ZINC OXIDE VARISTOR AND PRODUCTION THEREOF

Abstract: A zinc oxide varistor having an improved nonlinear voltage dependency and comprising a varistor element (1) based on zinc oxide and provided with at least two electrodes (2). The production process comprises diffusing a lead borosilicate glass containing at least one metallic oxide selected from the group consisting of oxides of cobalt, magnesium, yttrium, antimony, manganese, tellurium, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium into a fired varistor element (1) through the surface of the element (1).

Description: TECHNICAL FIELD The present invention relates to a zinc oxide varistor used for protecting various kinds of electronic instruments from unusually high voltages, and a process for producing the same. BACKGROUND TECHNIQUES Recently, there has been rapidly developed a high level integration of control circuits in instruments for general use and industry. When an extraordinarily high voltage (surge) is applied to electronic parts of semiconductors used in such control circuits, such parts may be destroyed. Accordingly, it becomes indispensable to take a countermeasure to meet the situation. As such a counterplan, varistors are generally employed. Among the rest, the zinc oxide varistor is widely available for the protection of various kinds of electronic instruments from unusually high voltages because the zinc oxide varistor has an excellent voltage non-linearity and surge absorbing ability. Hithertofore, there has been widely known a zinc oxide varistor provided with at least two electrodes on the surface of varistor element having zinc oxide as its main component. Further, materials for said electrodes, are disclosed in, for example, Patent Application Kokai SHO 62-290104 Official Gazette, etc., whose content is as follows: Electrode material for a zinc oxide varistor was produced by the process wherein 5.0% by weight of a lead borosilicate glass powder composed of 50.0 - 85.0% by weight of PbO, 10.0 - 30.0% by weight of B 2 O 3 and 5.0 - 25.0% by weight of SiO 2 was weighed out and then said powder together with Ag powder (65.0% by weight) were milled in a vehicle (30.0% by weight), in which ethyl cellulose was dissolved in butyl carbitol, to obtain a silver paste which is the electrode material. And then said electrode material was applied onto a surface of a fired varistor element and heated to form an electrode. Although the above zinc oxide varistor is excellent in voltage nonlinearity as mentioned above, further improvement in the voltage nonlinearity has been sought due to the desire of energy-saving and efficiency increase in the zinc oxide varistor. Thus, responding to the above requirements, the present invention aims to provide a zinc oxide varistor further improved in voltage nonlinearity. JP-A-1 030 204 discloses a resistor comprising a sintered body made of ZnO powder mixed with, i.e. SiO 2 on which a bismuth borosilicate glass paste is screen-pointed. Thereafter, sintering takes place. DISCLOSURE OF THE INVENTION In order to accomplish the above objective, according to the present invention, there is provided a zinc oxide varistor defined by the features of claim 1. Methods for producing such a varistor are defined by claims 4 to 23. There are also provided processes for producing a zinc oxide varistor as defined by claims 24 and 28. The following lead borosilicate-type glass was diffused into a fired varistor element from its surface, said lead borosilicate-type glass containing at least one metal oxide selected from cobalt oxide, magnesium oxide, yttrium oxide, antimony oxide, manganese oxide, tellurium oxide, lanthanum oxide, cerium oxide, praseodymium oxide, neodymium oxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, thulium oxide, ytterbium oxide and lutetium oxide. When the above constitution is adopted, it follows that there is interposed at particle boundaries between zinc oxide particles composing a varistor element, the chemical elements composing a lead borosilicate-type glass containing at least one metal oxide selected from cobalt oxide, magnesium oxide, yttrium oxide, antimony oxide, manganese oxide, tellurium oxide, lanthanum oxide, cerium oxide, praseodymium oxide, neodymium oxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, thulium oxide, ytterbium oxide and lutetium oxide. As a result, resistance values of the particle boundaries between zinc oxide particles will become higher, and a leakage current running between electrodes until reaching a varistor voltage becomes much lower. In conclusion, zinc oxide varistor improved in voltage nonlinearity can be obtained. BRIEF EXPLANATION OF THE DRAWINGS Fig. 1 is a front view showing one of the working examples of the zinc oxide varistor of the present invention. Fig. 2 is a sectional view of Fig. 1, and Fig. 3 is a front view showing varistor element of the zinc oxide varistor shown in Fig. 1. BEST MODES FOR CARRYING OUT THE INVENTION One of the working examples of the present invention is explained with reference to the drawings as follows: Fig. 1 and Fig. 2 show one of the working examples of the present invention. In the drawings, 1 is a disk-shape varistor element which is 13 mm in diameter and 1.5 mm in thickness. On both surfaces of