Heating apparatus

The induction-heating apparatus of this invention contains a conductive member, a power source, and a first and a second induction coil connected to the power source and adapted for causing the conductive member to generate an induced current, the first and the second induction coil being parallelly connected. The induction coil is formed by winding a copper wire round a core in such a manner that the numbers of turns of the copper wire gradually decrease from the lowermost layer approximating most closely to the core to the upper layers. This heating apparatus has the thickness of the induction coil in the range of 0.2-0.8 mm. The gap between the induction coil and the conductive member is in the range of 0.5-4.0 mm. This heating apparatus has the induction coil thrust out of the conductive member. The induction coil is formed of a Litz wire.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a heating apparatus and particularly to a heating 
apparatus of the principle of induction heating. 
2. Description of the Prior Art 
Heretofore it has been proposed to apply heating apparatuses of the 
principle of induction heating for such fixing apparatuses as are 
incorporated in electrophotographic copying devices, printers, and 
facsimile systems. 
As respects the application of a heating apparatus of the principle of 
induction heating for a fixing apparatus, JP-A-54-39,645 and 
JP-A-59-33,787, for example; disclose a fixing apparatus which, by means 
of an open magnetic circuit iron core formed by helically winding an 
induction coil and disposed inside a fixing roller made of a metallic 
conductor, is operated by flowing a high-frequency current through the 
induction coil approximating closely to the inner surface of the fixing 
roller, utilizing a high-frequency magnetic field consequently formed for 
inducing the fixing roller to generate an induced eddy current, and 
harnessing the skin resistance produced consequently by the fixing roller 
itself for enabling the fixing roller itself to emit Joule heat. 
JP-A-58-178,385 discloses a fixing apparatus which is provided with two 
induction coils and is adapted to switch the flow of current to the two 
induction coils depending on the size of paper to be used, i.e. cause the 
current to flow to only one of the induction coils when the paper has a 
small size or to both the two induction coils when the paper has a large 
size thereby enables induction heating exclusively to occur in the surface 
area that is in need of heating and promote conversion of energy. 
The induction heating of this nature is advantageous over the other forms 
of heating in the following respects. 
Firstly, it elevates the temperature of the fixing roller quickly and 
insignificantly incurs generation of heat in or conduction of heat to the 
components other than the fixing roller as compared with the indirect 
heating such as the near-infrared heating with a tungsten halogen lamp. It 
does not suffer any loss which is equivalent to the leakage of light 
incurred by the tungsten halogen lamp. Secondly, it generates heat with 
high efficiency because it has the skin effect peculiar to electromagnetic 
induction and it enjoys lasting reliability of the fixing apparatus 
because it uses no sliding contact as compared with the surface heating 
which makes use of a solid resistance heating element on the surface of a 
fixing roller. Thirdly, it excels in terms of temperature control because 
it incurs loss of heat conduction insignificantly due to contact 
resistance and enjoys easy detection of the temperature of the surface of 
heat generation as compared with the heating which makes use of a film 
belt and a solid resistance heating element. 
In spite of all these advantages, the fixing apparatus of the principle of 
induction heating has problems yet to be solved. 
The problem of noise is one of them. When the oscillation caused jointly by 
the induction coil intended for the generation of induced current and the 
resonance capacitor falls within an audio frequency band (about 15 Hz-20 
kHz), this oscillation forms the source of noise in the fixing apparatus. 
The frequency, f, of the oscillation caused by the induction coil and the 
resonance capacitor is found roughly by the calculation of the following 
formula (1) (which will be described more specifically hereinbelow). 
##EQU1## 
In the formula, Ip represents the peak value of an induction coil current, 
L the inductance of the induction coil, VI the voltage applied to the 
induction coil, and C the capacity of the resonance capacitor. 
It is noted from the formula (1) that the frequency of the oscillation 
varies with the inductance of the induction coil. 
The frequency of the oscillation, therefore, can be deviated from the audio 
frequency band by lowering the inductance of the induction coil. Where the 
induced current is generated by one induction coil as disclosed in 
JP-A-59-33,787 mentioned above, a reduction effected in the length of the 
induction coil for the purpose of decreasing the inductance of the 
induction coil prevents acquisition of an ampere turn necessary for 
induction heating and consequently widens the gap from the fixing roller 
(member subjected to heating) and degrades the efficiency of heat 
generation. This hardship may be coped with by increasing the thickness of 
the core and decreasing the gap or by increasing the thickness of the 
induction coil. This measure is economically unfavorable because the core 
gains in volume and the work of winding the induction coil of an increased 
thickness impairs the operational efficiency of production and the cost of 
production is inevitably increased. 
Where a plurality of induction coils are used as disclosed in 
JP-A-58-178,385 mentioned above, though the inductance of each of the 
induction coils is low, the combined inductance of the plurality of 
induction coils connected in series is as high as when just one induction 
coil is used and consequently entails the same problem as mentioned above. 
Where the plurality of induction coils are controlled separately from one 
another, as many control circuits as the induction coils are required and 
the cost is proportionately increased. 
Another problem is that the heat generated by the induction coil itself 
excessively elevates the temperature inside the fixing roller. This 
hardship arises because the heat generated by the resistance of the 
induction coil itself in consequence of the flow of current through the 
induction coil cannot be fully radiated in the narrow space inside the 
fixing roller. For the purpose of avoiding this drawback, it becomes 
necessary to lower the resistance by increasing the thickness of the 
induction coil or to form the fixing roller and neighboring members with 
materials of high heat resistance. In this respect, JP-A-54-39,645 
mentioned above contemplates avoiding the excessive elevation of 
temperature by winding the induction coil helically round the fixing 
roller thereby giving rise to an empty core part therein and flowing air 
through the empty core part and repressing the elevation of temperature in 
the fixing roller. 
It is indeed important to overcome the drawback of the excessive elevation 
of the temperature of the fixing roller. When the induction coil of a 
large thickness is used so as to lessen the copper loss of the induction 
coil itself for the purpose of lowering the magnitude of the resistance of 
the induction coil as described above, however, the empty space which is 
allowed for winding the induction coil is necessarily limited because the 
induction coil is disposed inside the fixing roller. The use of the 
induction coil of a large thickness, therefore, entails the problem that 
the number of turns of the coil is proportionately decreased and the 
necessary ampere turn is no longer obtained. When materials of high heat 
resistance are used for the members neighboring the fixing roller, these 
members become so expensive as to render the apparatus uneconomical. 
As means to cool the induction coil for the purpose of preventing the 
temperature thereof from rising, JUM-A-55-29,492, for example, proposes a 
technique of covering the core disposed inside the induction coil to be 
wound with a ventilation pipe and flowing air by means of a fan motor 
through the interior of the ventilation pipe and JUM-A-55-18,292 discloses 
a technique of disposing round the induction coil a radiation pipe adapted 
to supply air to the periphery of the induction coil. 
In JP-A-54-39,645 mentioned above, however, the supply of air to the empty 
core of the helically wound induction coil needs a means for ventilation 
and the power which is required for the ventilating means counters the 
demand for energy conservation mentioned above. The extra provision of the 
ventilating means entails the problem of inevitably increasing the cost. 
Likewise, JUM-A-55-29,492 and JUM-A-55-18,292 are at a disadvantage in 
complicating facilities and rendering them expensive because they require 
a ventilating pipe for covering the core and a fan motor for flowing air 
thereto or a radiation pipe for encircling the induction coil and a means 
for flowing air thereto. JUM-A-55-29,492 in particular contemplates the 
structure in which the induction coil is wound on the ventilating pipe 
covering the core. The means to generate a magnetic flux, therefore, has a 
large volume. When such a hollow heating member as the fixing roller of a 
small diameter is used, therefore, this structure has the problem that the 
magnetic flux generating means mentioned above cannot be installed within 
the fixing roller. 
One of the fixing apparatuses of the principle of induction heating has a 
structure in which an induction coil is produced by forming a winding 
round a core destined to constitute a magnetic circuit. The following 
relation exists between the distance from the core to the fixing roller 
and the efficiency with which the fixing roller is heated (efficiency of 
energy transmission). When the distance between the core and the fixing 
roller is shortened, since the magnetic coupling gains in intensity, the 
heating efficiency is improved and the amount of heat generated in the 
fixing roller is exalted. Conversely, when the distance is lengthened, 
since the magnetic coupling is consequently weakened, the efficiency of 
heating is degraded and the amount of heat generated is decreased. 
For the purpose of preventing the fixing apparatus from forming a 
short-circuit and consequently sustaining damage, it is necessary to 
secure thorough electric insulation between the fixing roller and the 
core. For the same reason, electric insulation is required to exist 
between the fixing roller and the induction coil. 
For the purpose of securing the thorough electric insulation mentioned 
above, it suffices to lengthen the distance between the fixing roller and 
the core. The structure which embodies the lengthened distance, however, 
fails to obtain a thorough heating efficiency as pointed out above. In 
this case, for the purpose of enabling the fixing roller to acquire a 
prescribed amount of heat generation, it is necessary to flow current 
amply to the induction coil. The flow of an ample amount of current, 
however, entails copper loss of the induction coil to the extent of 
increasing the amount of heat generated and elevating the temperature of 
the induction coil itself even to a level exceeding 200.degree. C. As a 
result, the holders for supporting the core and the induction coil as well 
as the induction coil itself are required to use expensive materials 
capable of enduring the temperature mentioned above. The use of these 
materials not only adds to the cost of the apparatus but also counters the 
demand for energy conversion. 
