the applied energy and therefore, it will become impossible to record large dots. Also, since at this time, great energy is applied to the heat generating element, the temperature of the vicinity of the junctions between the heat generating element and the electrodes in which the electric current concentrates will become very high as compared with the temperature of the other portion and the deterioration of the heat generating element by heat will become vehement. In a similar manner, if the length of the heat generating element from a position at which the exposed width of the electric current becomes narrowest to a position at which the exposed width of the electric current becomes greatest is about 1.5 times as great as the width of the heat generating element, the area of the portion in which there is no difference in the current density (the middle portion of the heat generating element) will become great and gradation cannot be reproduced. If conversely, the length of the heat generating element is too small, the electric current will not go round to the marginal portion of the heat generating element, but will flow substantially rectilinearly through the heat generating element and therefore, so great a difference will appear in the distribution of the current density in the direction of the electric current. Therefore, great energy must be applied to the heat generating element and the durability of the thermal head will become bad. As described above, according to the present embodiment, the ratio of the length of the heat generating element to the width of the electrodes in the junctions between the heat generating element and the electrodes and to the width of the heat generating element are limited within particular ranges, whereby there can be provided a thermal head of good gradation reproducibility. [Description of a Heat Transfer Recording Apparatus (Figs. 10 - 14)] Figure 10 is a block diagram schematically showing the construction of a heat transfer recording apparatus using the thermal head 10 according to the aforedescribed embodiment, and this figure shows the case of the thermal head 10 according to the first embodiment, however the use of a thermal head according to other embodiments can also realize such recording apparatus. In Figure 10, the reference numeral 110 designates a plain paper cassette containing therein plain paper, which is recording sheets, the reference numeral 111 denotes a sensor for detecting the presence or absence of the plain paper, and the reference numeral 106 designates a conveying motor for picking up and conveying the plain paper from the cassette 110. The reference numeral 123 denotes a stepping motor which rotatively drives a platen 34 through a reduction gear mechanism, not shown. The reference numeral 131 designates a motor for bringing the thermal head 10 up and down, and by the driving of this motor 131, the thermal head 10 is urged against the platen 34 with an ink sheet 32 and recording paper interposed therebetween (the down state), and is moved away from the platen 34 (the up state). The reference numeral 139 denotes a motor which is a drive source for a feeding mechanism for the ink sheet 32. The rotation of the motor 139 is transmitted to the drive shaft of a take-up roll 140, whereby the ink sheet 32 is taken up in the direction of the arrow. The reference numeral 141 designates a supply roll of the ink sheet 32. The reference numeral 35 denotes a buffer memory for temporarily holding input image data therein, and the reference numeral 36 designates an image data conversion table for converting the image data read out of the buffer memory 35. Usually the image data conversion table 36 is comprised of a look-up table such as ROM. The reference numeral 37 denotes a head driving pulse control circuit of which the details are shown in Figure 11. Figure 12 is a block diagram showing the construction of the thermal head 10. In Figure 12, the reference numeral 11 designates heat generating elements each of which is made on the basis of the aforedescribed embodiment, and which are provided for one line in the widthwise direction of the recording paper. The reference numeral 233 denotes a latch circuit for latching recording data for one line, and the reference numeral 234 designates a shift register which successively receives as inputs serial recording data (gradation data) 444 in synchronism with a clock signal CLK. The serial data thus input to the shift register 234 are latched in the latch circuit 233 by a latch signal 235 and are converted into parallel data. Thus, the recording data corresponding to each heat generating element is held in the latch circuit 233. The timing and time at which a voltage is applied are determined by a strobe signal STB 445. An output transistor 231 connected to an AND circuit 232 having data therein can thus be turned on. Thereby, the corresponding heat generating elements 11 are electrically energized from the common electrode 12 through the signal electrode 13, and the heat generating elements 11 are driven for heat generation. The head driving pulse control circuit 37 will now be described with reference to Figure 11. The reference numeral 450 designates an oscillator which outputs a clock signal CLK of a predetermined frequency, and the reference numeral 451 denotes a frequency divider circuit for frequency-dividing the clock signal CLK and outputting a latch signal 235 each time, for example, the number of the heat generating elements of the thermal head 10 for one line is counted. The reference numeral 440 designates a gradation conversion decoder which corresponds to each picture element of input picture element data and transports gradation data 444 to each shift register stage of a shift register 234 in synchronism with the signal CLK. Thereby, when for example, a color image is to be processed, gradation conversion is effected by the gradation conversion decoder 440 for each of colors Y, M and C. The reference numeral 441 denotes a gradation counter which counts up each time the latch signal 235 is input, and which carries out the counting of mod 64 (6 bits), for example, in the case of an ink sheet of sublimating property, and the counting of mod 16 (4 bits) in the case of an ink sheet of meltable property, on the basis of an instruction signal 443 from CPU 38 (Fig.10). The gradation conversion decoder 440 compares the count value from the gradation counter 441 with each input picture element data, and outputs "1" as gradation data 444 when the picture element data is greater than or equal to the count value, and outputs "0" when the picture element data becomes smaller than the count value. A strobe signal generating circuit 442 outputs a strobe signal STB 445 a little later than the latch signal 235, whereby the heat generating elements are driven and recording is effected. Figure 13 shows the driving of the thermal head 10 according to the aforedescribed embodiment of Figures 10 to 12, to which also reference will be made in the following, and the timing of the strobe signal STB. The thermal head 10 is a line type head, and the reference numeral 70 indicates the recording timing for one line. Assuming that the image data per picture element input to the gradation conversion decoder 440 comprises, for example, 6 bits, 64 kinds of data per picture element can be assumed. Thus, in this case, N of N gradation is "64". First, gradation data 444 for the first STB signal B₁ of the data for one line is transported to the shift register 234, and is latched in the latch circuit 233 by the latch signal 235. The strobe signal B₁ is then output and the heat generating element to which the data "1" has been output is driven by the pulse width of the strobe signal B₁. During this driving, the next gradation data 444 is input to the shift register 234, and when the STB signal 445 falls, said gradation data is latched in the latch circuit 233 by the latch signal 235. Thus, the STB signal is then output for B₂. Such an operation is executed 64 times (STB signals B₁ - B₆₄), whereby recording of one line is terminated. That is, the gradation conversion decoder 440 receives image data as an input, and when of the images, the value of the m th picture element data of the line to be recorded is "20", it outputs such data that the first 20 data are "1" and the second 44 (64 - 20) data are "0", 64 times in total, to the m th stage of the shift register 234 which corresponds to the position of that picture element data while referring to the value of the gradation counter 441. At this time, however, of course, data are set in the other stages of the shift register 234 in conformity with the degree of gradation of the corresponding picture element. At this time, each strobe signal STB has its pulse width changed correspondingly to the number of times of the outputting of the STB signal, as shown. It is the strobe signal generating circuit 442 that executes the adjustment of the pulse width of such a strobe signal STB. This strobe signal generating circuit 442, as previously described, receives as an input the gradation data 444 corresponding to the kind of the ink sheet 32 by a corresponding ROM table or the like, and adjusts the width, period, etc. of the STB signal 445 correspondingly to the kind of the ink sheet 32. Figure 14 is a flow chart showing the recording process in the heat transfer recording apparatus according to the present embodiment of Figures 10 to 13, to which also reference will be made in the following, and this recording process is stored in the ROM of the CPU 38. When at step S1, image data is input, advance is made to step S2, where the image data is stored in the buffer memory 35. At step S3, a recording sheet is picked up from the cassette 110 and conveyed to the recording position. At step S4, the ink sheet 32 is conveyed so that the desired position of the ink sheet 32 comes to the recording position. Advance is then made to step S5, where the motor 131 is driven to get down the thermal head 10. At step S6, picture elements for one line are read out from the buffer memory 35 and output to the head driving pulse control circuit 37 through the conversion table 36. Thereby, the gradation data 444, the latch signal 235 and the strobe signal STB are output with the timing as shown in Figure 13. Thereby the thermal head 10 is caused to generate heat, and transfer recording is effected on the recording sheet. Then step S7 is carried out, where the recording paper and the ink sheet 32 are conveyed by one line, and subsequently at step S8 is examined, whether the recording process for one page has been terminated. If the recording for one page is not terminated, return is made to the step S6, where picture element data for the next line is read out from the buffer memory 35, and the aforedescribed recording process is carried out again. In the case of color recording, recording is effected for one page unit of recording data for each color, and each time recording of each color is terminated, the color portion of the ink sheet to be recorded next time is conveyed to the recording position and the recording sheet also makes a round of the platen 34 and is returned to its original position, and recording is effected in another color. Such operation is