this varistor element 1, electrodes 2 are baked thereto as shown in Fig. 3. The electrodes 2 are also disk-shape of 10 mm in diameter, and an outside periphery part of varistor 1 projects out and around the whole circumference of the electrodes. In addition, upper end of lead wire 3 is fixed onto each electrode 2 by soldering. Under said state, the outside periphery of varistor element 1 is coated with an epoxy-type insulative resin 4. As shown in Fig. 1, only the lower end of the lead wire is drawn out to the outside of the insulative resin 4. It should be noted that the present working example is characterized by the material of electrode 2. That is, the present working example used the material formulated by milling a lead borosilicate-type glass frit into a Ag paste. This will be explained in detail hereinunder. (Working Example 1) At first, preparation of the glass frit will be mentioned. According to the composition table of the following Table 1, PbO, B 2 O 3 , SiO 2 and Co 3 O 4 were weighed each in a given amount, and then they were simultaneously mixed and ground in a ball-mill. Thereafter, said admixture was fused in a platinum crucible at a temperature condition of 1000°C - 1500°C, and then quenched to be glassified. The obtained glass was roughly ground, which was followed by fine milling in a ball-mill to obtain a lead borosilicate-type glass frit. On the other hand, as a lead borosilicate glass frit of conventional example, a glass frit composed of 70.0% by weight of PbO, 15.0% by weight of B 2 O 3 , and 15.0% by weight of SiO 2 was formulated in a similar manner. The glass transition point (Tg) of each glass prepared as above was as shown in the following Table 1. Hereupon, the glass transition point (Tg) was determined by using a thermal analysis apparatus. Designation of glass Component ratio (wt.%) Tg ( o C) PbO B 2 O 3 SiO 2 Co 3 O 4 A 70 15 15 0 405 B 69.9 15 15 0.1 405 C 60 15 15 10 420 D 45 15 15 25 465 E 40 15 15 30 475 F 35 15 15 35 490 G 30 34.9 35 0.1 545 H 40 29.9 30 0.1 520 I 89.9 5 5 0.1 315 J 60 0 15 25 445 K 55 5 15 25 450 L 50 30 15 5 480 M 40 40 15 5 500 N 60 15 0 25 440 O 55 15 5 25 445 P 50 15 30 5 495 Q* 40 15 40 5 515 Then, 5.0% by weight of the lead borosilicate-type glass frit was weighed which was followed by milling in the above-mentioned Ag paste (65% by weight of Ag powder was dissolved into 30% by weight of a vehicle in which ethyl cellulose is dissolved into butyl carbitol) to produce electrode material for a zinc oxide varistor. In order to evaluate the electrode material for zinc oxide varistor, which was produced as above, a zinc oxide varistor sintered-body (varistor element 1 in Fig. 3) (a disk-shape of 13 mm in diameter and 1.5 mm in thickness) was provided, said sintered-body consisting of bismuth oxide (Bi 2 O 3 ), cobalt oxide (Co 3 O 4 ), manganese oxide (MnO 2 ), nickel oxide (NiO) and titanium oxide (TiO 2 ) respectively in 0.5 mole%, and antimony oxide (Sb 2 O 3 ), and chromium oxide (Cr 2 O 3 ) respectively in 0.1 mole%, and 0.005 mole% of Al 2 O 3 , the rest being zinc oxide (ZnO). On both surfaces of said sintered-body, an electrode material for zinc oxide varistor was screen-printed to be 10 mm in diameter, and then baked at 800 o C for 10 min. to form electrodes 2 as shown in Fig. 3. After lead wires 3 indicated in Fig. 2 were soldered thereon, the outer periphery was coated with insulating resin 4 to obtain a sample. It is noted that when the above electrode material is applied onto a surface of the sintered-body (varistor element 1) and then heated, a lead borosilicate-type glass in the electrode material, which contains cobalt oxide will penetrate into the varistor element 1, thereby exerting its effect as under-mentioned. With respect to the thus-obtained samples, voltage ratio (V 1mA /V 10µA representing voltage nonlinearity), surge current resistance characteristic and high temperature load life performance are shown in the following Table 2. The above voltage ratio (voltage nonlinearity) was obtained through determination using a direct current constant current electric source. Further, surge current resistance characteristic was obtained by determining a variation ratio of varistor voltage (V 1mA ) occurring when an impact current of 8/20 µS standard waveform and 2500 A crest value was applied two times in the same direction. It is preferred that such a value is less than that in conventional example A. Further, high temperature load life performance was obtained by determining a variation ratio of varistor voltage (V 1mA ) after 1000 hrs. when direct current voltage corresponding to 90% of sample varistor voltage was applied between lead terminals 3 at an environment temperature of 125 o C. Such a value is preferably lower than that in conventional example A. The number of samples was 10 per lot. Further, the above voltage ratio (V 1mA /V 10µA ) indicates voltage nonlinearity. When the voltage ratio is less than that in conventional example A, a leakage current up to reaching a varistor voltage will become lower than conventional one. That is, V 1mA represents a voltage (varistor voltage) when 1mA current runs between electrodes 2. Likewise, V 10µA represents a voltage when 10µA current runs between electrodes 2. A small value of V 10µA