One of the fixing apparatuses of the principle of induction heating uses a 
flexible metallic sleeve of a small wall thickness as a hollow heating 
member in the place of the fixing roller. It nevertheless encounters the 
same situation as described above. 
SUMMARY OF THE INVENTION 
This invention, with a view to solving the problems of the prior art 
mentioned above, has an object of providing a heating apparatus for use in 
a fixing apparatus, which secures a necessary ampere turn and lowers the 
whole inductance of an induction coil and prevents an oscillation 
occurring in the audio frequency band without sacrificing the efficiency 
of heating. 
Another object of this invention is to provide a heating apparatus for use 
in an induction-heating fixing apparatus which is capable of economically 
and efficiently preventing the temperature of the fixing roller used in 
the fixing apparatus from being elevated excessively. 
Still another object of this invention is to provide a heating apparatus 
for use in a fixing apparatus which secures thorough electric insulation 
between a hollow metallic member and a core and allows highly efficient 
heating. 
Yet another object of this invention is to provide a heating apparatus for 
use in a fixing apparatus which is capable of cooling an induction coil 
with a simple and inexpensive mechanism and adaptable even to a fixing 
roller of a small diameter. 
In one preferred embodiment, the heating apparatus of this invention is 
intended to be used as a fixing apparatus which serves the purpose of 
enabling a toner image formed on a recording medium to be fixed thereto 
and is characterized by being provided with a conductive member and at 
least two parallelly connected induction coils for enabling the conductive 
member to produce an induced current and emit heat. 
By having at least two induction coils parallelly connected as described 
above, the heating apparatus is enabled to heighten the frequency of 
oscillation enough to depart from the audio frequency band, select an 
optimum frequency, and preclude the otherwise inevitable occurrence of 
noise. As a result, it attains prevention of noise easily without adopting 
such measures as enlarging the core and increasing the thickness of the 
induction coil which incur a rise of cost. 
In a further preferred embodiment, the heating apparatus of this invention 
is intended to be used as a fixing apparatus which serves the purpose of 
enabling a toner image formed on a recording medium to be fixed thereto 
and is characterized by being provided with a conductive member, a core 
formed inside the conductive member and disposed in a direction 
perpendicular to the direction of a rotary shaft of the conductive member, 
and an induction coil wound on the core along the direction of the rotary 
shaft of the conductive member, the induction coil being so wound that the 
numbers of turns gradually decrease from the lowest layer approximating 
most closely to the core to the upper layers. 
Since the induction coil is so wound that the numbers of turns gradually 
decrease from the lowermost layer approximating the core to the upper 
layers as described above, the heating apparatus enjoys a good utilization 
factor of the space inside the conductive member, derives a necessary 
ampere turn from the induction coil of a large thickness, and decreases 
the amount of heat generated by the induction coil during the flow of 
current, and consequently prevents the temperature of the conductive 
member from being excessively elevated. It further enjoys an exalted ratio 
of thermal conversion and a decrease of power consumption owing to the 
decreased distance between the induction coil and the conductive member. 
In another preferred embodiment, the heating apparatus of this invention is 
intended to be used as a fixing apparatus which serves the purpose of 
enabling a toner image formed on a recording medium to be fixed thereto 
and is characterized by being provided with a cylindrical conductive 
member, a core formed inside the conductive member and disposed in a 
direction perpendicular to the direction of a rotary shaft of the 
conductive member, and an induction coil wound on the core along the 
direction of the rotary shaft of the conductive member, the induction coil 
having an outside diameter in the range of 0.2-0.8 mm. 
By having the outside diameter of the induction coil fixed in the range of 
0.2-0.8 mm as described above, the heating apparatus is enabled to 
minimize the magnitude of effective resistance of the induction coil 
exposed to exertion of a high frequency, improve the energy efficiency of 
the current flowing through the induction coil, minimize the heat 
generation of the induction coil, and prevent the temperature of the 
inductive member from being elevated excessively. 
In still another preferred embodiment, the heating apparatus of this 
invention is intended to be used as a fixing apparatus which serves the 
purpose of enabling a toner image formed on a recording medium to be fixed 
thereto and is characterized by being provided with a conductive member, a 
core made of a magnetic material, an induction coil produced by forming a 
winding round the core through the medium of an insulating part and 
adapted for causing the conductive member to produce an induced current 
and consequently emit heat, and a non-rotatable retaining member 
supporting the core and the induction coil and stowed inside the 
conductive member, the core being disposed to interpose a distance of not 
less than 0.5 mm and not more than 4 mm between the core and the 
conductive member. 
Owing to the structure produced as described above, the heating apparatus 
is enabled to prevent the temperature of the induction coil from being 
elevated, obviate the necessity of using an expensive material, and 
repress the rise of cost. It further is enabled to curb the degradation of 
the efficiency of heating and contribute to energy conservation. It also 
secures thorough electric insulation between the conductive member and the 
core. In this structure, even when the core is approximated more closely 
to the inner surface of the hollow metallic roller formed of the 
conductive member, the electric insulation between the hollow metallic 
roller and the core and the induction roller is further ensured. When the 
coating of the induction coil happens to be broken as by the excessive 
temperature elevation, for example, the structure secures the electric 
insulation and prevents the fixing apparatus as a whole from being broken. 
In a further preferred embodiment, the heating apparatus of this invention 
is intended to be used as a fixing apparatus which serves the purpose of 
enabling a toner image formed on a recording medium to be fixed thereto 
and is characterized by being provided with an induction coil adapted to 
generate a magnetic flux inside the conductive member, the induction coil 
being enabled to admit the flow of a high-frequency current and generate 
electromagnetic induction in response thereto and consequently heat the 
conductive member by virtue of electromagnetic induction and the induction 
coil being projected into the ambience from the terminal part in at least 
one of the axial directions of the fixing apparatus. 
Since the induction coil is projected into the ambience from the terminal 
part in at least one of the axial directions of the fixing apparatus as 
described above, the outwardly projected and exposed part of the induction 
coil can be spontaneously cooled by the ambient air. The cooling effect is 
exalted by the fact that the induction coil itself is formed of copper, a 
substance excelling in the ability to radiate heat. As the radiation of 
heat from the entirety of the induction coil is thus promoted, the 
elevation of the temperature of the induction coil can be curbed.

DETAILED DESCRIPTION OF THE INVENTION 
These and other objects, advantages and features of the invention will 
become apparent from the following description thereof taken in 
conjunction with the accompanying drawings which illustrate specific 
embodiments of the invention. 
EXAMPLE 1 
Example 1 embodying this invention concerns a heating apparatus for use in 
a fixing apparatus. This heating apparatus has at least two induction 
coils for effecting induction heating of a fixing roller formed of 
conductive members parallelly connected and has the parallelly connected 
induction coils set in place inside the fixing roller. 
Specifically, this heating apparatus is produced, for example, by helically 
winding a plurality of induction coils 301, 302, . . . on one cylindrical 
core 2 inside a conductive fixing roller 5 and parallelly connecting the 
induction coils 301, 302, . . . as shown in FIG. 1, by helically winding 
induction coils 301, 302, . . . severally on a plurality of cores 201a, 
201b, . . . inside the conductive fixing roller 5 and parallelly 
connecting the induction coils 301, 302, . . . as shown in FIG. 2, by 
winding a plurality of induction coils 301, 302, . . . in the longitudinal 
direction on one prismatic core 202 and parallelly connecting the 
induction coils 301, 302, . . . as shown in FIG. 3, or by winding 
induction coils 301a, 301b, and 302a, 302b, . . . in the longitudinal 
direction severally on a plurality of prismatic cores 202a, 202b, . . . 
serially connecting the induction coils 301a, 301b, . . . and 302a, 302b. 
. . severally, and parallelly connecting the serially connected induction 
coils 301a, 301b, . . . and 302a, 302b, . . . as shown in FIG. 4. In each 
of the diagrams, an electric wiring 101 is connected to a high-frequency 
power source and a resonance capacitor. 
The embodiments of the fixing apparatus which vary the number of induction 
coils to be used will be illustrated below with the aid of a circuit 
diagram. They are formed by connecting two induction coils 301 and 302 
parallelly as shown in FIG. 5, by connecting induction coils 301a and 301b 
and induction coils 302a and 302b severally serially and connecting the 
two serially connected pairs parallelly in two rows as shown in FIG. 6, by 
connecting four induction coils 301, 302, 303, and 304 parallelly as shown 
in FIG. 7, by connecting the induction coils 301a, 301b, and 301c and 
induction coils 302a, 302b, and 302c severally serially and connecting the 
serially connected triples parallelly in two rows as shown in FIG. 8, by 
connecting induction coils 301a and 301b, induction coils 302a and 302b, 
and induction coils 303a and 303b severally serially and connecting the 
three serially connected pairs parallelly in three rows as shown in FIG. 
9, or by connecting six induction coils 301, 302, 303, 304, 305, and 306 
parallelly as shown in FIG. 10, for example. In each of the diagrams, the 
reference numeral 24 represents a resonance capacitor and the reference 
numeral 100 a high-frequency power source. 
In Example 1, such a cylindrical fixing roller as mentioned above is used 
and adapted to be directly subjected to induction heating. When this 
fixing roller is to be used as a fixing apparatus, the fixing roller may 
be made of a heat-resistant resin and a metallic plate for induction 
heating may be disposed inside the resinous fixing roller. Otherwise, a 
metallic plate may be disposed through the medium of a film following the 
motion of a recording medium and adapted for induction heating instead of 
using the fixing roller. 
Now, the operation obtained by parallelly connecting a plurality of 
induction coils as shown above will be described in detail below. 