performed for three colors such as Y, M and C, whereby full color recording can be accomplished on the recording sheet. Also, the gradation width of the aforementioned gradation data 444 and the pulse width of the strobe signal STB may be changed correspondingly to the kind of the ink sheet 32 and the kind of the recording sheet used. [Description of the Other Embodiments (Figs. 15 - 54)] A recording head according to another embodiment, which will now be described, is such that each of a plurality of heat generating resistance members includes at least one narrow portion narrower than the width of the heat generating resistance member and said narrow portion is formed by an electric conductor or an electric conductor portion provided on the heat generating resistance member. Therefore, the density of the electric current flowing through the heat generating resistance members is changed and the heat generation distribution is biased, whereby gradation expression is made possible. Also, according to still another recording head, a conductor portion is provided in a heat generating resistance member and the density of the electric current flowing through the heat generating resistance member can be changed to thereby change the heat generation distribution. Figures 15A - 15C show the shapes of the heat generating element and the electrodes of a thermal head according to another embodiment. Figure 15A is a top plan view of a heat generating element and the electrodes thereof, Figure 15B is a cross-sectional view taken along line X - X₁ of Figure 15A, and Figure 15C is a cross-sectional view taken along line Y - Y₁ of Figure 15A. The heat generating elements 111a, 111b and the electrode portions 112a, 112b of an electrode assembly 112 of this thermal head will hereinafter be described. In the thermal head according to the present embodiment, as shown in Figure 15C, a glaze layer 114 as a heat accumulating portion is formed on an alumina substrate 113. Further, a resistance layer 115 is formed on the glaze layer as by vacuum evaporation or sputtering. The central portion of the resistance layer 115 (a conductor layer portion 119) is cut as by photolithography or photoetching. An electrode layer 116 is formed on the resistance layer as by the aforementioned vacuum evaporation or sputtering. Further, as by photolithography or photoetching, the electrode layer 116 and the central conductor layer portion 119 are formed in the same process to thereby form the heat generating elements 111a, 111b and the electrode assembly 112. Further, a wear resisting layer 117 is formed on the electrode layer as by sputtering. The portion called the heat generating elements 111a, 111b forming the heat generating element assembly refers to the resistance layer in the portion wherein the electrode layer 116 is cut. As is apparent from Figure 15A, the thermal head 131 according to this embodiment is of a construction in which the heat generating element of the prior art thermal head is divided into two elements, which are connected together by a conductor portion 118 narrower in width than the heat generating element. Thus, the heat generating element in a picture element is divided into two heat generating elements 111a and 111b. Also, as is apparent from Figures 15B and 15C, the central glaze layer 114 in this picture element protrudes with respect to its surroundings and therefore, the thermal head 131 is convex in this portion. The reference character 112a designates a common electrode, and the reference character 112b denotes an individual electrode. An example of each size is shown here. The width (W₁) of the electrode assembly 112 is 147 µm, the width (W₂) of the conductor portion 118 is 40 µm, the length (L₁) of the conductor portion 118 is 52 µm, the distance (L₂) between the electrodes is 280 µm, and the distance (L₃) between the heat generating elements is 22 µm. The heat transfer recording portion will now be described. Figure 16 is a schematic view of the recording portion of a heat transfer recording apparatus using the thermal head 131 of Figure 15. The thermal head 131 provided with a plurality of heat generating elements 111a, 111b and electrodes 112a, 112b for supplying electrical energy to the heat generating elements 111a, 111b is disposed astride the full recording width and in opposed relationship with a platen 134. The thermal head is mounted in such a manner that it can be urged against and spaced apart from the surface of the platen by a mechanism, not shown. During recording, the heat generating portion of the thermal head 131 is urged against the platen 134 with a recording sheet 133 and an ink sheet 132 which are supported by the platen 134 being interposed therebetween. In this state, the heat generating elements 111a, 111b of the thermal head 131 are selectively driven on the basis of an image signal, whereby the ink of the ink sheet 132 is melted and transferred to the recording sheet 133 to thereby accomplish recording. The driving of the heat generating elements 111a, 111b is accomplished by a pulse voltage being applied to the electrodes 112a, 112b thereof. Each time predetermined recording is terminated, the recording sheet 133 and the ink sheet 132 are conveyed in the direction of the arrows in Figure 16. Figure 17A shows the flow of an electric current flowing from the upper common electrode toward the lower individual electrode of the electrode assembly 112 when a pulse voltage is applied to the heat generating elements 111a, 111b of the thermal head shown in Figure 15 through the electrode assembly 112. In Figure 17A, the reference numeral 120 represents the electric current flowing through the heat generating elements 111a, 111b, and the density of the electric current flowing through the heat generating elements 111a, 111b is great at the vicinity 111H of the junctions between the narrow conductor portion 118 and the heat generating elements 111a and 111b. Figure 17B shows the temperature distribution in the vicinity of the conductor portion 118 between the heat generating elements 111a, 111b (at the cross-section Z - Z₁ of Figure 17A). The temperature of the vicinity of this conductor portion 118 is particularly high in the heat generating elements 111a, 111b and the temperature difference thereof from the other portions of the heat generating elements 111a, 111b is very great. As the time of application of the pulse signal applied to the heat generating elements 111a, 111b is gradually lengthened to change the applied energy, the widthwise temperature distribution at the cross-section Z - Z₁ of e.g. the heat generating element 111b varies as indicated by curves 142 → 143 → 144 in Figure 17B. Since, with reference also to Figures 15C and 16, the glaze layer 114 of the thermal head 131 protrudes about the central portion in a picture element with respect to its surroundings, the pressure force applied to the recording sheet 133 concentrates in the vicinity of the junction between the conductor portion 118 and the heat generating elements 111a, 111b. Also, the glaze layer 114 is thick in the vicinity of this portion and therefore, the amount of accumulated heat in that portion becomes great and the concentration effect of the heat generating energy becomes more remarkable. Thus, in the portion near this junction, it becomes easy for the ink of the ink sheet 132 to be transferred. In Figure 17B, Tm indicates the melting point of the ink of the ink sheet 132, and in the temperature area higher than this, the ink of the ink sheet 132 is melted and transferred to the recording sheet 133. Accordingly, the area of a dot transferred widens as indicated by 142A, 143A and 144A in Figure 17C correspondingly to the variations in the temperature distributions 142, 143 and 144 of Figure 17B. At this time, the shape of the head in the vicinity of the junction between the conductor portion 118 and the heat generating elements 111a, 111b is convex. Therefore, the area of the dot is small and moreover, the reproducibility of image in the area of low recording density becomes good. Also, if as shown, for example, in Figure 17B, the applied energy is varied and, thereby, the temperature of the thermal head is varied, the transfer area will vary as shown in Figure 17C, so that for medium energy, a medium transfer area will be provided and continuous gradation will become possible. That is, multivalue information can be recorded by a heat generating element assembly comprised of the heat generating elements 111a, 111b. Also, as shown in Figure 17B, the temperature gradient of those portions of the heat generating elements 111a, 111b in the present embodiment which is near the electrodes, is sufficiently great as compared with the prior-art heat generating element shown in Figure 9, and the temperature of the heat generating elements 111a, 111b is of a value sufficiently greater than the melting point Tm of the ink. Therefore, the change in the recording density for a minute variation in the applied energy can be suppressed, and the fluctuation of the recording density for that applied energy assumes an inclination (a rate of variation) as shown in the graph of Figure 18. Also, the scattering width S W of the recording density D R for each applied energy E A is as shown by the length of the vertical line in the graph, and is sufficiently small as compared with the case shown in Figure 9A. Therefore, by controlling, for example, the time of application of the pulse signal to the thermal head or the voltage value thereof, i.e., the applied energy, the density of the picture element recorded by a heat generating element can be changed with good accuracy. Therefore, it becomes possible to express half tone, and the irregularity of the density in half tone recording can be made sufficiently small and stable. Also, the heat generating central portion is the central portion of the heat generating element assembly consisting of the heat generating elements 111a and 111b. Therefore, the amount of radiant heat to the upper (Fig. 17A) common electrode and the lower (Fig. 17A) individual electrode of the electrode assembly 112 becomes small and the mutual interference between adjacent dots becomes small. On the other hand, the conductor portion 118 which is near the heat generating portion is a heat radiating substance which takes the heat of the heat generating portion away and reduces the heat efficiency of the heat generating elements 111a, 111b, but since the capacity (area) thereof is very small, there is no problem such as the deterioration of the energy efficiency by radiant heat. Rather, it moderately takes the heat of the heat generating central portion away, and this leads to the effect that the temperature of the heat generating resistance member need not be locally too high. That is, the thermal head according to the present embodiment is excellent in resolution and can effectively use the heat generating energy and further, by being endowed with a heat generation distribution, multivalue recording becomes possible. While the present embodiment is of such a construction that the resistance layer 115 is not present in the lower layer of the narrow conductor portion 118, a similar effect has been recognized even when the resistance layer remains provided in the lower