is not preferable because a high leakage current runs from a low voltage. Sample No. Designation of glass V 1mA /V 10µA Surge current resistance characteristic ΔV 1mA (%) High temperature load life performance ΔV 1mA (%) Direction same as that of current Direction reverse to that of current Direction same as that of current Direction reverse to that of current 1 A 1.83 -22.3 -28.9 -3.9 -10.8 2 B 1.52 -10.9 -18.0 +1.5 -2.9 3 C 1.36 -9.7 -14.5 +1.4 +0.9 4 D 1.28 -5.9 -8.3 +2.0 +1.1 5 E 1.32 -8.8 -11.9 +2.1 +1.1 6 F* 1.71 -16.7 -21.7 +1.2 -1.7 7 G 1.51 -16.2 -23.5 +1.3 -2.4 8 H 1.46 -12.8 -17.3 +2.2 +0.3 9 I* 1.38 -25.5 -36.9 -10.5 -20.8 10 J 1.30 -20.4 -26.0 +0.8 -2.8 11 K 1.32 -10.2 -16.4 +1.7 +0.1 12 L 1.39 -11.5 -19.1 +1.8 +0.2 13 M 1.36 -18.4 -26.3 +1.9 -0.2 14 N 1.32 -21.0 -27.8 +1.1 -3.7 15 O 1.34 -11.3 -17.2 +1.8 +0.4 16 P 1.36 -10.1 -18.2 +1.0 +0.2 17 Q 1.45 -20.5 -28.4 +0.9 +0.1 At first, there is contemplated from Tables 1 and 2 the influence on voltage ratio (voltage nonlinearity), surge current resistance characteristic and high temperature load life performance by Co 3 O 4 content contained in a lead borosilicate-type glass frit in an electrode material for a zinc oxide varistor. As compared with the lead borosilicate glass of the conventional example containing no Co 3 O 4 (Designation of glass: A in Table 1), the composition systems having Co 3 O 4 content of 0.1% by weight or more are improved in voltage ratio (voltage nonlinearity) but those having Co 3 O 4 content of more than 30.0% by weight or more will deteriorate voltage nonlinearity and surge current resistance characteristic. Accordingly, it is a necessary condition that lead borosilicate glass in an electrode material for zinc oxide varistor is a composition system containing at least 0.1 - 30.0% by weight of Co 3 O 4 . On the other hand, since surge current resistance characteristic and high temperature load life performance are affected by contents of PbO, B 2 O 3 and SiO 2 in addition to Co 3 O 4 content, these compositions are required to be considered. Therefore, influence on surge current resistance characteristic and high temperature load life performance by constitution components of lead borosilicate-type glass contained in an electrode material for a zinc oxide varistor will be considered on the basis of Tables 1 and 2. Glass of a composition system having PbO content less than 40.0% by weight has a higher glass transition point (Tg in Table 1) and too small a fluidity of the glass, which results in a deteriotated solder-wetness of the glass. Contrarily, glass of a composition system having PbO content of more than 80.0% by weight has a lower glass transition point and too high a fluidity of the glass, which results in a lower adhesion strength of electrode 2 onto varistor element 1, this fact leads to a lack of reliability. In a composition system having B 2 O 3 content of less than 5.0% by weight, surge current resistance characteristic becomes inferior. On the other hand, in a composition system having B 2 O 3 content of more than 30.0% by weight, surge current resistance characteristic is also deteriorated. In a composition system having SiO 2 content of less than 5.0% by weight, surge current resistance characteristic is also lowered. In a composition system having SiO 2 content of more than 30.0% by weight, surge current resistance characteristic will also become lowered. From the above results, it is understandable that a composition of glass components of an electrode material for a zinc oxide varistor is optimum in a range of 40.0 - 80.0% by weight of PbO, 5.0 - 30.0% by weight of B 2 O 3 , 5.0 - 30.0% by weight of SiO 2 and 0.1 - 30.0% by weight of Co 3 O 4 . Although lead oxide, boron oxide, silicon oxide and cobalt oxide were used, as material of lead borosilicate-type glass, in the forms of PbO, B 2 O 3 , SiO 2 and Co 3 O 4 , respectively in the present working example, it was confirmed that similar characteristics could also have been obtained by using the other oxide forms. Further, the present working example referred only to the case in which lead borosilicate-type glass content in electrode material for a zinc oxide varistor was 5.0% by weight. However, so far as said content is within 1.0 - 30.0% by weight, no change is seen in the effect of the present invention. Furthermore, the zinc oxide varistor of system consisting of ZnO, Bi 2 O 3 , Co 3 O 4 , MnO 2 , NiO, TiO 2 , Sb 2 O 3 , Cr 2 O 3 and Al 2 O 3 was used as a sintered varistor element 1 for evaluation. However, even when the electrode material for a zinc oxide varistor according to the present invention is applied to a zinc oxide varistor containing Pr 6 O 11, CaO, BaO, MgO, K 2 O, SiO 2 , etc., no change is seen in effect. (Working Example 2) Hereinunder, detailed explanation is made for the second working example of the present invention. At first, the description refers to formulation of glass frit to be incorporated to electrode material for zinc oxide varistor. According to the composition list of the following Table 3, PbO, B 2 O 3 , SiO 2 and MgO weighed each in a given amount were mixed and simultaneously ground in a ball mill, and then fused under a temperature condition of 1000 o C - 1500 o C in a Pt-crucible, which was followed by quenched to be glassified. The thus-obtained glass was roughly crushed and then finely milled in a ball mill to obtain lead borosilicate-type glass frit. Also, glass powder composed of 70.0% by weight of PbO, 15.0% by weight of B 2 O 3 and 15.0% by weight of SiO 2 was prepared by a similar procedure, as a conventional example of lead borosilicate glass. The glass transition point (Tg) of the thus-obtained glass is shown in the following Table 3. Herein, the glass transition point (Tg) was determined using a thermal analysis apparatus. Designation of glass Component ratio (wt.