First, the operation for the induction heating of the fixing roller will be 
described. 
The induction heating arises from the oscillation which is generated 
between the induction coils and the resonance capacitor when the 
high-frequency source adapted to apply a high frequency to the induction 
coils is turned on. With reference to the voltage-current chart shown in 
FIG. 11, when the power source voltage V.sub.G is turned on, the current 
flows through the induction coils and the magnitude of current I.sub.D is 
consequently increased. The power source V.sub.G is turned off when the 
magnitude of current I.sub.D reaches its peak value Ip. Consequently, the 
energy accumulated in the induction coils charges the resonance capacity 
and the voltage V.sub.D of the capacitor is increased, with the result 
that the energy of the induction coils is wholly spent in charging the 
capacitor. Now, the capacitor plays the part of discharging. 
The period of the oscillation, therefore, is expressed as the sum of the 
time, Ta, required for the current flowing to the induction coils in 
consequence of the accumulation of energy in the induction coils to reach 
the peak value, Ip, and the time, Tb, required for the capacitor to be 
recharged and discharged. 
Thus, the determination of the frequency of the oscillation is initiated by 
finding the duration, Ta, between the time the accumulation of energy in 
the induction coils is started and the time the current reaches its peak 
value, Ip. 
The relation between the voltage, V.sub.I, applied to the induction coils 
and the inductance of the induction coils is expressed as V.sub.I 
=L.multidot.di/dt. 
The relation between the current and the time, therefore, is given as 
di/dt=V.sub.I /L. 
It follows that the time, Ta, required for the current to reach the value, 
Ip, is expressed as Ta=Ip.multidot.L/V.sub.i. 
Then, the time, Tb, which is required for the capacitor to be recharged and 
discharged is found. 
It is divided into the time for recharging, Tb', and the time for 
discharging, Tb". The time for discharging, Tb", is decided by the period 
of resonance generated by the induction coils and the capacitor. The 
period of resonance, T.sub.o, is found as T.sub.o =2.pi..sqroot.LC in 
accordance with the formula, f.sub.o =1/(2.pi..sqroot.LC, for the 
calculation of the resonance frequency, f.sub.o. 
Hence, Tb" is found as Tb"=.pi..sqroot.LC. 
Since Tb is about 1.5 times Tb", Tb is found by approximation as 
Tb=Tb"+Tb'=1.5Tb". 
The oscillation frequency f, therefore, is found by the following formula. 
##EQU2## 
Here, the oscillation frequency generated when the induction coils are 
parallelly connected as shown in FIG. 6 and FIG. 7 mentioned above will be 
calculated in accordance with the formula (A) by way of example. For the 
purpose of comparison, the oscillation frequency generated when four 
equally divided induction coils 300a, 300b, 300c, and 300d are serially 
connected as shown in FIG. 13 where a necessary ampere turn is derived by 
one induction coil 300 as shown in FIG. 12 will be calculated. 
Here, for the calculation of the oscillation frequency, it is assumed that 
the voltage V.sub.I applied to the induction coils is 100 V, the value of 
peak current I.sub.p is 7 A, the capacity C of the resonance capacitor is 
0.033 .mu.F, and the inductance L of one induction coil is 400 .mu.H. 
Thus, the inductance of all the induction coils is found as 
400.times.2.times.1/2=400 .mu.H in the circuit shown in FIG. 6 and 
400.times.1/4.times.100 .mu.H in the circuit shown in FIG. 7, both 
embodying this invention. In contrast, the inductance is found as 
400.times.4=1600 .mu.H in the circuit having four induction coils serially 
connected as shown in FIG. 13. 
In the case of the circuit shown in FIG. 13, the oscillation frequency f 
calculated in accordance with the formula (A) is found to be 6.8 kHz, a 
frequency which falls in the audio frequency band. In contrast, the 
oscillation frequency f is 22.2 kHz in the circuit shown in FIG. 6 and 
41.4 kHz in the circuit shown in FIG. 7, both embodying this invention. 
The oscillation frequencies in the circuits embodying this invention both 
deviate from the audio frequency band. In the other circuits embodying 
this invention, when such induction coils as produce a fixed ampere turn 
are used, the oscillation frequencies to be generated are invariably high 
enough to deviate from the audio frequency band because the inductance of 
the induction coil is lowered owing to the parallel connection as compared 
with the serial connection. In executing this invention, it suffices to 
set the oscillation frequency at a level in the optimum range, preferably 
20 kHz-40 kHz, by combining the number of induction coils to be parallelly 
connected and the form of connection as already described through the 
medium of the capacity of the resonance capacitor and the necessary ampere 
turn. 
EXAMPLE 2 
Example 2 constitutes one example of the fixing apparatus possessed of a 
heating apparatus having a plurality of induction coils parallelly 
connected as described in Example 1 above. 
FIG. 14 is a schematic cross section illustrating an induction-heating 
fixing apparatus embodying this invention, FIG. 15 is a schematic 
perspective view illustrating a fixing roller and a pressing roller used 
in the apparatus, and FIG. 16 is a perspective view of the fixing roller 
used in the apparatus. 
As illustrated in FIG. 14, the induction-heating fixing apparatus which is 
incorporated as in a printer, for example, is provided with a fixing 
roller 5 disposed so as to be rotated in the direction of an arrow a and a 
pressing roller 6 pressed against the fixing roller 6 and consequently 
enabled to follow the rotation of the fixing roller 5. The fixing roller 5 
is a conductive cylindrical hollow pipe and has disposed therein as shown 
in FIG. 16 four coil assemblies 15 adapted for enabling the fixing roller 
5 to generate an induced current. The induction coils 301, 302, 303, and 
304 severally of the coil assemblies 15 are parallelly connected. The coil 
assemblies 15 are stowed in a holder unit 4 which is fixed as separated 
from the fixing roller 5 by a slight gap for allowing free rotation of the 
fixing roller 5. Owing to the parallel connection of the plurality of 
induction coils in the manner described above, the oscillation frequency 
in the present fixing apparatus is allowed to fall outside the audio 
frequency band similarly to the fixing apparatus of Example 1. 
The coil assemblies 15 each comprise a core 2, a bobbin 1 containing a 
through hole 16, and an induction coil 3 formed by winding a copper wire 
round the bobbin 1 (hereinafter the term "induction coil 3" used in 
Example 2 will refer individually or collectively to the induction coils 
301-304) as shown in FIG. 17. The core 2 is inserted in the through hole 
16 of the bobbin 1 and the induction coil 3 is formed by winding a copper 
wire round the bobbin 1 so as to encircle the core 2. 
The core 2 is a ferrite core or a laminated core, for example. The bobbin 1 
is formed of a ceramic substance or a heat-resistant insulating 
engineering plastic substance and fulfills the role of pressing the 
induction coil 3 and adjusting the shape thereof. The induction coil 3 is 
produced by winding a single wire or a Litz wire, 0.8 mm in diameter, 
provided on the surface thereof with a fused layer and an insulating layer 
round the bobbin 1 in the direction along the rotary axis of the fixing 
roller 5. 
The holder unit 4 for accommodating the coil assembly 15, as illustrated in 
FIG. 18, is composed of a holder stay 4a and a holder cover 4b to be 
fitted to the holder stay 4a, both formed of a heat-resistant insulating 
engineering plastic substance. The holder stay 4a and the holder cover 4b 
have formed on the inner surfaces thereof depressed parts 71 for holding 
the coil assembly 15 and in the opposite terminal parts thereof slipping 
parts 72 for fixing the holder unit 4 to the fixing unit frame of the 
apparatus proper. The holder unit 4 is completed by inserting the bobbin 1 
having parallelly connected induction coils 301, 302, 303, and 304 wound 
thereon in the depressed part 71 formed in the holder stay 4a, inserting 
the cores 2 in the through holes 16 of the bobbin 1, disposing an 
insulating film 75 on the peripheries of the induction coils 301, 302, 
303, and 304, and fitting the holder cover 4b to the holder stay 4a. The 
holder unit 4 is formed of a heat-resistant insulating material such as, 
for example, polyphenylene sulfide (PPS) or liquid crystal polymer. 
The fixing roller 5 is formed of such a conductive member as, for example, 
a carbon steel tube, stainless alloy tube, or aluminum alloy tube and has 
formed on the circumferential surface thereof a heat-resistant releasable 
layer obtained by coating the surface with such a fluorine resin as 
polyethylene tetrafluoride (PTFE). Preferably the fixing roller 5 is 
formed of conductive magnetic members. The pressing roller 6 has a 
silicone rubber layer 62, a surface-release type heat-resistant rubber 
layer, formed on the periphery of an axial core 61. The fixing roller 5 is 
provided, in the part toward which the sheet 4 is discharged from the 
fixing roller 5, with a separation claw 7 so disposed as to have the 
leading part thereof slide on the surface of the fixing roller 5 and peel 
the sheet 14 from the fixing roller 5. 
The fixing roller 5 has slider bearing parts 51 formed one each at the 
opposite terminals thereof and is rotatably fixed to the fixing unit 
frame. Further, the fixing roller 5 has a drive gear (not shown) fixed at 
one end thereof and is rotated by a drive source such as a motor (not 
shown) connected to the drive gear. The holder unit 4 is stowed inside the 
fixing roller 5 as separated by the minimum gap of a prescribed size from 
the inner wall surface of the fixing roller 5 and is fixed to the fixing 
unit frame and consequently rendered non-rotatable. The slider bearing 51 
and the separation claw 7 are formed of a heat-resistant slidable 
engineering plastic substance. 