layer of the conductor portion 118 for the simplification or the like of the method of manufacturing the head. Also, the conductor portion 118 can be formed of a material having an electric conductivity substantially equal to that of the material of the electrode layer 116 forming the electrode portions 112a, 112b (Fig. 15A), and need not always be the same material. Also, in this embodiment, the heat generating element assembly has been described as being divided into two heat generating elements, but may be divided into more elements. In such case, of course, the number of conductor portions for connecting these heat generating elements will be increased. Figure 19 is a top plan view of the heat generating elements and the electrodes of a thermal head according to still another embodiment. In this thermal head, as compared with the thermal head according to the aforedescribed embodiment, the number of the narrow conductor portions is changed from one to two, and in Figure 19, portions common to those in Figure 15 are designated by the same reference characters. In the thermal head of Figure 19, two heat generating elements 111a and 111b are connected together by two conductor portions 118a and 118b provided on the opposed sides of these heat generating elements 111a and 111b and provided in spaced apart relationship with each other. The heat generating elements 111a and 111b and the conductor portions 118a and 118b are connected together, respectively, at two points (153, 154 and 155, 156) and therefore, the electric current concentrates in the vicinity of these junctions and the current density near the junctions becomes great. Thus, the heat energy applied to the thermal head is varied by a principle similar to that of the aforedescribed embodiment, whereby the dot area of ink transferred can be varied as indicated by hatched portions in Figures 20A - 20C. In Figures 20A - C, portions common to those in Figure 19 are designated by the same reference characters, and the applied energy is A < B < C. In this manner, the dot area is changed correspondingly to the applied energy, whereby half tone expression becomes possible. Thus, according to said still another embodiment, in addition to the individual electrode 112b and the common electrode 112a, conductor portions narrower than the heat generating elements are formed and the heat generating elements on the individual electrode side and the common electrode side are electrically connected together by the conductor portions. By controlling the pulse width of the driving pulse applied by the heat generating elements, the amount of ink transferred to the recording sheet can be varied to express half tone. The number of the conductor portions is not limited to two. Figure 21A shows an example of the structure of the heat generating element and the electrodes of a thermal head according to a further embodiment, and in Figure 21A, portions common to those in Figure 15 are designated by the same reference characters. Here, the heat generating element 111c of the thermal head is not divided into two, and a conductor 121 is provided in the central portion of the heat generating element 111c. An example of each size in the present embodiment is shown: the width (W₁) of the electrodes 112a, 112b is 147 µm, the width (W₂) of the conductor is 40 µm, the length (L₁) of the heat generating element is 280 µm, and the length (L₂) of the conductor is 52 µm. In Figure 21A, the conductor 121 is shown as being long sideways, but the vertical size thereof is shown shortened, and in reality the above values W₂ and L₂ are realized in this exemplary embodiment. Figure 21B shows the flow of the electric current in the thermal head of Figure 21A. Here, as indicated by 122, the electric current concentrates in the conductor 121 at the center of the heat generating element 111c which is small in resistance value and therefore, the current density near the central conductor 121 becomes high and heat generation takes place concentratedly about this portion. So, if the energy applied to the thermal head is varied, the transfer area of the ink sheet changes as indicated by hatched portions in Figures 22A-22C and therefore, multivalue recording becomes possible. Thus, in the thermal head according to said further embodiment, as compared with the aforedescribed embodiment, the ink transfer in the opposite end portions becomes easy to effect. Of course, the location and number of the conductor 121 are not limited to those shown in this embodiment. As described above, according to the present embodiment, a shape in which a conductor portion is provided in the heat generating element of the thermal head is adopted, whereby the thermal head is formed so that there is provided a difference in the density of the electric current flowing through the heat generating element, and the glaze layer in the portion wherein the density of the electric current becomes great is protruded into a convex shape, whereby the control of the heat generating area, i.e., gradation expression, can be accomplished easily. Other embodiments will further be described by the use of some further embodiments. The embodiment which will now be described is a thermal head provided with heat generating elements and electrodes for supplying electrical energy to said heat generating elements, and the shape of said heat generating elements or said electrodes is formed so that the density of the electric current flowing through said heat generating elements may be rough and fine, and the glaze layer is protruded with respect to its surroundings in the portion thereof wherein the density of the electric current is fine. Figures 23A - C show the shapes of the heat generating element and the electrodes of a thermal head according to one more embodiment. Figure 23A is a top plan view of a heat generating element and the electrodes thereof, Figure 23B is a cross-sectional view taken along line Y-Y₁ of Figure 23A, and Figure 23C is a cross-sectional view taken along line X-X₁ of Figure 23A. The heat generating element 211 and the electrodes 212 of this thermal head will hereinafter be described. In the thermal head according to the present embodiment, a glaze layer 214 as a heat accumulating portion is formed on an alumina substrate 213. Further, a resistance layer 215 and an electrode layer 216 are formed on the glaze layer as by vacuum evaporation or sputtering. The electrode layer 216 is formed as by photolithography or photoetching, and a wear resisting layer 217 is further formed thereon as by sputtering. Here, the portion called the heat generating element 211 refers to the resistance layer in which the electrode layer 216 is cut. As is apparent from Figure 23A, the width of the electrode layer 216 (the electrodes 212) is smaller than the width of the heat generating element 211. As is clear from Figure 23B, the glaze layer 213 corresponding to the width of the electrodes 212 protrudes with respect to its surroundings with the vicinity of the junctions between the heat generating element 211 and the electrodes 212 (the point at which the electrode layer 216 is cut) as the vertex and therefore, this portion is convex. Figure 24A shows the flow of an electric current when a pulse voltage is applied to the heat generating element 211 of the thermal head shown in Figures 23A-23C through the electrodes 212 narrower than the width of this heat generating element 211. In Figure 24A, the density of the electric current flowing through the heat generating element 211 is great near the junctions between the narrow electrodes 212 and the heat generating element 211. Figure 24B shows the temperature distribution in the portions of the heat generating element 211 which are near the electrodes 212 (at the cross-section Z-Z₂ of Figure 24A). The temperature of the vicinity of the electrodes 212 is particularly high in the heat generating element 211, and the temperature difference thereof from the other portion of the heat generating element 211 is very great. As the time of application of the pulse signal applied to the heat generating element 211 is gradually lengthened to change the applied energy, the widthwise temperature distribution at the cross-section Z-Z₂ of the heat generating element 211 varies as indicated by curves 242 → 243 → 244 in Figure 24B. Here, the thermal head is of such a construction that the glaze layer 213 thereof is protuberant with the vicinity of the junctions between the heat generating element 211 and the electrodes 212 as the vertex and is somewhat concave in the central portion of the heat generating element 211. Thus, the pressure force applied to the recording sheet 133 (Figure 16) concentrates in the vicinity of the junctions between the electrodes 212 and the heat generating element 211. Also, the glaze layer 213 is thick near this portion and therefore, the amount of accumulated heat in that portion becomes great and the concentration effect of the heat generating energy becomes more remarkable. Thus, in the portions near the junctions, it becomes easy for the ink of the ink sheet to be transferred. In Figure 24B, Tm indicates the melting point of the ink of the ink sheet 132 (Figure 16), and in a temperature area higher than this, the ink of the ink sheet is melted and transferred to the recording sheet. Accordingly, the area of a dot transferred widens as indicated by 242A, 243A and 244A in Figure 24C, corresponding to the variations in the temperature distributions 242, 243 and 244 of Figure 24B. At this time, the shape of the head in the vicinity of the junctions between the electrodes 212 and the heat generating element 211 is convex and therefore, the area of the dot becomes small and moreover, the reproducibility of image in the area wherein the recording density is low becomes good. Also, when the applied energy is varied as shown, for example, in Figure 24B, the transfer area varies as shown in Figure 24C. Therefore, multivalue information (for example, in the case of Figure 24C, 2-bit data) can be recorded by such a heat generating element 211. Also, as shown in Figure 24B, the temperature gradient of the portions of the heat generating element 211 in the present embodiment which are near the electrodes is sufficiently great as compared with the prior-art heat generating element shown in Figure 9, and the temperature of the heat generating element 211 is of a value sufficiently greater than the melting point Tm of the ink. Therefore, the change in the recording density for a minute variation in the applied energy can be suppressed, and the fluctuation of the recording density D R for that applied energy E A assumes an inclination (a rate of variation) as shown in the graph of Figure 25. Also, the scattering width S W of the recording density D R for each applied energy E A is as indicated by the length of the vertical line in the graph, and is sufficiently small as compared with the case shown in Figure 9A. Therefore, by controlling, for example, the time of application of the pulse signal to the thermal head, or the voltage value thereof, i.e., the applied energy, the density of picture elements recorded by a heat generating element can be changed with good accuracy. Therefore, it becomes possible to express half tone and the irregularity