%) Tg ( o C) PbO B 2 O 3 SiO 2 MgO A 70 15 15 0 405 B 69.9 15 15 0.1 405 C 60 15 15 10 420 D 50 15 15 20 410 E 40 15 15 30 420 F 40 10 10 40 410 G 30 34.9 35 0.1 545 H 40 29.9 30 0.1 520 I 89.9 5 5 0.1 315 J 65 0 15 20 390 K 60 5 15 20 395 L 50 30 15 5 470 M 40 40 15 5 490 N 65 15 0 20 410 O 60 15 5 20 415 P 50 15 30 5 490 Q 40 15 40 5 510 Then, the lead borosilicate-type glass frit was weighed by 5.0% by weight, which was followed by milling in the above-mentioned Ag paste (65% by weight of Ag powder was dissolved into 30% by weight of a vehicle, in which ethyl cellulose is dissolved into butyl carbitol) to produce electrode material for a zinc oxide varistor. In order to evaluate the electrode material for a zinc oxide varistor, which was produced as above, a zinc oxide varistor sintered-body (varistor element 1) (a disk-shape of 13 mm in diameter and 1.5 mm in thickness) was provided, said sintered-body consisting of bismuth oxide (Bi 2 O 3 ), cobalt oxide (Co 3 O 4 ), manganese oxide (MnO 2 ), nickel oxide (NiO) and titanium oxide (TiO 2 ) respectively in 0.5 mole%, and antimony oxide (Sb 2 O 3 ) and chromium oxide (Cr 2 O 3 ) respectively in 0.1 mole%, and 0.005 mole% of Al 2 O 3 , the rest being zinc oxide (ZnO). On both surfaces of said sintered-body, an electrode material for zinc oxide varistor was screen-printed to be 10 mm in diameter, and then baked at 800 o C for 10 min. to form electrodes 2 and then lead wires 3 were soldered thereon, and thereafter the outer periphery was molded with insulative resin 4 to obtain a sample. With respect to the thus-obtained samples, voltage ratio (V 1mA /V 10µA ) and limit voltage ratio and surge current resistance characteristic are shown in the following Table 4. Herein, the voltage ratio and limit voltage ratio were obtained through determination using a direct current constant current electric source. Further, the surge current resistance characteristic was obtained by determining a variation ratio of varistor voltage (V 1mA ) occurring when an impact current of 8/20 µS standard waveform and 2500 A crest value applied two times in the same direction. The number of samples was 10 per lot. Sample No. Designation of glass V 1mA /V 10µA Limit voltage ratio V 5A /V 1mA Surge current resistance characteristic ΔV 1mA (%) Direction same as that of current Direction reverse to that of current 1 A 1.83 1.93 -22.3 -28.9 2 B 1.50 1.77 -11.2 -18.3 3 C 1.32 1.66 -9.6 -15.4 4 D 1.24 1.51 -5.3 -7.8 5 E 1.35 1.71 -7.4 -11.7 6 F 1.56 1.85 -16.6 -21.8 7 G 1.51 1.76 -17.8 -24.1 8 H 1.45 1.74 -11.4 -18.4 9 I 1.39 1 88 -26.4 -33.8 10 J 1.31 1.59 -20.7 -25.1 11 K 1.30 1.56 -10.3 -15.8 12 L 1.37 1.66 -11.4 -18.7 13 M 1.39 1.68 -19.6 -26.8 14 N 1.28 1.59 -17.1 -25.8 15 O 1.31 1.58 -11.0 -16.4 16 P 1.38 1.65 -10.8 -17.9 17 Q 1.43 1.66 -21.4 -29.7 At first, there is contemplated from Tables 3 and 4, the influence on voltage ratio (voltage nonlinearity), limit voltage ratio characteristic and surge current resistance characteristic by MgO content contained in a lead borosilicate-type glass frit in an electrode material for a zinc oxide varistor. As compared with the lead borosilicate glass of the conventional example containing no MgO, the composition systems having MgO content of 0.1% by weight or more are improved in voltage ratio (voltage nonlinearity) but those having MgO content of more than 30.0% by weight will deteriorate in limit voltage characteristic and surge current resistance characteristic. Accordingly, it is a necessary condition that a lead borosilicate-type glass in an electrode material for a zinc oxide varistor is a composition system containing at least 0.1 - 30.0% by weight of MgO. On the other hand, since the limit voltage ratio characteristic (V 5A /V 1mA ) and surge current resistance characteristic are affected by contents of PbO, B 2 O 3 and SiO 2 in addition to MgO content, these compositions are required to be considered. Therefore, influence on limit voltage ratio characteristic and surge current resistance characteristic by constitution components of lead borosilicate glass contained in an electrode material for zinc oxide varistor will be considered on the basis of Tables 3 and 4. Glass of a composition system having PbO content of less than 40.0% by weight has a higher glass transition point and too little a fluidity of glass, which result in a lower solder-wetness of glass. Contrarily, glass of a composition system having PbO content of more than 80.0% by weight has a lower glass transition point and too great a fluidity of glass, which results in a lower adhesion strength of an electrode. Therefore, this fact leads to lack of reliability. In a composition system having B 2 O 3 