Above the fixing roller 5, a temperature sensor such as, for example, a 
thermistor 8 for detecting the temperature of the fixing roller 5 is 
disposed. This thermistor 8 is held in contact with the surface of the 
fixing roller 5 as opposed to the lateral surface of the induction coil 3 
across the fixing roller 5. The flow of current to the induction coil 3 is 
controlled so as to optimize the temperature of the fixing roller 5, which 
is monitored by the thermistor 8 meanwhile. Besides the thermistor 8, a 
thermostat 9 for breaking the flow of current to the induction coil 3 on 
detecting an abnormal rise of temperature is provided as a safeguard 
against the abnormal temperature elevation. 
The induction-heating fixing apparatus which is constructed as described 
above operates as follows. 
First, the sheet 14 to which an unfixed toner image has been transferred is 
conveyed from the left direction in the bearings of FIG. 14 and forwarded 
in the direction of a nip part between the fixing roller 5 and the 
pressing roller 6. The sheet 14 is conveyed in the nip part as 
simultaneously exposed to the heat of the fixing roller 5 which is heated 
by the principle to be specifically described below and to the pressure 
produced jointly by the two rollers 5 and 6. As a result, the unfixed 
toner is fixed and the fixed toner image is formed on the sheet 14. The 
sheet 14 which has passed through the nip part is spontaneously separated 
from the fixing roller 5, forcibly separated from the fixing roller 5 by 
means of the separation claw 7 or a separation guide, and conveyed in the 
right direction in the bearings of FIG. 14. The sheet 14 is conveyed by a 
paper discharging roller (not shown) and released onto a paper discharge 
tray. 
Then, the control system of this fixing apparatus will be described. FIG. 
19 is a block diagram of the control system of the fixing apparatus. 
The high-frequency current is produced by causing the alternating current 
of a commercial power source 10 to be rectified by means of a rectifying 
circuit 11 and the rectified current to be converted into a high frequency 
by means of a self-exciting inverter circuit 12. The current to the 
induction coil 3 is supplied via the thermostat 9 pressed against the 
surface of the fixing roller 5. The circuit of this current is broken by 
the thermostat 9 when the surface temperature of the fixing roller 5 has 
reached an abnormal temperature set in advance. 
A control circuit 13 is formed of a microprocessor and a memory and adapted 
to control the temperature of the fixing roller 5 by monitoring this 
temperature based on the potential of the thermistor 8 and emitting an 
ON/OFF signal in response to the monitor to a drive circuit 80 laid inside 
the inverter circuit 12. 
The inverter circuit 12 receives the DC current from the rectifying circuit 
11, converts it into a high-frequency current, and supplies the 
high-frequency current to the induction coil 3. 
In the inverter circuit 12, when the control signal (heating signal) issued 
from the control circuit 13 changes to the status of ON, the drive circuit 
80 turns on a switching element 81 formed of a transistor, FET, or IGBT 
and consequently starts the flow of current to the induction coil 3. 
Meanwhile, a current detecting circuit 82, on detecting the fact that the 
current has reached the prescribed value, Ip, issues to the drive circuit 
80 a signal for turning off the switching element 81. The waveform of a 
drain current I.sub.D flowing to the induction coil 3 and detected by the 
current detecting circuit 82 is shown in FIG. 11 as already described in 
Example 1 above. A resonance current flows between the induction coil 3 
and a resonance capacitor 84 when the switching element 81 is turned off. 
Then, a voltage detecting circuit 83, on detecting the fact that the drain 
voltage V.sub.D on the induction coil 3 side of the switching element 81 
has fallen to the neighborhood of 0 V in consequence of the resonance, 
issues to the drive circuit 80 a signal for turning the switching element 
81 on again. Thereafter, the flow of the high-frequency current to the 
induction coil 3 is continued by repeating the switching cycle described 
above. The waveform of the voltage V.sub.D detected by the voltage 
detecting circuit 83 and the ON/OFF signal V.sub.G of the switching 
element 81 (the ON/Off signal of the gate in the case of FET, for example) 
are as shown in FIG. 11. 
When the current of high frequency (several kHz--some tens of kHz) is 
consequently flown to the induction coil 3, the fixing roller 5 generates 
heat in conformity with the principle which will be specifically described 
hereinafter and effects the fixation of the toner image on the sheet 
(recording medium) 14. 
FIG. 20 is a diagram for aiding in the description of the principle of 
heating of the fixing roller 5 which is used in the induction-heating 
fixing apparatus. When the current of high frequency (several kHz--some 
tens of kHz) is flown to the induction coil 3, the core 2 emits a magnetic 
flux 31a in a direction perpendicular to the direction of the longitudinal 
axis of the fixing roller 5 as shown in the diagram in compliance with the 
"ampere's corkscrew rule." This magnetic flux 31a is also a high-frequency 
magnetic flux. 
The magnetic flux 31b, on arriving at the fixing roller 5 of conductive 
members, is bent along the fixing roller 5 and converted into a magnetic 
flux 31c which passes along the inner wall surface of the fixing roller 5 
at a ratio which depends on the specific permeability of the conductive 
members. The magnetic flux 31c which is concentrated on the 
circumferential surface of the fixing roller 5 shows the maximum density 
in the part opposed to the induction coil 3. 
In the structure under discussion, the magnetic flux density within the 
circumferential surface is maximized at points P and R and conversely 
minimized at points Q and S of the fixing roller 5. Since the density of 
the induced current assumes the same trend, therefore, the generation of 
heat in the fixing roller 3 is not uniform throughout the entire 
circumferential surface. The fixing roller 5 generates heat locally in 
parts 32a and 32b which are enclosed with an alternate one-dash two-dot 
line. The parts 32a and 32b for the local heating correspond to the upper 
and the lower part of the fixing roller 5 in the diagram of FIG. 14. The 
nip part and either of the parts (areas) of heat generation, therefore, 
overlap at least partly. The other part (area) of heat generation is 
allowed to contact the thermistor 8 owing to the structure. Incidentally, 
the position for seating the thermistor 8 may be either in the upper part 
or in the lower part of the fixing roller 5, whichever better suits the 
occasion. In the illustrated embodiment, the thermistor 8 is seated 
outside the upper part. When the thermistor 8 is a miniature product, it 
may be seated inside the upper part or inside the lower part of the fixing 
roller 5. 
Owing to the action of the magnetic flux 31c which is concentrated as 
described above, the fixing roller 5 generates inside the wall surface 
thereof such an eddy induced current as produces a magnetic flux directed 
opposite the magnetic flux 31c and suffered to obstruct this magnetic flux 
31c in accordance with the "Lenz's rule". Since this induced current is 
converted by the skin resistance of the fixing roller 5 into a Joule heat, 
the fixing roller 5 generates heat and consequently effects the fixation 
of the toner image on the sheet 14 as described above. 
EXAMPLE 3 
FIG. 21 is a cross section illustrating another embodiment of the 
induction-heating fixing apparatus embodying the present invention, FIG. 
22 is a perspective view for aiding in the description of a coil assembly, 
and FIG. 23 is a perspective view of a fixing roller. Example 3 differs 
from Example 2 in the manner of winding the induction coil and in the 
structure of the induction coil. In all the other particulars of 
structure, these two examples are identical. The structure and the 
operation which are identical to those of Example 2, therefore, will be 
omitted from the following description. 
The induction-heating fixing apparatus of Example 3, as illustrated in FIG. 
21, is provided with a fixing roller 5 which is disposed rotatably in the 
direction of an arrow a and a pressing roller 6 pressed against the fixing 
roller 5 and allowed to follow the rotation of the fixing roller 5. The 
fixing roller 5 is a cylindrical hollow pipe formed of conductive members. 
A coil assembly 15a for causing the fixing roller 5 to produce an induced 
current is disposed inside the fixing roller 5 along the direction of the 
axis of rotation (direction of the axis of the cylinder). 
The coil assembly 15a, as illustrated in FIG. 22 and FIG. 23, is provided 
inside the fixing roller 5 with one core 2a elongate in the direction of 
the axis of the fixing roller 5, a bobbin 1a disposed so as to encircle 
the core 2a, and an induction coil 3a formed by winding a copper wire 
round the bobbin 1a. This coil assembly 15a is encased in a cylindrical 
holder 40 and stowed ultimately inside the fixing roller 5. 
The holder 40 is formed of a heat-resistant insulating material such as, 
for example, PPS (polyphenylene sulfide) or a liquid crystal polymer. 
The core 2a is formed of a ferrite core or a laminated core, for example. 
The bobbin 1a is formed of a ceramic substance or a heat-resistant 
insulating engineering plastic substance and is intended to fulfill the 
role of pressing at least the lowest layer part of the induction coil 3a 
approximating most closely to the core 2a and adjusting the shape of the 
induction coil 3a. This bobbin 1a has practically the same width as the 
core 2a so that in the lowest layer part near the core 2a, the induction 
coil 3a may be wound in the largest possible number of turns. 
The induction coil 3a is produced by winding a single wire or a Litz wire, 
0.8 mm in diameter, provided on the surface thereof with a fused layer and 
an insulating layer round the bobbin 1a in the direction along the rotary 
axis of the fixing roller 5. The induction coil 3a is so wound that the 
numbers of turns gradually decrease from the lowest layer approximating 
most closely to the core 2a to the upper layers. The induction coil 3a, 
therefore, winds along the circumference of the holder 40 and fills the 
inner empty space substantially completely. 
When the induction coil 3a is so wound that the numbers of turns thereof 
decrease from the lowest layer to the upper layers as described above, the 
induction coil to be used is allowed to have a greater thickness than when 
the induction coil is wound in one same number of turns in all the layers. 