of the density in half tone recording can be made sufficiently small and stable. Figures 26A - 26C show an example of the structure of the heat generating element 271 and the electrodes 272 of a thermal head according to a ninth embodiment, wherein Figure 26A is a top plan view thereof, Figure 26B is a cross-sectional view showing the cross-section H-H₁ of Figure 26A, and Figure 26C is a cross-sectional view showing the cross-section G-G₁ of Figure 26A. In these figures, portions common to those in Figures 23A-23C are designated by the same reference characters. In the thermal head of Figures 26A - C, electrodes 272 are connected to the heat generating element 271 at two locations toward the opposite sides of the heat generating element 271 and at two spaced apart points (273, 274 and 275, 276). Therefore, the electric current concentrates in those portions of the heat generating element 271 which are near the junctions at which the electrodes 272 are connected to the heat generating element 271, and the density of the electric current in the vicinity thereof becomes great. Portions designated by 277 and 278 in Figure 26A are, for example, opening portions or insulating portions. So, as shown in Figures 26B and 26C, the glaze layer 213 corresponding to the vicinity of the portions of the heat generating element in which the electrodes 272 are connected to the heat generating element is made thick and protrudes with respect to its surroundings. Thus, the heat energy applied to the head is varied by a principle similar to that of the aforedescribed embodiment, whereby the dot area of the ink transferred can be varied as shown in Figures 27A-27C. In Figures 27A-27C, portions common to those in Figures 26A-26C are designated by the same reference characters, and the applied energy is A < B < C. The transferred area is represented by the portions of the heat generating element 271 which are indicated by hatching and thus, as is apparent from the figures, the transfer of the ink widens from the four corners of the heat generating element 271 (the portions corresponding to 273-276 in Figures 26A-26C). In this manner, the dot area is changed correspondingly to the applied energy, whereby half tone expression becomes possible. Thus, according to the present embodiment, the shapes, sizes, widths, etc. of the heat generating element and the electrodes are formed so that the electric current flowing through the heat generating element may be partly dense, and the glaze layer 213 in the portion wherein the density of the electric current becomes great (the heat generation temperature becomes high) is protruded into a convex shape with respect to its surroundings. By controlling the pulse width of the driving pulse applied to the heat generating element, the amount of ink transferred to the recording sheet is varied and half tone can be expressed. As described above, according to the present embodiment, the shapes of the heat generating element and the electrodes of the thermal head are formed so that there may be provided a difference in the density of the electric current flowing through the heat generating element, and the glaze layer in that portions wherein the density of the electric current becomes great are protruding in a convex shape, whereby the control of the heat generating area, i.e., gradation expression, can be accomplished easily. Other embodiments will further be described by the use of tenth to fourteenth embodiments. The embodiment of the thermal head which will now be described is one which is provided with a plurality of heat generating resistance members arranged in a row and electrodes for supplying electrical energy to respective ones of said plurality of heat generating resistance members and in which the width of said electrodes is less than the effective width of said heat generating resistance members at the junctions between said electrodes and said heat generating resistance members and the width of the electrodes in the portion thereof remote from said junctions is greater than the width of the electrodes at said junctions. Figure 28 shows the shapes of a heat generating element and electrodes of a thermal head 310 according to still another embodiment, and Figure 29 is a cross-sectional view showing the cross-section E-E₁ of Figure 28. In Figure 28, the reference numeral 311 designates a heat generating element of the thermal head 310, and this thermal head 310 is comprised of a plurality of such heat generating elements 311 arranged in a row, and image recording is accomplished by these heat generating elements being selectively electrically energized corresponding to recording data. The reference numeral 312 denotes a common electrode for supplying electric power to each heat generating element, and the reference numeral 313 designates a signal electrode provided correspondingly for each heat generating element, and by the voltage level thereof being changed corresponding to the recording data, the control of the electrical energization of the heat generating elements 311 is executed. The heat generating element 311 and the electrodes 312 and 313 of the thermal head 311 according to the present embodiment will now be described with reference to Figure 29. A glaze layer 316 as a heat accumulating portion is formed on an alumina substrate 317. A resistance layer 315 and an electrode layer 312 are further formed on the glaze layer as by vacuum evaporation or sputtering. The electrodes 312 and 313 and the heat generating element 311 are formed as by photolithography or photoetching, and a wear resisting layer 314 is further formed thereon as by sputtering. The portion called the heat generating element 311 refers to the resistance layer portion laid free between the electrode layers 312 and 313. As is apparent from Figure 28, the electrode width of junction portions 312a and 313a at which the heat generating element 311 is connected to the electrodes 312 and 313 is smaller than the width of the heat generating element 311, and the electrode width in the portion remote from the junction portions 312a and 313a is substantially equal to the width of the heat generating element 311. Figure 30A shows the flow of an electric current when a pulse voltage is applied to the heat generating element 311 of the thermal head 310 shown in Figure 28 through the electrodes 312 and 313 by means of the junction portions 312a and 313a smaller in width than the heat generating element 311. In Figure 30A, the reference numeral 320 designates an electric current flowing through the heat generating element 311, and the density of the electric current flowing through the heat generating element 311 is great near the narrow junctions or junction portions 312a and 313a between the electrodes 312, 313 and the heat generating element 311. Figure 30B shows the temperature distribution near the electrode 312 in the heat generating element 311 (at the cross-section X-X₁ of Figure 30A). The temperature of the vicinity of the electrode 312 (or the electrode 313) is particularly high in the heat generating element 311. As the time of application of the pulse signal applied to the heat generating element 311 is gradually lengthened to change the applied energy to the heat generating element 311, the widthwise temperature distribution at the cross-section X-X₁ of the heat generating element 311 varies as indicated by curves 342 → 343 → 344 in Figure 30B. In Figure 30B, Tm indicates the melting point of the ink of the ink sheet 132 (Figure 16), and in a temperature area higher than this, the ink of the ink sheet 132 is melted and transferred to the recording sheet 133 (Figure 16). Accordingly, the area of a dot transferred widens as indicated by 342A, 343A and 344A in Figure 30C, corresponding to the variations in the temperature distributions 342, 343 and 344 of Figure 30B. The width of the electrodes 312 and 313 is small near the junctions 312a and 313a by which the electrodes are connected to the heat generating element 311, but in the other portions the width of the electrodes is substantially equal to the width of the heat generating element 311. Therefore, as compared with the heat generating element of the prior-art thermal head, no special accuracy is required of this heat generating element and therefore, the yield during the manufacture changes very little. Consequently, there can be provided a thermal head which is good in productivity and low in cost. How a variation in the recording density appears by varying the applied energy to such a thermal head has been measured by the use of the thermal head. The result is shown in Figure 31. As the result, the recording density D R assumes an inclination (a rate of variation), as shown in the graph of Figure 31, for the magnitude of the applied energy E A , and the scattering width S W of the density becomes small as indicated by the length of vertical lines in the graph. Therefore, by controlling, for example, the time of application of the pulse signal applied to the thermal head 310, or the voltage value thereof, i.e., the applied energy, the density of picture elements recorded by a heat generating element can be changed with good accuracy. Thus, it becomes possible to express half tone and the irregularity of the density in half tone recording can be made sufficiently small and stable. Figure 32 shows the shapes of the heat generating element 311 and the electrodes 312b and 313b of a thermal head according to an eleventh embodiment. In this heat generating element 311, the electrode width in the vicinity of the junctions between the common electrode 312b and the heat generating element 311 and between the signal electrode 313b and the heat generating element 311 is smaller than the width of the heat generating element 311, and in the portion remote from the heat generating element 311, the electrode widths are substantially equal to the width of the heat generating element 311. Generally, the heat of the heat generating element 311 is radiated into the air and in addition, is radiated through the electrodes 312b and 313b which are high in heat conductivity. Accordingly, by adopting a shape as shown in Figure 32, the heat radiation of the heat generating element 311 takes place through the electrodes 312b and 313b proximate thereto by a distance L. Therefore, the heat radiation of the heat generating element 311 does not concentrate in the vicinity of the junctions between the electrodes and the heat generating element 311. Thus, the heat radiation of the heat generating element 311 becomes substantially uniform over the entire element 311, whereby the heat concentration effect can be enhanced more. The distance L between the heat generating element 311 and the electrodes 312b, 313b need be made longer than the limit value of the manufacturing accuracy of the head in order to prevent the contact between the electrodes and the heat generating element 311. If conversely, the distance L is too long, the heat radiation effect by the electrodes 312b and 313b will not be obtained and thus, the heat radiation of the heat generating element 311 will concentrate in the vicinity of the junctions between the heat generating element and the electrodes. Further, if the distance L is too long, the portion in which the width