content of less than 5.0% by weight, surge current resistance characteristic becomes inferior. On the other hand, in a composition system having B 2 O 3 content of more than 30.0% by weight, surge current resistance characteristic is also deteriorated. In a composition system having SiO 2 content of less than 5.0% by weight, surge current resistance characteristic is also deteriorated. In a composition system having SiO 2 content of more than 30.0% by weight, surge current resistance characteristic will also become deteriorated. From the above results, it is understandable that composition of glass components of electrode material for zinc oxide varistor is optimum to be in a range of 40.0 - 80.0% by weight of PbO, 5.0 - 30.0% by weight of B 2 O 3 , 5.0 - 30.0% by weight of SiO 2 and 0.1 - 30.0% by weight of MgO. Although lead oxide, boron oxide, silicon oxide and magnesium oxide were used, as materials of lead borosilicate-type glass, in the forms of PbO, B 2 O 3 , SiO 2 and MgO, respectively in the present working example, it was confirmed that the similar characteristics could have also been obtained by using the other oxide forms. Further, the present working example referred only to the case in which the lead borosilicate-type glass content in electrode material for zinc oxide varistor was 5.0% by weight. However, so far as said content is within 1.0 - 30.0% by weight, no change is seen in the effect of the present invention. Furthermore, the zinc oxide varistor of a system consisting of ZnO, Bi 2 O 3 , Co 3 O 4 , MnO 2 , NiO, TiO 2 , Sb 2 O 3 , Cr 2 O 3 and Al 2 O 3 was used as a sintered-body for evaluation. However, even when the electrode material for the zinc oxide varistor according to the present invention is applied to a zinc oxide varistor containing Pr 6 O 11 , CaO, BaO, MgO, K 2 O, SiO 2 , etc., no change is seen in effect. (Working Example 3) Hereinunder, detailed explanation is made for the third working example of the present invention. At first, the description refers to formulation of glass frit to be incorporated to electrode material for zinc oxide varistor. According to the composition list of the following Table 5, PbO, B 2 O 3 , SiO 2 and MnO 2 each weighed in a given amount were mixed and simultaneously ground in a ball mill, and then fused under a temperature condition of 1000 o C - 1500 o C in a Pt-crucible, which was followed by quenching to be glassified. The thus-obtained glass was roughly crushed and then finely milled in a ball mill to obtain lead borosilicate-type glass frit. Also, glass powder composed of 70.0% by weight of PbO, 15.0% by weight of B 2 O 3 and 15.0% by weight of SiO 2 was prepared by a similar procedure, as a conventional example of lead borosilicate glass. The glass transition point (Tg) of the thus-obtained glass is shown in the following Table 5. Herein, the glass transition point (Tg) was determined using a thermal analysis apparatus. Then, the lead borosilicate-type glass powder was weighed in a given amount (5.0% by weight), which was followed by milling in the above-mentioned Ag paste (65% by weight of Ag powder was dissolved into 30% by weight of a vehicle in which ethyl cellulose was dissolved into butyl carbitol) to produce an electrode material for zinc oxide varistor. In order to evaluate the electrode material for zinc oxide varistor, which was produced as above, a zinc oxide varistor sintered-body (varistor element 1) (a disk-shape being 13 mm in diameter and 1.5 mm in thickness) was provided, said sintered-body consisting of bismuth oxide (Bi 2 O 3 ), cobalt oxide (Co 3 O 4 ), manganese oxide (MnO 2 ), nickel oxide (NiO), antimony oxide (Sb 2 O 3 ), and chromium oxide (Cr 2 O 3 ) respectively in 0.5 mole%, and 0.005 mole% of Al 2 O 3 , the rest being zinc oxide (ZnO). On both surfaces of said sintered-body, an electrode material for zinc oxide varistor was applied to be 10 mm in diameter, and then baked at 800°C for 10 min. to form electrodes 2. Then, lead wires 3 were soldered thereon, and thereafter, molded with insulating resin 4 to obtain a sample. With respect to the thus-obtained samples, voltage ratio (V 1mA /V 10µA ), surge current resistance characteristic and high temperature load life performance are shown in the following Table 6. Herein, the above voltage ratio (voltage nonlinearity) was obtained through determination using a direct current constant current electric source. Further, surge current resistance characteristic was obtained by determining a variation ratio of varistor voltage (V 1mA ) occurring when an impact current of 8/20 µS standard waveform and 5000 A crest value was applied two times in the same direction. Further, high temperature load life performance was obtained by determining a variation ratio of varistor voltage (V 1mA ) after 1000 hrs. under the conditions of 125°C of environment temperature and 90% of applied voltage ratio. The number of samples was 10 per lot. Designation of glass Component ratio (wt.