When an induction coil having the same ampere turn as is contemplated by 
Example 3 is to be obtained by winding the coil with one same number of 
turns in all the layers, for example, the thickness of the induction coil 
must be decreased to about 0.5 mm, a size smaller than in Example 3. 
Since the use of an induction coil of a greater thickness is allowed as 
mentioned above, it is made possible to reduce the copper loss of the 
induction coil, decrease the amount of heat generated by the induction 
coil during the flow of current, and prevent the temperature of the fixing 
roller 5 from excessively rising. 
The relation between the copper loss and the thickness of the induction 
coil offers a clear explanation for the advantages just mentioned. The 
following formulas (1) and (2) represent the relation between the copper 
loss and the thickness of the induction coil. 
EQU P=I.sup.2 R (1) 
EQU R=.rho..multidot.L/S (2) 
(wherein P represents the copper loss, I the current flown to the induction 
coil, R the resistance of the copper loss, .rho. the resistivity of the 
copper wire destined to form the induction coil, L the length of the 
copper wire destined to form the induction coil, and S the cross section 
of the copper wire destined to form the induction coil). 
From the formulas given above, it is clearly noted that the copper loss of 
the induction coil decreases in proportion as the thickness (cross 
section) of the induction coil (the copper wire destined to form the 
induction coil) increases. The generation of heat in the induction coil, 
therefore, results in a decrease of the copper loss. 
EXAMPLE 4 
FIG. 24 is a cross section illustrating still another example of the 
induction-heating fixing apparatus embodying the present invention. 
Example 4 differs from Example 3 in the manner of winding the induction 
coil. In all the other particulars of structure, they are identical. Thus, 
the particulars that characterize Example 4 will be covered in the 
following description and the structure and the operation that are 
identical to those of Example 3 will be omitted from the following 
description. 
In the induction-heating fixing apparatus of Example 4, an induction coil 
3a wound round a bobbin 1 which is disposed so as to encircle a core 2a 
has one same number of turns in almost all the layers from the lowermost 
layer to the upper layers as shown in FIG. 24. Further, the induction coil 
3a is given an outside diameter in the range of 0.2-0.8 mm and a plurality 
of copper wires are bundled to form a Litz wire. 
By setting the outside diameter of the induction coil 3a in the range of 
0.2-0.8 mm as mentioned above, the magnitude of the effective resistance 
of the induction coil in the induction-heating fixing apparatus to which a 
high frequency in the approximate range of 10-100 kHz is applied can be 
minimized, the generation of heat in the induction coil can be decreased 
to the fullest possible extent, and the fixing roller can be prevented 
from excessively elevating its temperature. Further, the use of the Litz 
wire facilitates the production which is implemented by winding the thin 
induction coil, 0.2-0.8 mm, on a bobbin. 
Now, the reason for limiting the outside diameter of the induction coil to 
the range of 0.2-0.8 mm as described above will be given below. 
Between the outside diameter of the induction coil and the generation of 
heat due to the copper loss of the induction coil, there exists a close 
relation as described in Example 3 with the aid of the formulas (1) and 
(2). Simply it is concluded that the resistance is lowered, the loss due 
to the self-generation of heat is decreased, and consequently the 
generation of heat in the induction coil is reduced and, at the same time, 
the proportion of energy to be used for the induction-heating of the 
fixing roller increases in proportion as the outside diameter of the 
induction coil increases. 
Specifically, when the thickness of one coated copper wire is increased, 
the copper loss is decreased because the cross section of the copper wire 
is increased proportionately to the increase in the thickness thereof. At 
a high frequency, however, the effect in reducing the copper loss is 
diminished by the skin effect in proportion as the outside diameter of the 
induction coil is increased. When the thickness is unduly increased, the 
induction coil 3a bulges at the corners of turns as shown in FIG. 25 and 
is not easily wound tightly on the bobbin 1a in terms of production. 
When the induction coil is formed with one coated copper wire, the fact 
that this invention imparts an alternating current of high frequency (10 
kHz-100 kHz) to the induction coil brings about the contrary effect that 
the effective resistance of the copper wire is not appreciably increased 
for an increase in the cross section of the copper wire owing to the skin 
effect. This fact implies that the effect derived from increasing the 
thickness of the copper wire diminishes in proportion as the copper wire 
gains in thickness. For the sake of decreasing the loss, therefore, it is 
more advantageous to form the induction coil with a Litz wire resulting 
from bundling a plurality of a thin coated copper wires than to form the 
induction coil with one thick coated copper wire. 
The increase in the outside diameter of the copper wire also entails the 
problem that the induction coil produced by winding the copper wire is not 
easily stowed in the holder of a stated space because the copper wire 
bulges at the corners of turns thereof as illustrated in FIG. 25. When a 
multiplicity of very fine coated copper wires are bundled and are forcibly 
stowed in the holder of a stated space, the same contrary effect of 
decrease in loss as mentioned above ensues that the ratio of the copper 
wire part (the cross section used for the flow of current) to the cross 
section of the Litz wire is increased by the bulges of bundles, the 
coating of the copper wire, and the fused layer (which is not always 
necessary). The attempt proves unfavorable in terms of the cost and the 
efficiency of production (the size of the holder inside the rotary member 
generally falls in the range of 10-60 mm as outside diameter). 
When the induction coil is formed, as contemplated by this invention, in a 
limited space of the interior of the rotary member in the fixing 
apparatus, the optimum outside diameter of the copper wire has its limits. 
It has been found experimentally that the Litz wire using a copper wire 
with an outside diameter of not less than 0.2 mm and not more than 0.8 mm 
is appropriate for the induction coil in terms of efficiency, cost, and 
ease of production. 
The relation of the induction coil loss to the outside diameter of the 
copper wire is shown in FIG. 26. It is clearly noted from the diagram that 
the coil loss unduly increases when the outside diameter of the copper 
wire is smaller than 0.2 mm or larger than 0.8 mm. Since the ratio of the 
self-generation of heat in the induction coil increases and the elevation 
of the temperature of the induction coil also increases as a result, the 
temperature of the induction coil surpasses the limit of its own heat 
resistance and the induction coil is required to be made of an expensive 
material of high grade of heat resistance. Further, the self-generation of 
heat impairs the efficiency of heating and exerts an adverse effect on the 
heat resistance of circumferential parts such as of the bobbin. 
When the induction coil is to be stowed in a holder, 30 mm in outside 
diameter, for example, the outside diameter of the copper wire is 
appropriately in the range of from 0.3 mm to 0.5 mm. When the coated 
copper wire is unduly thin, the efficiency of bundling is not very high 
because of the inevitable formation of bulges and the production of the 
Litz wire grows in difficulty because of the necessity of bundling a 
multiplicity of coated copper wires. When the holder has a smaller outside 
diameter, the range of the outside diameter of the copper wire is shifted 
toward a decreasing direction because of a decrease in the number of wires 
to be bundled. 
The minimum magnitude of the coil loss exists in the range, 0.2 mm-0.8 mm, 
of the outside diameter of the copper wire. The heating of high energy 
efficiency can be realized by forming the induction coil within this 
range. In terms of the cost and the efficiency of production, the number 
of Litz wires to be bundled is appropriately not more than 100. When the 
Litz wire is formed by bundling a plurality of coated copper wires, it is 
necessary that the wire be bundled as twisted meantime. The reason for 
this necessity is that the current flowing through the coated copper wire 
deviates from one to another ply of the copper wire when the copper wire 
is wound in an untwisted form on the bobbin. It has been experimentally 
found that the pitch of this twisting must be kept below 200 mm. The 
relation between the twisting pitch and the induction coil loss at a fixed 
frequency is shown in FIG. 27. The induction coil loss is increased by the 
deviation of the current when the twisting pitch is unduly large. When the 
twisting pitch exceeds 200 mm, the temperature of the induction coil 
surpasses the limit of its own heat resistance (generally the limit of 
heat resistance of the induction coil is about 220.degree. C. at most) 
similarly to the outside diameter of the copper wire mentioned above. 
EXAMPLE 5 
FIG. 28 is a cross section schematically illustrating an induction-heating 
fixing apparatus of Example 5 embodying this invention. 
Part of the components of the structure of this example which are identical 
with those of Example 2, Example 3, and Example 4 will be omitted from the 
following description. 
The fixing apparatus of Example 5, as illustrated in FIG. 28, is provided 
with a fixing roller 5 and a pressing roller 6 pressed against the fixing 
roller 5 and consequently allowed to follow the rotation of the fixing 
roller 5. 
The fixing roller 5 is a hollow pipe formed of conductive members. A coil 
assembly 15b is disposed inside the fixing roller 5. This coil assembly 
15b is retained by a holder 25 and to complete a holder unit 40a as a 
whole. 
The fixing roller 5 is provided at the opposite terminals thereof each with 
a slider bearing part and it is rotatably attached to a fixing unit frame 
(not shown). Further, the fixing roller 5 is provided at one of the 
terminals thereof with a drive gear and is rotated by a drive source such 
as a motor (not shown) which is connected to the drive gear. The holder 25 
is fixed to the fixing unit frame and therefore rendered non-rotatable. It 
is stowed inside the roller 5 as separated by a gap of a stated size from 
the inner wall surface of the fixing roller 5. 
Then, the fixing roller 5 is provided with a separation claw 7 adapted to 
make sliding contact with the surface of the fixing roller 5. Above the 
fixing roller 5, a thermistor 8 for detecting the temperature of the 
fixing roller 5 and a thermostat 9 as a safeguard against abnormal 
temperature elevation are disposed. The thermistor 8 and the thermostat 9 
are pressed against the surface of the fixing roller 5 so as to be opposed 
to the induction coil 22 across the fixing roller 5. 