of the electrodes at the junctions between the heat generating element 311 and the electrodes 312b, 313b will become long and therefore, the yield of manufacture will become bad. The above result can be collectively shown in the Table below. Distance L Gradation Yield of manufacture 0.05 µm - X (bad) 0.1 µm o ○ ○ 1 µm o ○ o ○ 10 µm o ○ o ○ 50 µm o ○ o ○ 3 mm ○ o ○ 10 mm ▵ X As is apparent from the above table, the gradation, when thermal heads in which the distance L is 0.1 µm - 3 mm are used to vary the amount of the applied energy, is good. In addition, there is no problem in the yield of manufacture of these thermal heads. However, when the distance L is less than 0.1 µm, manufacture is impossible in respect of the manufacturing accuracy, and when the distance L is greater than 3 mm, the portion in which the width of the electrodes is small becomes long and therefore, the yield of manufacture becomes bad. Figure 33 is an enlarged front view showing the shapes of the heat generating element 311 and the electrodes 312c and 313c of a thermal head 321 according to a twelfth embodiment. The electrode 312c is a common electrode, and the electrode 313c is a signal electrode. As shown, in the junctions between the heat generating element 311 and the electrodes 312c and 313c, the width of the electrodes 312c and 313c is sufficiently small as compared with the width of the heat generating element 311. In the portions remote from the junctions of the electrodes 312c and 313c and moreover, in the portions near the marginal portion of the heat generating element 311, the electrodes 312c and 313c are proximate to the heat generating element 311 with a minute space formed therebetween. The heat generation temperature distribution when this heat generating element 311 is electrically energized is shown in Figure 34B. Figure 34A shows the distribution of an electric current flowing through the heat generating element 311, and in the vicinity of the junctions between the heat generating element 311 and the electrodes 312c and 313c in which the width of the electrodes is small, the density of the electric current is high. Further, the narrow portions of the electrodes 312c and 313c are in contact with the heat generating element 311, but as indicated by 312x and 312y as well as 313x and 313y, portions of the electrodes are proximate to the vicinity of the marginal portion of the heat generating element 311. Therefore, the heat of the marginal portions of the heat generating element 311 is conducted and radiated through the portions 312x and 312y as well as 313x and 313y of the electrodes. Thus, in a state in which the heat generating element 311 is generating heat, the temperature of the marginal portions of the heat generating element 311 becomes particularly lower than that of the other portions. Figure 34B shows the heat generation temperature distribution of the portion of the heat generating element 311 which is near the electrode 312c or analogous 313c and particularly high in temperature (the portion Y-Y₁ of Figure 34A), and the temperature difference between this portion and other portions of the heat generating element is particularly great. So, as the time of application of the pulse voltage is gradually lengthened to change the applied energy, the widthwise temperature distribution of the heat generating element 311 varies as indicated by curves 323 → 324 → 325 in Figure 34B. If at this time, the melting point of the ink of the ink sheet 132 (Figure 16) is Tm, the ink is melted in a temperature area higher than Tm and is transferred to the recording sheet 133 (Figure 16). Accordingly, the dot area transferred widens as indicated by 323A, 324A and 325A in Figure 36C, in conformity with the variation in the temperature distribution shown in Figure 34B. The time of application of the pulse is in the relation that 323 < 324 < 325. Also, the relation between the applied energy E A and the recording density D R by this thermal head 321 assumes an inclination corresponding to the applied energy as in the aforedescribed case of Figure 31, and the scattering width S W of each density is small as indicated by a vertical line in the graph. Figures 35 and 36 show the shapes of the heat generating elements and the electrodes of thermal heads according to thirteenth and fourteenth embodiments. In the heat generating element 311 and electrodes 312d and 313d of Figure 35, the portions of the electrodes 312d and 313d which protrude toward the heat generating element 311 are made greater in width away from the heat generating element 311 in order to increase the heat radiation effect of the marginal portions of the heat generating element 311. Figure 36 shows the shapes of the heat generating element 311 and the electrodes 312e and 313e of the thermal head according to the fourteenth embodiment. In this embodiment, the electrodes 312e and 313e are bifurcated halfway and connected to the marginal portions of the heat generating element 311. The protruded portion 326, respectively 327, of each electrode is proximate to the corresponding central portion of the electrode which is remote from the junctions. The temperature distribution when this heat generating element 311 is electrically energized is shown in Figure 37. In this figure, in the vicinity of the junctions between the electrodes 312e, 313e and the heat generating element 311 the density of the electric current flowing through the heat generating element 311 becomes highest in temperature, and becomes the center of heat generation (transfer) as indicated by 328 in Figure 37. Thus, according to the heat generating element of Figure 36, the areas 328 which are the centers of transfer are present at four locations in this heat generating element and therefore, there is the effect that gradation increases and resolution increases. As described above, in the thermal head according to the present embodiment, the width of the electrodes at the junctions between the heat generating element and the electrodes is made smaller than the width of the heat generating element and the width of the electrodes in the portions thereof remote from the junctions is made greater than the width of the electrodes at the junctions, whereby the yield during the manufacture of the head is good, and by controlling the applied energy, gradation expression can be accomplished easily. Also, in the thermal head according to the present embodiment, a portion of the electrodes is made proximate to the heat generating element, whereby the heat radiation of the heat generating element can be changed to make the temperature of the heat generating element more uniform. Other embodiments will further be described by the use of still other embodiments. The embodiment of the thermal head which will now be described is one which is provided with a plurality of heat generating resistance members arranged in a row and electrodes for supplying electrical energy to respective ones of said plurality of heat generating resistance members and in which the width of said electrodes is smaller than the effective width of said heat generating resistance members at the junctions between said electrodes and said heat generating resistance members and the resistance value of said heat generating resistance members is higher in the vicinity of said junctions. Figure 38 is an enlarged front view showing the shapes of a heat generating element 11 and the electrodes 12 and 13 of a thermal head 10 according to still another embodiment, and Figure 39 is a cross-sectional view showing the cross-section E-E₁ of Figure 38. In Figure 38, the reference numeral 11 designates a heat generating element of the thermal head 10, and this thermal head 10 is comprised of a plurality of such heat generating elements 11 arranged in a row, and these heat generating elements are selectively electrically energized corresponding to recording data, whereby image recording is accomplished. The electrode 12 is a common electrode for supplying electric power to each heat generating element, and the electrode 13 is a signal electrode provided correspondingly to each heat generating element, and by the voltage level thereof being changed correspondingly to the recording data, the control of the electrical energization of the heat generating elements 11 is executed. The heat generating element 11 and the electrodes 12 and 13 of the thermal head 10 according to the present embodiment will now be described with reference to Figure 39. A glaze layer 16 as a heat accumulating portion is formed on an alumina substrate 17. Resistance layers 15a, 15b, 15c and an electrode layer are further formed on the glaze layer as by vacuum evaporation or sputtering. The electrode layer and the resistance layers are partly cut away as by photolithography or photoetching to thereby form the electrodes 12 and 13 and the heat generating element 11, and a wear resisting layer 14 is further formed thereon as by sputtering. Here, the portion called the heat generating element 11 refers to the resistance layer portion laid free between the electrode layers 12 and 13. As is apparent from Figure 38, the width of the electrodes in the portions 12a and 13a thereof wherein the heat generating element 11 and the electrodes 12 and 13 are connected together is smaller than the width of the heat generating element 11. Furthermore, in Figure 38, the letter a indicates the length of the resistance layer 15a in the direction of flow of the electric current, and likewise, the letter b indicates the length of the resistance layer 15b in the direction of flow of the electric current. The letter c indicates the length between the electrodes 12 and 13 which form the heat generating element 11. By such a construction, the resistance layer portions comprising resistance layers 15a - 15c are of a shape in which a level difference is provided in the thickness along the direction of flow of the electric current flowing from the electrode 12 toward the electrode 13, and the resistance value of the heat generating element 11 varies non-continuously along the direction of flow of the electric current. Thus, the resistance value of the portions of the heat generating element 11 which are near to the electrodes 12 and 13 is great and the central portion of the heat generating element 11 is smallest in resistance value. The glaze layer in the central portion of the heat generating element 11 is formed concavely in advance so that the central portion of the heat generating element 11 may not be made convex by a level difference being thus provided in the resistance layer. Figure 40A shows the flow of the electric current when a pulse voltage is applied to the heat generating element 11 of the thermal head 10 shown in Figure 38, through the electrodes 12 and 13 which are smaller in width than this heat generating element 11. In Figure 40A, the reference numeral 20 designates the electric current flowing through the heat generating element 11, and the density of the electric current flowing through the heat generating element 11 is great near the junctions 12a and 13a between the narrow electrodes 12, 13 and the heat generating element 11. Figure 40B is a graph showing the temperature distribution of that portion of the heat generating element 11 which is near