%) Tg (°C) PbO B 2 O 3 SiO 2 MnO 2 A 70 15 15 0 405 B 69.9 15 15 0.1 405 C 60 15 15 10 430 D 45 15 15 25 480 E 40 15 15 30 495 F 35 15 15 35 530 G 30 34.9 35 0.1 545 H 40 29.9 30 0.1 520 I 89.9 5 5 0.1 315 J 60 0 15 25 460 K 55 5 15 25 465 L 50 30 15 5 480 M 40 40 15 5 495 N 60 15 0 25 455 O 55 15 5 25 465 P 50 15 30 5 515 Q 40 15 40 5 525 Sample No. Designation of glass V 1mA /V 10µA Surge currently resistance characteristic ΔV 1mA (%) High temperature load life performance ΔV 1mA (%) Direction same as that of current Direction reverse to that of current Direction same as that of current Direction reverse to that of current 1 A 1.33 -18.4 -27.5 -3.9 -8.8 2 B 1.13 -14.5 -25.3 +1.3 -3.1 3 C 1.06 -9.4 -15.5 +1.4 +0.5 4 D 1.09 -4.3 -7.3 +2.0 +1.6 5 E 1.12 -12.3 -15.9 +2.2 +1.8 6 F 1.24 -20.5 -24.7 +1.2 -2.7 7 G 1.10 -22.4 -28.3 +1.1 -2.8 8 H 1.12 -15.9 -26.4 +1.0 +0.3 9 I 1.34 -38.6 -49.7 -5.5 -9.8 10 J 1.25 -20.4 -26.0 -1.8 -3.8 11 K 1.17 -9.2 -16.1 +1.0 +0.2 12 L 1.10 -10.5 -19.2 +1.8 -0.1 13 M 1.13 -22.3 -38.7 +1.7 -1.2 14 N 1.12 -21 .0 -27.9 +1.3 -3.7 15 O 1.13 -10.3 -17.1 +1.5 +0.6 16 P 1.15 -9.8 -18.2 +2.0 +0.7 17 Q 1.16 -22.5 -33.4 +1.9 +0.3 At first, there is contemplated from Tables 5 and 6 the influence on voltage nonlinearity by MnO 2 content contained in a lead borosilicate-type glass in an electrode material for a zinc oxide varistor. The composition systems having MnO 2 content of 0.1% by weight or more are improved in voltage nonlinearity. Those in which MnO 2 content is more than 30.0% by weight take a bad turn in voltage ratio (voltage nonlinearity) as well as surge current resistance characteristic. Accordingly, it is a necessary condition that lead borosilicate-type glass in an electrode material for zinc oxide varistor is a composition system containing at least 0.1 - 30.0% by weight of MnO 2 . On the other hand, since surge current resistance characteristic and high temperature load life performance are affected by contents of PbO, B 2 O 3 and SiO 2 in addition to Co 3 O 4 content, these compositions are required to be considered. Next, influence on surge current resistance characteristic and high temperature load life performance by constituents of lead borosilicate-type glass contained in an electrode material for zinc oxide varistor will be considered referring to Tables 5 and 6. Glass of a composition system having PbO content less than 40.0% by weight has a higher glass transition point Tg and too low a fluidity of glass, which result in a deteriorated solder-wetness of glass. Contrarily, glass of a composition system having PbO content of more than 80.0% by weight has a lower glass transition point and too high a fluidity of glass, which result in a lower adhesion strength of electrode, and therefore, lacks reliability. In a composition system having B 2 O 3 content of less than 5.0% by weight, high temperature load life performance becomes inferior. On the other hand, in a composition system having B 2 O 3 content of more than 30.0% by weight, surge current resistance characteristic is also deteriorated. In a composition system having SiO 2 content of less than 5.0% by weight, surge current resistance characteristic is also deteriorated. In a composition system having SiO 2 content of more than 30.0% by weight, surge current resistance characteristic will also become deteriorated. From the above results, it is understandable that composition of glass components of electrode material for zinc oxide varistor is optimum to be in a range of 40.0 - 80.0% by weight of PbO, 5.0 - 30.0% by weight of B 2 O 3 , 5.0 - 30.0% by weight of SiO 2 and 0.1 - 30.0% by weight of MnO 2 . Although lead oxide boron oxide, silicon oxide and manganese oxide were used, as material of lead borosilicate-type glass, in the forms of PbO, B 2 O 3 , SiO 2 and Co 3 O 4 , respectively in the present working example, it was confirmed that the similar characteristics could have also been obtained by using the other oxide forms. Further, the present working example referred only to the case in which lead borosilicate-type glass content in electrode material for zinc oxide varistor was 5.0% by weight. However, so far as said content is within 1.0 - 30.0% by weight, no change is seen in the effect of the present invention. Furthermore, the zinc oxide varistor of a system consisting of ZnO, Bi 2 O 3 , Co 3 O 4 , MnO 2 , NiO, Sb 2 O 3 , Cr 2 O 3 and Al 2 O 3 was used as a sintered-body (varistor element 1) for evaluation. However, even when the electrode materials for a zinc oxide varistor according to the present invention are applied to a zinc oxide varistor containing Pr 6 O 11 , CaO, BaO, MgO, K 2 O, SiO 2 , etc., no change is seen in effect. (Working Example 4) Hereinunder, detailed explanation is made for the 4th working example of the present invention. At first, the description refers to the formulation of glass frit to be incorporated in the electrode material for zinc oxide varistor. According to the composition list of the following Table 7, PbO, B 2 O 3 , SiO 2 and Sb 2 O 3 weighed each in a given amount were mixed and simultaneously ground in a ball mill, and then fused under a temperature condition of 1000 o C - 1500 o C in a Pt-crucible, which was followed by quenching to be glassified. The thus-obtained glass was roughly crushed and then finely milled in a ball mill to obtain lead borosilicate-type glass frit. Also, glass powder composed of 70.0% by weight of PbO, 15.0% by weight of B 2 O 3 and 15.0% by weight of SiO 2 was prepared in the similar procedure, as a conventional example of lead borosilicate