The fixing roller 5 is formed of such a conductive member as, for example, 
a steel tube, stainless alloy tube, nickel tube, carbon steel tube, or 
aluminum alloy tube and has formed on the circumferential surface thereof 
a heat-resistant releasable layer obtained by coating the surface with a 
fluorine resin. The fixing roller 5 preferably is formed of conductive 
magnetic members. The pressing roller 6 has a silicone rubber layer 62, a 
surface-release type heat-resistant rubber layer, formed on the periphery 
of an axial core 61. Then, the slider bearing and the separation claw 7 
are formed of a heat-resistant sliding engineering plastic substance. 
The holder unit 40a, as illustrated in FIG. 29, is composed of a coil 
assembly 15b and a holder 25 as described above. The coil assembly 15b is 
provided with a core 23 formed of a magnetic material and an induction 
coil 22 formed by winding a copper wire 21 through the medium of an 
insulating part round the core 23 and adapted for causing the fixing 
roller 5 to produce an induced current and generate heat. The core 23 and 
the induction coil 22 are supported by the holder 25. 
In Example 5 in particular, the holder 25 is provided with an integrating 
part formed in a substantially cylindrical shape of an electrically 
insulating material integrally with the holder 25 and adapted to insulate 
the core 23 from the induction coil 22. The core 23 has formed therein an 
end face 38 which continues into the circumferential surface 33 of the 
holder 25. 
To be more specific, the holder 25, as illustrated in FIG. 30, is provided 
with a holder proper 30 formed in a substantially cylindrical shape and 
terminal stoppers 32 adapted to close openings 31 which are provided one 
each at the opposite terminals in the axial direction of the holder proper 
30. The holder proper 30 is composed of arcuate parts 34 seated in the 
upper and the lower position in the bearings of the diagram and having 
arcuate outer circumferential surfaces 33 and a rectangular bobbin part 35 
extending in the axial direction and serving to connect the arcuate parts 
34. In the bobbin part 35, through holes 36 for allowing insertion of the 
cores 23 are formed through the arcuate parts 34. The holder proper 30 and 
the terminal stoppers 32 are both formed of a heat-resistant insulating 
engineering plastic substance. The holder proper 30 has the two arcuate 
parts 34 and the bobbin part 35 formed integrally therewith. The terminal 
plugs 32 are each provided with a projecting part 37 to be fixed to the 
fixing unit frame. The induction coil 22 is formed by winding a copper 
wire 21 a plurality of turns in one direction round the bobbin part 35. 
The four wall surfaces of the bobbin 35 function as the insulating part 
formed integrally on the holder 25 as mentioned above. 
The core 23 assumes the shape of a relatively elongate plate. A terminal 
surface 38 of the core 23 lying along the direction of the opening in the 
through hole 36, as illustrated on a magnified scale in FIG. 31, has the 
same radius of curvature as the arcuate circumferential surface 33 of the 
arcuate part 34 so that, when the core 23 is inserted in the through hole 
36 and fixed therein, the terminal surface 38 smoothly continues into the 
arcuate circumferential surface 33. The core 23 is formed of a ferrite 
core or a laminated core, for example, and is disposed to intersect 
perpendicularly the copper wire 21 of the induction coil 22 and allowed to 
form a magnetic circuit. 
FIG. 30 depicts a structure in which the terminal stoppers 32 are fitted to 
the openings 31 in the holder proper 30 after the induction coil 22 is 
formed. The terminal stoppers 32 are not essential members for the 
formation of the holder 25. It is allowable to form on the fixing unit 
frame such members as fit into the openings 31 and use these members for 
retaining the holder proper 30. For the induction coil 22, it is 
appropriate to use a single wire or a Litz wire provided on the surface 
thereof with a fused layer and an insulating layer. The copper wire 21 is 
wound within the range not surpassing the circumferential surface of the 
holder. 
Further in Example 5, the holder 25 is wholly enveloped with a 
heat-resistant electric insulating member 39 of a small wall thickness. 
The electric insulating member 39 of a small wall thickness is formed of 
polyimide, polyamideimide, or fluorine resin, for example. Specifically, 
it is an insulating film or an insulating tube having a thickness in the 
approximate range of 30 .mu.m-100 .mu.m. The electric insulating member 39 
is not limited to a tubular shape. It is only required to be so shaped as 
to cover at least the portions of the core 23 and the induction coil 22 
that are exposed from the holder 25. For example, the electric insulating 
member formed in the shape of a sheet capable of covering not the holder 
25 wholly but the terminal surface 38 of the core 23 and the lateral 
surface of the induction coil 22 partly may be attached to the arcuate 
part 34 of the holder proper 30 by adhesion or coating. 
The holder 25 is stowed in a non-rotatable state inside the fixing roller 5 
and the core 23 is disposed as separated by a distance (hereinafter 
referred to as "gap") of not less than 0.5 mm and not more than 4 mm from 
the inner surface of the fixing roller. 
The reason for specifying the size of the gap as mentioned above is given 
below. 
The holder 25 supporting the induction coil 22 and the core 23 is stowed in 
the fixing roller 5 and is fixed at the opposite terminals thereof to the 
fixing unit frame. Thus, it sags to a certain extent under the weight of 
the induction coil 22 and that of the core 23. The possibility exists that 
the fixing roller 5 and the holder 25 will suffer from such dimensional 
dispersions as are not infrequently observed in articles of manufacture, 
i.e. the former having its inside diameter and the latter its outside 
diameter fluctuate. All these factors considered, the gap must be kept 
above 0.5 mm. The amount of the sag which the holder 25 produces depends, 
though not entirely, on the length thereof. So long as the gap has the 
maximum size of 4 mm, the possibility of the core 23 colliding against the 
fixing roller 5 is nil, even in consideration of the dimensional 
dispersions of relevant parts. 
When the size of the gap increases, the efficiency of heating is degraded 
and the amount of the current flown to the induction coil must be 
increased. FIG. 32 is a graph showing the relation between the distance 
from the fixing roller to the core and the temperature of the induction 
coil. It is noted from this diagram that the amount of heat generation due 
to the copper loss of the induction coil is fairly increased when the gap 
exceeds 4 mm. The temperature of the induction coil itself exceeds 
200.degree. C., for example, when the temperature of the fixing roller is 
160.degree. C. In this case, the power applied is 160 W for the roller of 
an outside diameter of 20 mm, 300 W for the roller of an outside diameter 
of 30 mm, and 700 W for the roller of an outside diameter of 40 mm. When 
the temperature of the induction coil itself exceeds 200.degree. C., an 
expensive material capable of enduring the temperature mentioned above 
must be used for the holder for supporting the core and the induction coil 
or for the induction coil. The use of this material not merely entails an 
addition to the cost but also counters the trend toward energy 
conservation. 
In view of the points mentioned above, it is safe to conclude that the 
temperature of the induction coil can be kept below 200.degree. C. by 
giving the gap a size of not more than 4 mm and the core can be prevented 
from colliding against the fixing roller and the fixing roller can be 
safely rotated by giving the gap a size of not less than 0.5 mm. 
The induction-heating fixing apparatus of Example 5 which specifies a size 
of not more than 4 mm for the gap, therefore, can prevent the cost from 
increasing and contribute to the conservation of energy. Further, by 
specifying a size of not more than 0.5 mm for the gap, the fixing 
apparatus can ensure smooth rotation of the fixing roller 5 and secure 
electric insulation between the core 23 and the fixing roller 5. Further, 
since the holder 25 is wholly enveloped with an electric insulating member 
39 such as an insulating film or insulating tube, the electric insulation 
between the core 23 and the fixing roller 5 and between the induction coil 
22 and the fixing roller 5 can be further ensured. Even when the coating 
of the induction coil 22 is broken as by excessive temperature elevation, 
for example, the electric insulation can be infallibly secured and the 
fixing apparatus can be prevented from being wholly fractured. 
Incidentally, the fixing apparatus incorporated in a copying device or a 
printer generally uses a fixing roller having an outside diameter in the 
approximate range of 10 mm-60 mm. The necessity of enlarging the outside 
diameter of the fixing roller for the purpose of securing an ample nip 
width for heating paper grows in proportion as the number of pages to be 
printed per unit time or the speed of printing operation increases. When 
the outside diameter of the fixing roller is large, the coil assembly of 
the holder unit can be proportionately enlarged. Since this enlargement 
results in an increase in the power required for the operation, the 
situation of the elevation of the temperature of the induction coil is 
substantially invariable without reference to the outside diameter of the 
roller as shown in FIG. 32. This fact indicates that the elevation of the 
temperature of the induction coil is copiously affected by the size of the 
gap rather than the outside diameter of the roller. The fixing apparatus 
which uses a fixing roller having an outside diameter departing from the 
range mentioned above, therefore, requires to use a size of not more than 
4 mm for the gap without reference to the outside diameter of the fixing 
roller. The results of other experiments indicate that, in spite of the 
difference in material of the fixing roller, the size of the gap notably 
affects the elevation of the temperature of the induction coil rather than 
the difference in material. 
Incidentally, in the structure in which a holder unit 45 having a core 43 
and an induction coil 42 wholly enveloped with a holder 44 is stowed in a 
fixing roller 46 as illustrated in FIG. 33, when the distance between the 
outer circumferential surface of the holder 44 and the inner 
circumferential surface of the fixing roller 46 is the same as is 
contemplated by Example 5, the core 43 is fated to be separated from the 
inner surface of the fixing roller 46 by a distance equalling the 
thickness of the holder 44. An idea of decreasing the wall thickness of 
the holder 44 and consequently allowing the core 43 to approximate to the 
inner surface of the fixing roller 46 may be conceivable. This measure, 
however, causes a shortage of the mechanical strength for fixing the 
holder unit 45 to the fixing unit frame. 