the electrode 12 (at the cross-section X-X₁ of Figure 40A). The temperature of the vicinity of the electrode 12 (or the electrode 13) (i.e. the vicinity of the junctions 12a and 13a) is particularly high in the heat generating element 11. As the time of application of the pulse signal applied to the signal electrode 13 of the heat generating element 11 is gradually lengthened to change the applied energy to the heat generating element 11, the widthwise temperature distribution at the cross-section X-X₁ of the heat generating element 11 varies as indicated by curves 42 → 43 → 44 in Figure 40B. In Figure 40B, Tm indicates the melting point of the ink of the ink sheet 132 (Figure 16), and in a temperature area higher than this, the ink of the ink sheet 132 is melted and transferred to the recording sheet 133 (Figure 16). The relation of the pulse width of the applied pulse signal is 42 < 43 < 44. Figure 40C shows the temperature distribution at the cross-section Y-Y₁ of Figure 42A, and correspondingly to 42-44 in Figure 40B, the temperature distribution is shown by 42a, 43a and 44a corresponding to the respective times of application. As is apparent from this figure, the temperature assumes peaks near the junctions between the heat generating element 11 and the electrodes 12 and 13, and the heat generation temperature is low in the central portion of the heat generating element 11. Also, the curve indicative of the temperature distribution is substantially vertical in the portion which intersects the temperature Tm and therefore, the irregularity of the transfer area is small. This is because the junctions 12a and 13a are highest in the density of the electric current and the resistance value of the heat generating element 11 is higher toward the vicinity of the junctions and moreover, the resistance value of the heat generating element 11 varies non-continuously along the direction of flow of the electric current. Again in Figure 40C, Tm indicates the melting point of the ink of the ink sheet 132, and a, b and c indicate the lengths of the resistance layers of Figure 38 in the direction of flow of the electric current. Also, the glaze layer is concave in the central portion of the heat generating element 11 and the thickness thereof is increased at the junctions between the heat generating element 11 and the electrodes and therefore, the amount of radiant heat is great in the central portion of the heat generating element 11 and conversely, the amount of radiant heat is small near the electrodes. Thus, the amount of the heat generated near the junctions between the electrodes and the heat generating element and radiated through the electrodes and the alumina substrate becomes smaller and the concentration effect of the heat generating energy becomes more remarkable. By such an effect, the shape of a dot transferred by the heat generating element 11 of this thermal head 10 becomes such as shown by 42b, 43b and 44b in Figure 41. As is apparent from this figure, a fine edge portion is formed at the level difference in the thickness provided by the resistance layers 15a and 15b. Therefore, the transfer area can be changed with good accuracy correspondingly to the applied energy and the reproducibility of harmony becomes good. Figure 42 is a graph showing the relation of the recording density D R to the applied energy E A in this thermal head 10 of Figure 38. In Figure 42, the inclination (the rate of variation) of the recording density D R relative to the magnitude of the applied energy E A is sufficiently small as compared with the case of Figure 9A, and the scattering width S W of the recording density D R relative to each applied energy E A also is small as indicated by the length of vertical lines in the graph. Particularly, the variation in the recording density in areas indicated by A, B and C in Figure 42 is small, and these areas correspond to 42b, 43b and 44b, respectively, indicative of the transfer areas of Figure 41. Thus, by controlling the applied energy to this thermal head 10, half tone can be expressed and the irregularity of the recording density in this half tone recording becomes sufficiently small and stable. Also, the thickness of the heat generating element 11 of this thermal head 10 is varied at two stages and therefore, the scattering width of the density is particularly small at two points of medium density. Therefore, if multivalue recording of total four (2+2) values is effected at said two points and two points of solid density and density "0", the density reproducibility will become very good. For such a reason, gradation recording can be accomplished with good reproducibility even on coarse paper having a small degree of smoothness. Figure 43 is an enlarged front view showing the shapes of a heat generating element 11a and the electrodes 12b and 13b of a thermal head 21 according to a sixteenth embodiment. In this thermal head 21, the width of the heat generating element 11a is smaller near the common electrode 12b and the signal electrode 13b, and is greater in the central portion than of the electrodes 12b and 13b. In other words, the width of the electrodes 12b and 13b is smaller than that of the central portion of the heat generating element 11a. Therefore, the resistance value is higher in the portions of the heat generating element 11a which are near the electrodes, and the heat generation temperature of those portions is also higher. Figures 44A and 44B show the transfer area when this thermal head 21 is electrically energized. Here is shown an area in which, as described with reference to Figure 40, the heat generation temperature of the heat generating element 11a becomes higher than the melting point Tm of the ink of the ink sheet 132. The reference numerals 22 and 23 indicate the transfer areas when the applied energy to the heat generating element 11a is made small, and the reference numeral 24 indicates the transfer area when the applied energy is made great, and in this case, the transfer area covers substantially the entire area of the heat generating element 11a. In this manner, the energy applied to the heat generating element 11a is adjusted to change the transfer area of the ink sheet transferred, whereby half tone expression becomes possible. There is, for example, one level difference in the resistance value of this head and therefore, if multivalue recording of total three values (i.e., 1+2 = 3) is effected at one point of medium density and two points of solid density and density "0", the density reproducibility will become very good. Figure 45 is an enlarged view showing the shapes of a heat generating element 11b and the electrodes 12c and 13c of a thermal head 27 according to a seventeenth embodiment, and by making the shape of the heat generating element 11b such as shown, more gradation expression are made possible. That is, the width of the heat generating element 11b is varied at one more stages than in the case of the heat generating element 11a of Figure 43 and the heat energy applied to the heat generating element 11b is varied, whereby two level differences are provided in the resistance value in the heat generating element and four-value (2+2 = 4) recording can be accomplished. As described above, in the thermal head according to the present embodiment, the width of the electrodes at the junctions between the heat generating element and the electrodes is made smaller than the width of the heat generating element, and the resistance value of the heat generating element is made higher in the portions thereof which are near the electrodes and lower in the central portion thereof, and by controlling the applied energy to this thermal head, gradation expression can be accomplished with good accuracy. Other embodiments will further be described by the use of eighteenth to twenty-second embodiments. The embodiment of the thermal head which will now be described is one which has a plurality of heat generating resistance members arranged in a row, electrodes narrower than the effective width of said heat generating resistance members and supplying electrical energy to the respective ones of said heat generating resistance members, and a glaze layer which is thick in the portions wherein said heat generating resistance members are connected to said electrodes and thin substantially in the central portions of said heat generating resistance members. Figures 46A - 46C show the shapes of a heat generating element 11 and the electrodes 12 and 13 of a thermal head 10 according to a eighteenth embodiment, wherein Figure 46A is an enlarged front view thereof, Figure 46B is a cross-sectional view showing the cross-section E-E₁ of Figure 46A, and Figure 46C is a cross-sectional view showing the cross-section F-F₁ of Figure 46A. In Figures 46A and 46C, the reference numeral 11 designates a heat generating element of the thermal head 10, and this thermal head 10 is comprised of a plurality of such heat generating elements 11 arranged in a row, and by these heat generating elements being selectively electrically energized corresponding to recording data, image recording is accomplished. The electrode 12 is a common electrode for supplying electric power to each heat generating element, and the electrode 13 is a signal electrode provided correspondingly to each heat generating element, and by the voltage level thereof being changed correspondingly to the recording data, the control of the electrical energization of the heat generating elements 11 is executed. The heat generating element 11 and the electrodes 12 and 13 of the thermal head 10 according to the present embodiment will now be described with reference to Figures 46B and 46C. A glaze layer 16 as a heat accumulating portion is formed on an alumina substrate 17. A resistance layer 15 and an electrode layer 12 respectively 13 are further formed on the glaze layer as by vacuum evaporation or sputtering. The electrodes and the resistance layer are partly cut away as by photolithography or photoetching to thereby form the electrodes 12 and 13 and the heat generating element 11, and a wear resisting layer 14 is further formed thereon as by sputtering. The portion called the heat generating element 11 refers to the portion of the resistance layer 15 which is laid free between the electrodes 12 and 513. The wear resisting layer includes all of what serves as a protective film such as an antioxidation layer. As is apparent from Figures 46A-46C, the width of the electrodes in the portions wherein the heat generating element 11 is connected to the electrodes 12 and 13 is smaller than the width of the heat generating element 11. Also, the glaze layer corresponding to the width of these electrodes 12 and 13 protrudes with the junctions between the heat generating element 11 and the electrodes 12 and 13 as the vertices and therefore, the shape of the heat generating element 11 at these junctions is convex. Figure 47A shows the flow of the electric current when a pulse voltage is applied to the heat generating element 11 of the thermal head 10 shown in Figures 46A - 46C, through the electrodes 12 and 13 which are narrower than the width of the heat generating element 11. In Figure 47A, the reference numeral 20 designates the electric current flowing through the heat