glass. Glass transition point (Tg) the thus-obtained glass was shown in the following Table 7. Herein, glass transition point (Tg) was determined using a thermal analysis apparatus. Then, the lead borosilicate-type glass frit was weighed by 5.0% by weight, which was followed by milling in the above-mentioned Ag paste (65% by weight of Ag powder was dissolved into 30% by weight of a vehicle in which ethyl cellulose is dissolved into butyl carbitol) to produce electrode material for a zinc oxide varistor. In order to evaluate the electrode material for zinc oxide varistor, which was produced as above, a zinc oxide varistor sintered-body (varistor element 1) (a disk-shape being 13 mm in diameter and 1.5 mm in thickness) was provided, said sintered-body consisting of bismuth oxide (Bi 2 O 3 ), cobalt oxide (Co 3 O 4 ), manganese oxide (MnO 2 ), nickel oxide (NiO), antimony oxide (Sb 2 O 3 ) and chromium oxide (Cr 2 O 3 ) respectively in 0.5 mole%, and 0.005 mole% of Al 2 O 3 , the rest being zinc oxide (ZnO). On both surfaces of said sintered-body, an electrode material for zinc oxide varistor was screen-printed to be 10 mm in diameter, and then baked at 800 o C for 10 min. to form electrodes 2. After lead wires 3 were soldered thereon, the outer periphery was molded with insulating resin 4 to obtain a sample. With respect to the thus-obtained samples, voltage ratio (V 1mA /V 10µA ), limit voltage ratio (V 25A /V 1mA ) and surge current resistance characteristics are shown in the following Table 8. The voltage ratio and limit voltage ratio were obtained through determination using a direct current constant current electric source. Further, surge current resistance characteristic was obtained by determining a variation ratio of varistor voltage (V 1mA ) occurring when an impact current of 8/20 µS standard waveform and 5000 A crest value was applied two times in the same direction. The number of samples was 10 per lot. Designation of glass Component ratio (wt.%) Tg ( o C) PbO B 2 O 3 SiO 2 Sb 2 O 3 A 70 15 15 0 405 B 69.9 15 15 0.1 405 C 60 15 15 10 435 D 45 15 15 25 470 E 40 15 15 30 480 F 35 15 15 35 510 G 30 34.9 35 0.1 545 H 40 29.9 30 0.1 520 I 89.9 5 5 0.1 315 J 60 0 15 25 450 K 55 5 15 25 465 L 50 30 15 5 490 M 40 40 15 5 515 N 60 15 0 25 445 O 55 15 5 25 455 P 50 15 30 5 520 Q 40 15 40 5 535 Sample No. Designation of glass V 1mA /V 10µA Limit voltage ratio V 25A /V 1mA Surge current resistance characteristic ΔV 1mA (%) Direction same as that of current Direction reverse to that of current 1 A 1.33 1.57 -18.4 -27.5 2 B 1.16 1.42 -17.5 -25.3 3 C 1.09 1.40 -8.4 -14.9 4 D 1.07 1.35 -6.3 -9.8 5 E 1.13 1.34 -4.6 -7.7 6 F 1.28 1.36 -21.7 -26.4 7 G 1.10 1.53 -22.5 -28.1 8 H 1.12 1.46 -10.4 -25.3 9 I 1.34 1.51 -38.9 -49.5 10 J 1.22 1.55 -20.7 -25.1 11 K 1.15 1.40 -10.3 -16.8 12 L 1.10 1.43 -10.4 -18.7 13 M 1.10 1.50 -22.4 -27.7 14 N 1.08 1.49 -24.1 -27.8 15 O 1.11 1.45 -9.5 -16.1 16 P 1.15 1.43 -9.8 -15.9 17 Q 1.14 1.48 -21.4 -29.7 At first, there is contemplated from Tables 7 and 8 the influence on voltage ratio (voltage nonlinearity), limit voltage ratio characteristic and surge current resistance characteristic by an Sb 2 O 3 content contained in a lead borosilicate-type glass frit in an electrode material for a zinc oxide varistor. As compared with the lead borosilicate glass of the conventional example containing no Sb 2 O 3 , the composition systems having an Sb 2 O 3 content of 0.1% by weight or more are improved in voltage ratio (voltage nonlinearity) but those having an Sb 2 O 3 content of more than 30.0% by weight will deteriorate in surge current resistance characteristic. Accordingly, it is a necessary condition that lead borosilicate-type glass in an electrode material for zinc oxide varistor is a composition system containing at least 0.1 - 30.0% by weight of Sb 2 O 3 . On the other hand, since limit voltage ratio characteristic (V 25A /V 1mA ) and surge current resistance characteristic are affected by contents of PbO, B 2 O 3 and SiO 2 in addition to Sb 2 O 3 content, these compositions are required to be considered. Therefore, influence on limit voltage ratio characteristic and surge current resistance characteristic and high temperature load life performance by constituents of lead borosilicate-type glass contained in an electrode material for zinc oxide varistor will be considered referring to Tables 7 and 8. Glass of a composition system having PbO content less than 40.0% by weight has a higher glass transition point (Tg) and too little a fluidity of glass, which result in a deteriorated solder-wetness of glass. Contrarily, glass of a composition system having a PbO content of more than 80.0% by weight has a lower glass transition point Tg and too high a fluidity of glass, which result in a lower adhesion strength of an electrode. This lacks reliability. In a composition system having a B 2 O 3 content of less than 5.0% by weight, surge current resistance characteristic becomes greatly inferior. On the other hand, in a composition system having a B 2 O 3 content exceeding 30.0% by weight, surge current resistance characteristic is also deteriorated. In a composition system having a SiO 2 content of less than 5.0% by weight, surge current resistance characteristic is also deteriorated. In a composition system having SiO 2 content exceeding 30.0% by weight, surge current resistance characteristic will also become deteriorated. From the above results, it is understandable that composition of glass components of electrode material for zinc oxide varistor is optimum to be in a range of 40.0 - 80.0% by weight of PbO, 5.0 - 30.0% by weight of B 2 O 3 , 5.0 - 30. 