In contrast, Example 5 can secure ample mechanical strength of the holder 
25 as a whole by integrating the bobbin part 35 with the holder proper 30 
and can cause the core 23 to approximate to the inner surface of the 
fixing roller 5 to the fullest possible extent by enabling the terminal 
surface 38 of the core 23 to continue to the arcuate outer circumferential 
surface 33 of the holder proper 30. Moreover, since the holder 25 is 
wholly enveloped with an electric insulating member 39 of a thin wall such 
as an insulating film or insulating tube, the present embodiment can 
secure an ample electric insulation even when the core 23 is approximated 
more closely to the inner surface of the fixing roller 5. 
EXAMPLE 6 
FIG. 34 is a cross section schematically illustrating an induction-heating 
fixing apparatus of Example 6 embodying this invention. In this diagram, 
like parts found in Example 5 will be denoted by like reference numerals 
and will be omitted from the following description. 
Example 6 differs from Example 5 in respect that a flexible metallic sleeve 
50 of a small wall thickness is used as a conductive member in the place 
of the fixing roller 5 of Example 5. Their difference also resides in the 
structure of a holder unit 53. 
This induction-heating fixing apparatus, as illustrated in FIG. 34, is 
provided with a non-rotatable holder unit 53 fixed to a fixing unit frame, 
a pressing roller 6 disposed so as to be rotated in the direction of an 
arrow c and pressed against the holder unit 53, and the metallic sleeve 50 
nipped between the pressing roller 6 and the holder unit 53 and adapted to 
follow the rotation of the pressing roller 6. 
The metallic sleeve 50 is formed of conductive members such as of nickel 
and is provided on the outer circumferential surface thereof with a 
heat-resistant releasable layer formed by coating the surface with a 
fluorine resin. The metallic sleeve 50 has a wall thickness in the range 
of 20 .mu.m-60 .mu.m. 
Inside the metallic sleeve 50, a coil assembly 52 adapted for causing the 
metallic sleeve 50 to generate an induced current (eddy current) is 
disposed. This coil assembly 52 is supported by a holder 54. All these 
components jointly form a holder unit 53. 
The holder unit 53 of Example 6 has a structure in which the coil assembly 
52 is wholly encircled with the holder 54. The coil assembly 52 is 
provided with a core 55 made of a magnetic material, a bobbin 58 
containing a through hole 57 for allowing insertion of the core 55, and an 
induction coil 56 formed by winding a copper wire 21 round the bobbin 58 
and adapted for enabling the metallic sleeve 50 to generate an induced 
current and emit heat. The bobbin 58 functions as an insulating part for 
insulating the core 55 from the induction coil 56. The coil assembly 52 is 
stowed as thoroughly concealed inside the holder 54 which is formed as 
divided in two halves separately from the bobbin 58. 
In the fixing apparatus using the metallic sleeve 50, since the holder unit 
53 is pressed against the inner circumferential surface of the metallic 
sleeve 50 and consequently caused to produce friction, the fixing 
apparatus finds it difficult to use the holder 25 shown by Example 5 and 
the insulation film or the insulation tube adapted to envelop the holder 
25. 
In the case of the fixing apparatus which has such a structure as shown 
above, the holder must be formed with an appreciable thickness (not less 
than 1 mm where it is made of resin) so as to secure mechanical strength 
enough to withstand the pressure to be exerted on the pressing roller 6. 
Meanwhile, the nip part, because of its structure involving relative slide 
between the metallic sleeve 50 and the holder 54, does not need to keep a 
gap from the viewpoint of securing smooth rotation as in the case of the 
fixing roller 5. Thus, the core 55 can be disposed as separated by a gap, 
d, of not less than 0.5 mm and not more than 4 mm from the inner surface 
of the metallic sleeve 50 in the same manner as in Example 5 by giving a 
size, which is the sum of the wall thickness of the holder 54 and a space, 
.alpha., arising from dispersions of the outside diameters of the metallic 
sleeve 50 and the holder 54, to the gap between the core 55 and the inner 
surface of the metallic sleeve 50. 
Incidentally, in Example 6, the gap in the area (parts indicated by a slash 
in FIG. 34) in which the shorter edge of the core 55 (the lateral edge in 
the bearings of FIG. 34) is opposed to the metallic sleeve 50 always 
equals the size mentioned above. 
Since a size of not more than 4 mm is specified for the gap, the 
induction-heating fixing apparatus of Example 6 can prevent the cost from 
rising and contribute to the conservation of energy without incurring the 
situation in which the temperature of the induction coil 56 itself 
surpasses 200.degree. C. Since the outer circumferential wall of the 
holder 54 intervenes between the core 55 and the metallic sleeve 50, the 
gap infallibly assumes a size of not less than 0.5 mm and ample electric 
insulation is secured between the core 55 and the metallic sleeve 50. Even 
when the coating of the induction coil is broken as by excessive 
temperature elevation, for example, the electric insulation can be secured 
and the fixing apparatus can be prevented in its entirety from breakage. 
EXAMPLE 7 
FIG. 35 is a schematic cross section of a fixing apparatus of Example 7 
embodying this invention, FIG. 36 a perspective view of a holder unit of 
the apparatus mentioned above, and FIG. 37 a perspective view illustrating 
the manner in which the holder unit is stowed inside a fixing roller. 
The fixing apparatus of Example 7 is substantially identical in its basic 
structure shown in FIG. 35 to those of Example 3 through Example 5. The 
aspect which particularly characterizes Example 7 resides in the fact that 
an induction coil is thrust out and exposed to view from at least one 
terminal part in the axial direction of the fixing apparatus as 
illustrated in FIG. 36 and FIG. 37. The term "axial direction of the 
fixing apparatus" as used herein refers to the same direction as the axial 
direction of the fixing roller 5 or the pressing roller 6. The operation 
of the fixing apparatus is equal to that described in Example 2 above and 
will be omitted from the following description. 
The fixing apparatus of Example 7, as pointed out above, is substantially 
equal to those of FIG. 3 through FIG. 5. It is provided, as illustrated in 
FIG. 35, with a fixing roller 5 and a pressing roller 6 pressed against 
the fixing roller 5 and enabled to follow the rotation of the fixing 
roller 5. 
The fixing roller 5 is a hollow pipe formed of conductive members. Inside 
the fixing roller 5, a coil assembly 15c is set in place. This coil 
assembly 15c is covered with a holder unit 40c which possesses an 
insulating property. 
The holder unit 40c, as shown in FIG. 38 which depicts the holder unit 40c 
as viewed from the axial direction, is divided into upper and lower halves 
and is composed of an upper holder 401 and a lower holder 402 which are 
joined to each other by means of hooked engaging parts K formed in the 
dividing points. Since it is divided into upper and lower halves, it can 
be easily formed with resin. The upper holder 401 and the lower holder 402 
can be brought into perfect and strong union by being slid against each 
other in the axial direction through the medium of the hooked engaging 
parts K. Thus, the holder unit 40c is produced easily and inexpensively as 
endowed with high strength and rigidity. The hooked engaging parts K do 
not need to be limited to the shape shown in FIG. 38 but may be formed in 
any other shape so long as the divided halves will be slid into 
engagement. They may be formed in matching concave and convex shapes, for 
example. 
The fixing roller 5 is provided at the opposite terminals thereof each with 
a slider bearing part and is rotatably attached to a fixing unit frame 
(not shown). Further, the fixing roller 5 is provided at one terminal 
thereof with a drive gear (not shown) and is driven by a drive source (not 
shown) such as a motor connected to the drive gear. The holder unit 40c is 
fixed to the fixing unit frame and hence rendered non-rotatable and is 
stowed inside the fixing roller 5 as separated by a gap of a stated size 
from the inner wall surface of the fixing roller 5. 
The fixing roller 5 is provided with a separation claw 7 adapted to make a 
sliding contact with the surface thereof. Above the fixing roller 5, a 
thermistor 8 for detecting the temperature of the fixing roller 5 and a 
thermostat 9 as a safeguard against abnormal temperature elevation are 
disposed. The thermistor 8 and the thermostat 9 are pressed against the 
surface of the fixing roller 5 so as to be opposed to the induction coil 
22 across the fixing roller 5. 
The fixing roller 5 is formed of such a conductive member as, for example, 
a steel tube, stainless alloy tube, nickel tube, carbon steel tube, or 
aluminum alloy tube and has formed on the outer circumferential surface 
thereof a heat-resistant releasable layer obtained by coating the surface 
with a fluorine resin. The fixing roller 5 preferably is formed of 
conductive magnetic members. The pressing roller 6 has a silicone rubber 
layer 62, a surface-release type heat-resistant rubber layer, formed on 
the periphery of an axial core 61. Then, the slider bearing and the 
separation claw 7 are formed of a heat-resistant sliding engineering 
plastic substance. 
Then, in Example 7, the induction coil 3c is thrust out and exposed to view 
from at least one terminal part in the axial direction of the fixing 
apparatus as illustrated in FIG. 36 and FIG. 37. 