generating element 11, and the density of the electric current flowing through the heat generating element 11 is great near the junctions between the narrow electrodes 12, 13 and the heat generating element 11. Figure 47B is a graph showing the temperature distribution in the portion of the heat generating element 11 which is near the electrode 12 (at the cross-section G-G₁ of Figure 47A). The temperature of the vicinity of the electrode 12 (or the electrode 13) (which is near the junctions) is particularly high in the heat generating element 11, and the temperature difference from the other portion of the heat generating element 11 exhibits a great value. As the time of application of the pulse signal applied to the heat generating element 11 is gradually lengthened to change the applied energy to the heat generating element 11, the widthwise temperature distribution at the cross-section X-X₁ of the heat generating element 11 varies as indicated by curves 21 → 22 → 23 in Figure 47B. In Figure 47B, Tm indicates the melting point of the ink of the ink sheet 132 (Figure 16), and in a temperature area higher than this, the ink of the ink sheet 132 is melted and transferred to the recording sheet 133. Here, the thermal head 10 is of a shape in which the wear resisting layer 14 thereof is protuberant with the junctions between the heat generating element 11 and the electrodes 12, 13 as the vertices and the central portion of the heat generating element 11 is somewhat concave. Therefore, the pressure force applied between the thermal head 10 and the recording sheet 133 (Fig. 16) concentrates in and near the junctions between the electrodes 12, 13 and the heat generating element 11. Also, in those portions, the glaze layer 14 is thicker, whereby the heat accumulation in those portions becomes great and therefore, the concentration effect of the heat generation temperature becomes more remarkable. Thus, it becomes easier for the ink of the ink sheet in this portion to be transferred. As a result, the temperature area (the transfer area) of the heat generating element 11 in which the temperature becomes higher than the melting point Tm of the ink of the ink sheet 132 (Fig. 16) is such as indicated by 21A, 22A and 23A in Figure 47C, corresponding to the temperature distribution of Figure 47B. Also here, the shape of the heat generating element 11 is a convex shape as shown in Figures 46A-46C and therefore, this thermal head becomes excellent in the transfer and reproducibility in the low recording density area wherein the dot area is small. Figure 48 is a graph showing the relation of the recording density to the applied energy in this thermal head 10. In Figure 48, the inclination (the rate of variation) of the recording density D R relative to the magnitude of the applied energy E A to the thermal head 10 is sufficiently small as compared with the case of Figure 9A, and the scattering width S W of the recording density D R relative to each applied energy E A is also small as shown by the length of vertical lines in the graph. It is thus seen that half tone can be expressed by controlling the applied energy and the irregularity of the recording density in half tone recording becomes sufficiently small and stable. Figures 49A-49C show the shapes of the heat generating element 11a, the common electrode 12b and the signal electrode 13b of a thermal head 24 according to a nineteenth embodiment, wherein Figure 49A being an enlarged front view thereof, Figure 49B being a cross-sectional view showing the cross-section H-H₁ of Figure 49A, and Figure 49C being a cross-sectional view showing the cross-section I-I₁ of Figure 49A. In this thermal head 24, like the heat generating element 11 of Figure 46, the width of the electrodes 12a and 13a is smaller than the width of the heat generating element 11a. The construction of this heat generating element 11a will now be described with reference to Figures 49B and 49C. A glaze layer 16a as a heat accumulating portion is formed on an alumina substrate 17a. A resistance layer 15a and an electrode layer 12a respectively 13a are further formed on the glaze layer as by vacuum evaporation or sputtering. The electrode layer and the resistance layer are partly cut away as by photolithography or photoetching to thereby form the electrodes 12a and 13a and the heat generating element 11a, and a wear resisting layer 14a is further formed thereon as by sputtering. Here, the portion called the heat generating element 11a refers to that portion of the resistance layer 15a which is laid free between the electrodes 12a and 13a. Also, as shown in Figure 49C, the glaze layer 16a is thin in the central portion of the heat generating element 11a and thick under the electrodes 12a and 13a, and the central portion of the heat generating element 11a is concave. Thus, the amount of accumulated heat in the vicinity of the junctions between the electrodes and the heat generating element 11a which is the center of heat generation becomes great and the amount of radiant heat of the central portion (the concave portion) of the heat generating element 11a becomes great. Consequently, as already described with respect to the eighteenth embodiment, the transfer area widens about the junctions, and by controlling the applied energy to the head, half tone expression can be accomplished. Figures 50A-50C show the shapes of a heat generating element 11b and the electrodes 12b and 13b of a thermal head 25 according to a twentieth embodiment, wherein Figure 50A being an enlarged front view thereof, Figure 50B being a cross-sectional view showing the cross-section J-J₁ of Figure 50A, and Figure 50C being a cross-sectional view showing the cross-section K-K₁ of Figure 50A. In Figures 50A-50C, the reference character 11b designates a heat generating element of the thermal head 25, and this thermal head 25 is comprised of a plurality of such heat generating elements 11b arranged in a row, and by these heat generating elements being selectively electrically energized corresponding to recording data, image recording is accomplished. The electrode 12b is a common electrode for supplying electric power to each heat generating element, and the electrode 13b is a signal electrode provided correspondingly to each heat generating element, and by the voltage level thereof being changed correspondingly to the recording data, the control of the electrical energization of the heat generating elements 11b is executed. The heat generating element 11b and the electrodes 12b and 13b of the thermal head 25 according to the twentieth embodiment will now be described with reference to Figures 50B and 50C. As in the aforedescribed embodiment, a glaze layer 16b as a heat accumulating portion is formed on an alumina substrate 17b. A resistance layer 15b and an electrode layer 12b respectively 13b are further formed on the glaze layer as by vacuum evaporation or sputtering. The electrode layer and the resistance layer are partly cut away as by photolithography or photoetching to thereby form the electrodes 12b and 13b and the heat generating element 11b, and a wear resisting layer 14b is further formed thereon as by sputtering. Here, the portion called the heat generating element 11b refers to that portion of the resistance layer 15b which is laid free between the electrode layers 12b and 13b. As is apparent from Figures 50A-50C, the width of the electrodes at the junctions between the heat generating element 11b and the electrodes 12b and 13b is smaller than the width of the heat generating element 11b, and the wear resisting layer 14b corresponding to the width of these electrodes 12b and 13b protrudes with the junctions between the heat generating element 11b and the electrodes 12b and 13b as the vertices and therefore, the shape of the heat generating element 11b in this portion is convex. Figure 51A shows the flow of the electric current when a pulse voltage is applied to the heat generating element 11b of the thermal head 25 shown in Figures 50A-50C, through the electrodes 12b and 13b which are narrower than the width of the heat generating element 11b. In Figure 51A, the reference numeral 26 designates the electric current flowing through the heat generating element 11b, and the density of the electric current flowing through the heat generating element 11b is great near the junctions between the narrow electrodes 12, 13b and the heat generating element 11b. Figure 51B is a graph showing the temperature distribution in that portion of the heat generating element 11b which is near the electrode 12b (at the cross-section L-L₁ of Figure 51A). The temperature of the vicinity of the electrode 12b (or the electrode 13b) (the vicinity of the junctions) is particularly high in the heat generating element 11b, and the temperature difference of this portion from the other portion of the heat generating element 11b exhibits a great value. As the time of application of the pulse signal applied to the heat generating element 11b is gradually lengthened to change the applied energy to the heat generating element 11b, the widthwise temperature distribution at the cross-section L-L₁ of the heat generating element 11b varies as indicated by curves 42 → 43 → 44 in Figure 51B. In Figure 51B, Tm indicates the melting point of the ink of the ink sheet 132 (Figure 16), and in a temperature area higher than this, the ink of the ink sheet 132 is melted and transferred to the recording sheet 133 (Figure 16). Here, the thermal head 25 is of a shape in which the wear resisting layer 14b thereof is protuberant with the junctions between the heat generating element 11b and the electrodes 12b, 13b as the vertices and the central portion of the heat generating element 11b is somewhat concave and therefore, the pressure force applied to between the thermal head 25 and the recording sheet concentrates in and near the junctions between the electrodes 12b, 13b and the heat generating element 11b. Also, the wear resisting layer 14b is thicker in that portions, whereby the heat accumulation in that portions becomes great and therefore, the concentration effect of the heat generation temperature becomes more remarkable. Thus, it becomes easier for the ink of the ink sheet in this portion to be transferred. As a result, the temperature area (the transfer area) of the heat generating element 11b of the thermal head 25 in which the temperature becomes higher than the melting point Tm of the ink of the ink sheet as shown in Figure 51C becomes such as shown by 42A, 43A and 44A in Figure 51C corresponding to the temperature distribution of Figure 51B. Also, the shape of the heat generating element 11b is convex as shown in Figures 50A-50C and therefore, this thermal head becomes excellent in the transfer and reproducibility in the low recording density area wherein the dot area is small. The relation between the applied energy and the recording density by the thus formed thermal head 25 is as shown in Figure 48. This thermal head 25 is also excellent in durability, because generally, the deterioration of a thermal head due to the friction between the thermal head and a recording sheet (or an ink sheet) tends to progress more rapidly as the temperature of the head becomes higher. In contrast, in the heat generating element 11b of