FIG. 39 is a perspective view illustrating the layout of the induction coil 
relative to the core in the fixing apparatus of Example 7 embodying this 
invention, FIG. 40 is a longitudinal section of the holder unit of the 
apparatus mentioned above, FIG. 41 is a perspective view illustrating the 
layout of the induction coil relative to the core in the fixing apparatus 
as posed in a state not exposing the induction coil to view, and FIG. 42 
is a longitudinal section of the holder unit of the apparatus mentioned 
above. The insulation such as a bobbin which is interposed between the 
core and the induction coil is omitted from the diagrams. In the 
longitudinal section of the holder unit, the holder alone has its section 
shown so as to permit clear comprehension of the layout of the holder 
relative to the induction coil stowed within (similarly applicable to 
FIGS. 44-49). 
Specifically, the induction coil 3c, as illustrated in FIG. 40, is disposed 
as extended so as to protrude in a length, L (mm), each from the opposite 
terminal parts in the axial direction of the holder unit 40c. The reason 
for projecting the induction coil 3c from the holder unit 40c in the 
manner mentioned above is that the coil assembly 15c including the 
induction coil 3c is retained inside the cylindrical holder unit 40c and 
the length of the holder unit 40c is generally so set as to equal or 
slightly exceed the length of the fixing roller 5 for the purpose of 
securing necessary electric insulation between the fixing roller 5 and the 
induction coil 3c. Optionally, the induction coil 3c may be so formed as 
to protrude from one terminal part in the axial direction of the holder 
unit 40c. 
In the fixing apparatus of Example 7, the induction coil 3c is 
spontaneously cooled owing to the exposure of the projected parts thereof 
to the ambient air. More often than not, the induction coil 3c is so 
formed as to be enveloped with the cylindrical holder unit 40c of an 
electrically insulating material from the viewpoint of safety as in the 
case of Example 7. In this structure, the effect of spontaneous cooling 
through the exposed parts is manifested particularly conspicuously because 
the radiation of heat is obstructed and the heat tends to build up within. 
Besides, the cooling effect is very large because the induction coil 3c 
itself is made of copper, a substance with an outstanding heat-radiating 
property. 
FIG. 43 is a graph showing the relation between the amount of projection of 
the induction coil and the temperature of the induction coil. 
It is clearly noted from this graph that the cooling effect of the 
induction coil is enhanced in proportion as the amount of projection of 
the induction coil is increased as evinced by the fact that the fixing 
apparatus of Example 7 illustrated in FIG. 39 and FIG. 40 enjoys a better 
cooling effect than the fixing apparatus of the type having the induction 
coil 3c not exposed to view as illustrated in FIG. 41 and FIG. 42. 
It is further appropriate practically from the viewpoint of reconciling 
quality and cost to keep the temperature of the induction coil below 
180.degree. C. in consideration of the heat resistance of such parts as 
the cover and the holder of the induction coil. Generally, therefore, it 
may be safely concluded that more appropriately the amount of projection, 
L, of the induction coil is not less than 10 mm. 
Thus, the fixing apparatus can promote the radiation of heat from the whole 
of the induction coil and prevent the induction coil from elevating the 
temperature of its own. Further, since the induction coil is prevented 
from excessive heating, the peripheral parts of the induction coil do not 
need to possess unduly high heat resistance. This fact contributes to 
further saving of the cost. 
Further, the fixing apparatus can be formed only by adding such a simple 
and inexpensive mechanism that the induction coil 3c merely protrudes 
slightly from the holder unit 40c with the same fixing roller 5 and is 
adaptable even to a fixing roller of a small diameter. 
EXAMPLE 8 
FIG. 44 is a perspective view illustrating the essential part of a fixing 
apparatus of Example 8 embodying the present invention and FIG. 45 is a 
longitudinal section of a holder unit of the apparatus mentioned above. In 
these diagrams, like parts found in FIG. 35-FIG. 40 used above to aid in 
the description of Example 7 will be denoted by like reference numerals. 
These parts will be omitted from the following description. 
The fixing apparatus of Example 8, as illustrated in FIG. 44, is provided 
with a pair of metallic plates 91 as metallic members adapted to contact 
an induction coil 3c and stowed in the fixing roller in conjunction with 
the induction coil 3c. The metallic plates 91, as illustrated in FIG. 45, 
are disposed as extended so as to protrude from the opposite terminal 
parts in the axial direction of the holder unit 40c. The metallic plates 
91 are formed of copper or a copper alloy and, therefore, exhibit an 
excellent heat-radiating property. They are provided in the terminal parts 
thereof each with a part 91a having the surface of contact with the 
ambient air enlarged to exalt the heat-radiating property. Example 8 is at 
an advantage not only in obtaining the same effect as the aforementioned 
embodiments of the invention by simply disposing the metallic plates 
without altering the shape of the induction coil but also in facilitating 
the adjustment of the degree of cooling. 
Incidentally, the metallic plates 91 may be formed of some other metallic 
material of good thermal conductivity such as, for example, aluminum or an 
alloy thereof. And the number of such metallic plates 91 to be installed 
is a matter of free choice. Instead of using these metallic plates, a 
cylindrical copper tube may be slipped over the induction coil 3c as held 
in contact therewith and the copper tube may be extended so as to have the 
terminal parts thereof thrust out of the opposed terminal parts in the 
axial direction of the holder unit 40c. It is also allowable to have the 
metallic plates 91 so disposed as to contact not only the induction coil 
3c but also the core 10. 
EXAMPLE 9 
FIG. 46 is a perspective view illustrating the essential part of a fixing 
apparatus of Example 9 embodying this invention and FIG. 47 is a 
longitudinal section of a holder unit of the apparatus mentioned above. In 
these diagrams, like parts found in FIG. 35-FIG. 40, FIG. 44, and FIG. 45 
will be denoted by like reference numerals. These parts will be omitted 
from the following description. 
Example 9 differs from Example 8 in respect that it uses a heat pipe 95 as 
a metallic member in the place of the metallic plates 91 of Example 8. The 
heat pipe 95, as is universally known, consists of a sealed metal tube 
with a lining of capillary material and a working fluid held in the tube 
in a decompressed state and operates on the principle that the working 
fluid vaporizes on exposure to heat at one end of the tube, the formed 
vapor flows to the other end and condenses with release of heat there, and 
the working liquid consequently restored returns to the heating part by 
virtue of capillarity. According to Example 9, therefore, the heat pipe 95 
laid out in a scanty empty space promotes quick transfer of the heat of 
the induction coil and further exalts the effect of spontaneous cooling of 
the induction coil by means of the ambient air. 
EXAMPLE 10 
FIG. 48 is a perspective view illustrating the essential part of a fixing 
apparatus of Example 10 embodying this invention and FIG. 49 is a diagram 
illustrating a longitudinal section of a holder unit of the apparatus 
mentioned above together with cooling means thereof. In these diagrams, 
like parts found in FIG. 35-FIG. 40, FIG. 44-FIG. 47 will be denoted by 
like reference numerals. These parts will be omitted from the following 
description. 
Example 10 differs from Example 8 in respect that it uses metallic pipes 96 
as metallic members in the place of the metallic plates 91 of Example 8 
and circulates a liquid of a large heat capacity inside the metallic pipes 
96 in the direction indicated by an arrow mark in the diagram. Example 10, 
as illustrated in FIG. 49, contemplates positively cooling the induction 
coil by the use of a cooling means which is formed by transfixing the 
interior of a holder unit 40c with the metallic pipes 96 and connecting a 
pumping mechanism 98 as a means for circulating the liquid through the 
medium of a communicating hose 97 to the metallic pipes 96. According to 
Example 10, therefore, by circulating the cooling liquid through the 
metallic pipes 96 laid out in a scanty empty space, the heat accumulated 
in the induction coil 3c can be positively and quickly absorbed by the 
cooling liquid and transferred and the cooling effect of the induction 
coil can be markedly exalted. 
EXAMPLE 11 
FIG. 50 is a diagram schematically illustrating the structure of a fixing 
apparatus of Example 11 embodying this invention. In this diagram, like 
parts found in FIG. 35-FIG. 40, FIG. 44-FIG. 49 will be denoted by like 
reference numerals. These parts will be omitted from the following 
description. 
In Example 11, an air blower 99 which is generally used in the main body of 
such an image forming apparatus as a copying machine for the purpose of 
cooling the interior of the main body is disposed in the proximity of one 
terminal part in the axial direction of the fixing apparatus, namely in 
the proximity of the part of the induction coil 3c thrust out and exposed 
to view. The air blower 99 is intended to cool mainly a conventionally 
installed power source (not shown) and the fixing apparatus. When this air 
blower 99 is laid out as described above, the fixing apparatus no longer 
requires any extra fan for the purpose of cooling and further exalts the 
cooling effect of the induction coil. In the diagram, the symbol "A" 
indicates the flow of air inside the main body of the image-forming 
apparatus and the symbol "B" the flow of air discharged out of the main 
body. Of course, it is permissible to install additionally an induction 
coil-cooling fan which is adapted to force a flow of air in the axial 
direction through the interior of the fixing roller for the purpose of 
further exalting the cooling effect. 
It should be understood that the invention is not limited to the particular 
embodiments shown and described hereinabove but that it may be implemented 
by suitably combining the embodiments, with various changes and 
modifications made therein without departing from the spirit and scope of 
this novel concept as defined by the following claims. For example, 
structures which combine the characteristic features of the embodiments 
mentioned above and, at the same time, use a multiplicity of induction 
coils arranged in a plurality of parallelly connected rows, structures 
which use induction coils, 0.2-0.8 mm in outside diameter, in the 
embodiments mentioned above, and structures which have the numbers of 
turns of the induction coil gradually decreased from the lowermost layer 
to the upper layers in the embodiments mentioned above may be cited.