the thermal head 25 according to this embodiment, heat generation takes place concentratedly and the wear resisting layer 14b in the vicinity of the junctions between the electrodes 12b, 13b and the heat generating element 11b wherein the temperature rises is made thicker and therefore, the thermal head 25 as a whole is excellent in wear resistance. Figures 52A-52C show the shapes of a heat generating element 11e and the electrodes 12e and 13e of a thermal head according to a twenty-first embodiment, wherein Figure 52A being an enlarged front view thereof, Figure 52B being a cross-sectional view showing the cross-section Q-Q₁ of Figure 52A, and Figure 52C being a cross-sectional view showing the cross-section R-R₁ of Figure 52A. In the figures, the reference numerals 71 and 72 denote insulating portions provided near the junctions between the electrodes 12e, 13e and the heat generating element 11e and thus, the electric current flowing through the heat generating element 11e concentrates in the portions wherein the electrodes 12e and 13e are connected to the heat generating element 11e (i.e. the four corners of the heat generating element 11e). So, the portions of a wear resisting layer 14e which correspond to these portions in which the density of the electric current becomes great are made thick to cause said portions to protrude with respect to the surroundings thereof. For the same reason as that set forth in the previous embodiment (see Fig. 50C), the area transferred by this thermal head varies and half tone expression becomes possible, and by the wear resisting layer 14e in this heat generating central portion being made thick, the durability of the thermal head can also be improved. Figures 53A-53C show the shapes of a heat generating element 11f and the electrodes 12f and 13f of a thermal head according to a twenty-second embodiment, wherein Figure 53B being a cross-sectional view showing the cross-section S-S₁ of Figure 53A, and Figure 53C being a cross-sectional view showing the cross-section T-T₁ of Figure 53A. In Figures 53A-53C, the reference numerals 73 and 74 designate insulating portions and thus, the electric current flowing through the heat generating element 11f concentrates in the portions wherein the electrodes 12f and 13f are connected to the heat generating element 11f (i.e. the four corners of the heat generating element 11f), and these portions become the center of heat generation. Accordingly, a glaze layer 16f in these portions which are the center of heat generation is made thick to cause the glaze layer to protrude with respect to its surroundings. Thus, in the same manner as that described in the previous embodiment (see Fig. 56C), the applied energy can be changed to thereby change the transfer area and realize half tone expression by area gradation. Figures 54A-54C shows the transfer area by the heat generating element shown in Figures 52A-52C or Figures 53A-53C corresponding to the applied energy thereto. In these figures, the transfer area widens from the four corners of the heat generating element 11e (11f) corresponding to the amount of applied energy. By thus adjusting the applied energy, the transfer area can be changed to accomplish gradation expression. As described above, in the thermal head according to the present embodiment, the glaze layer in the heat generating central portion of the heat generating element is made thick and the heat accumulation effect and the pressure force to the ink sheet, etc. are increased, whereby gradation expression can be accomplished with good accuracy corresponding to the applied energy. Also, in the thermal head according to the present embodiment, the density of the electric current flowing through the heat generating element is endowed with a variation, and the wear resisting layer in the portion wherein the density of the electric current becomes great is made thick to cause the wear resisting layer to protrude with respect to its surroundings, whereby the pressure force and the heat accumulation effect of the head are increased and accurate gradation expression for the applied energy can be accomplished. The thermal heads according to the aforedescribed first to twenty-second embodiments can be applied to the heat transfer recording apparatus shown in Figures 10-12. The thermal heads are controlled along the timing of the driving and strobe signals shown in Figure 13 and also, the recording process is carried out in accordance with the flow chart shown in Figure 14. Accordingly, Figures 10-14 and the description thereof are invoked as the description of each embodiment of the heat transfer recording apparatus in each of the aforedescribed embodiments. In each of the aforedescribed embodiments, as the electrodes, use can be made, for example, of aluminum, gold, copper or the like, and as the resistance members, use can be made, for example, of nickel chromium polysilicon, tantalum nitride, tantalum or the like. While in each of the above-described embodiments, the thermal recording system using a thermal head has been described as an example, the present invention is not restricted thereto, but of course can also be applied to a recording system using heat to effect image recording, such as the ink jet recording system using heat to discharge ink liquid and thereby accomplish image recording. That is, the present invention can of course be applied to a recording head which can accomplish image recording with the heat generating elements thereof caused to generate heat and to a recording apparatus provided with such recording head. So, the present invention can of course be applied, for example, to an ink jet recording apparatus or the like in which ink is caused to fly by means of heat generating elements caused to generate heat to thereby accomplish image recording. Of course, the present invention can also be applied to the bubble jet recording system in which at least one driving signal for providing a rapid temperature rise exceeding nuclear boiling is applied to electro-thermal converting members as heat generating elements to thereby create heat energy in the electro-thermal converting members and the heat-acting surface of the recording head is film-boiled to form a bubble in ink and by the growth and contraction of this bubble, the ink is discharged through discharge openings. Accordingly, the recording head is not limited to a thermal head, but includes, for example, an ink jet head or the like. As described above, according to each of the aforedescribed embodiments, there can be provided a thermal head which is formed so that a difference may be provided in the density of the electric current flowing through the heat generating element and in which the applied energy in varied, whereby the heat generation temperature distribution can be changed easily. Also, there can be provided a heat transfer recording apparatus in which the heat generating area of the thermal head is changed corresponding to the degree of gradation of image data to change the transfer area, thereby resulting in good reproducibility of gradation. As described above, in the recording head according to each of the aforedescribed embodiments, the recording density can be reliably changed by changing the applied energy and therefore, this recording head is suitable for half tone recording and also is excellent in durability. Also, according to the heat transfer recording apparatus of the present invention, multivalue recording is possible and recording at multi-gradation can be realized easily. As described above, according to the present invention, there can be provided a recording head which can accomplish clear-cut half tone recording and a recording apparatus provided with such recording head.

Claims: A recording head (10) for use in a recording apparatus for effecting recording on a recording medium, having a plurality of heat generating elements (11) and electrodes (12, 13) for applying energy to said heat generating elements (11), characterized in that when the effective width of said heat generating elements (11) is Wr and the width of the electrodes connected to said heat generating elements is We, 1/10 ≦ We/Wr ≦ 1/2 , and when the length, in the direction of an electric current, between the portion of said heat generating elements (11) in which the current density is maximum and the portion of said heat generating elements (11) in which the current density is minimum, is Lr, the condition that 1/4 ≦ Lr/Wr ≦ 3/2 is satisfied. The recording head according to claim 1, wherein the junctions between said heat generating elements (11) and said electrodes (12, 13) overlap each other. The recording head according to claim 1 or 2, comprising: control means for controlling the applied energy to said recording head; and conveying means for conveying said recording medium. The recording head according to anyone of claims 1 to 3, wherein the heat generating elements are resistance elements arranged in a row. The recording head according to claim 4, wherein the electrode portion connected to said heat generating resistance elements (11) comprises a common electrode (12) and a signal electrode (13), and at least one of said common electrode (12) and said signal electrode (13) is narrower than the width of said heat generating resistance elements (11). The recording head according to claim 5, wherein each of said heat generating resistance elements (11) is substantially equally divided into at least two portions, namely a common electrode side portion and a signal electrode side portion, and said portions are connected together by a conductor portion (13d, 118, 118a, 118b) narrower than the effective width of said heat generating resistance elements. The recording head of anyone of claims 1 to 5, having an electrical conductor provided on said heat generating elements (11). The recording head according to claim 6, with the modification that the electrodes have the same width as the heat generating resistance elements, and it is the width of the conductor portion (13d, 118, 118a, 118b) which is deemed to be the value for We in the conditions of claim 1. The recording head according to claim 8, wherein said conductor portion comprises two separate parts (118a, 118b). The recording head according to claim 5, with the modification that the electrodes have the same width as the heat generating resistance elements, each of the heat generation resistance elements has in its central portion a conductor portion (121) connected thereto, and it is the width of the conductor portion (121) which is deemed to be the value for We in the conditions of claim 1. A heat recording apparatus using the head of anyone of claims 1 to 10, characterized by gradation determining means for receiving a multivalue image signal as an input and determining the degree of gradation of each of said heat generating resistance elements (11) of said recording head (10) correspondingly to the degree of gradation of said image signal; and recording means for electrically energizing each of said heat generating resistance elements (11) in accordance with said degree of gradation and effecting recording.

Inventors: AOKI MAKOTO, AOKI, MAKOTO
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1989
Main classification: B41J2
IPC codes: B41J2