0% by weight of SiO 2 and 0.1 - 30.0% by weight of Sb 2 O 3 . Although lead oxide,boron oxide, silicon oxide and antimony oxide were used, as material of lead borosilicate-type glass, in the forms of PbO, B 2 O 3 , SiO 2 and Sb 2 O 3 , respectively in the present working example, it was confirmed that the similar characteristics could have also been obtained by using the other oxide forms. Further, the present working example referred only to the case in which lead borosilicate-type glass content in electrode material for a zinc oxide varistor was 5.0% by weight. However, so far as said content is within 1.0 - 30.0% by weight, no change is seen in the effect of the present invention. Furthermore, a zinc oxide varistor of a system consisting of ZnO, Bi 2 O 3 , Co 3 O 4 , MnO 2 , NiO, Sb 2 O 3 , Cr 2 O 3 and Al 2 O 3 was used as a sintered-body for evaluation. However, even when the electrode material for zinc oxide varistor according to the present invention is applied to a zinc oxide varistor containing Pr 6 O 11 , CaO, BaO, Sb 2 O 3 , K 2 O, SiO 2 , etc., no change is seen in effect. (Working Example 5) Hereinunder, detailed explanation is made for the 5th working example of the present invention. At first, the description refers to the formulation of glass frit to be incorporated to electrode material for a zinc oxide varistor. According to the composition list of the following Table 9, PbO, B 2 O 3 , SiO 2 and Y 2 O 3 each weighed in a given amount were mixed and simultaneously ground in a ball mill, and then fused under a temperature condition of 1000 o C - 1500 o C in a Pt-crucible, which was followed by quenching to be glassified. The thus-obtained glass was roughly crushed and then finely milled in a ball mill to obtain lead borosilicate-type glass frit. Also, glass powder composed of 70.0% by weight of PbO, 15.0% by weight of B 2 O 3 and 15.0% by weight of SiO 2 was prepared by a similar procedure, as a conventional example of lead borosilicate glass. A glass transition point (Tg) of the thus-obtained glass is shown in the following Table 9. Herein, glass transition point (Tg) was determined using a thermal analysis apparatus. Then, 5.0% by weight of the lead borosilicate-type glass frit was weighed, which was followed by milling in the above-mentioned Ag paste (65% by weight of Ag powder was dissolved into 30% by weight of a vehicle in which ethyl cellulose is dissolved into butyl carbitol) to produce electrode material for a zinc oxide varistor. In order to evaluate the electrode material for zinc oxide varistor, which was produced as above, a zinc oxide varistor sintered-body (varistor element 1) (a disk-shape being 13 mm in diameter and 1.5 mm in thickness) was provided, said sintered-body consisting of bismuth oxide (Bi 2 O 3 ), cobalt oxide (Co 3 O 4 ), manganese oxide (MnO 2 ), nickel oxide (NiO), antimony oxide (Sb 2 O 3 ) and chromium oxide (Cr 2 O 3 ) respectively in 0.5 mole%, and 0.005 mole% of Al 2 O 3 , the rest being zinc oxide (ZnO). On both surfaces of said sintered-body, an electrode material for a zinc oxide varistor was screen-printed to be 10 mm in diameter, and then baked at 800 o C for 10 min. to form electrodes 2. After lead wires 3 were soldered thereon, the outer periphery was with insulative resin 4 to obtain a sample. With respect to the thus-obtained samples, voltage ratio (V 1mA /V 10µA ), limit voltage ratio and surge current resistance characteristic are shown in the following Table 10. The voltage ratio and limit voltage ratio were obtained through determination using a direct current constant current electric source. Further, surge current resistance characteristic was obtained by determining a variation ratio of varistor voltage (V 1mA ) occurring when an impact current of 8/20 µS standard waveform and 5000 A crest value was applied two times in the same direction. The number of samples was 10 per lot. Designation of glass Component ratio (wt.%) Tg ( o C) PbO B 2 O 3 SiO 2 Y 2 O 3 A 70 15 15 0 405 B 69.9 15 15 0.1 405 C 60 15 15 10 425 D 45 15 15 25 470 E 40 15 15 30 490 F 35 15 15 35 525 G 30 34.9 35 0.1 545 H 40 29.9 30 0.1 520 I 89.9 5 5 0.1 315 J 60 0 15 25 455 K 55 5 15 25 465 L 50 30 15 5 475 M 40 40 15 5 500 N 60 15 0 25 460 O 55 15 5 25 470 P 50 15 30 5 510 Q 40 15 40 5 530 Sample No. Designation of glass V 1mA /V 10µA Limit voltage ratio V 25A /V 1mA Surge current resistance characteristic ΔV 1mA (%) Direction same as that of current Direction reverse to that of current 1 A 1.33 1.57 -18.4 -27.5 2 B 1.18 1.43 -15.7 -24.4 3 C 1.10 1.41 -7.6 -15.3 4 D 1.08 1.36 -3.1 -6.2 5 E 1.15 1.36 -5.3 -8.8 6 F 1.27 1.39 -15.9 -30.4 7 G 1.15 1.55 -21.3 -31.1 8 H 1.18 1.46 -15.3 -24.9 9 I 1.29 1.52 -37.3 -47.5 10 J 1.27 1.53 -17.1 -26.2 11 K 1.18 1.45 -10.8 -17.4 12 L 1.12 1.42 -10.2 -18.6 13 M 1.11 1.53 -19.7 -28.7 14 N 1.19 1.49 -18.3 -28.2 15 O 1.18 1.43 -12.4 -16.9 16 P 1.16 1.45 -10.9 -18.3 17 Q 1.19 1.47 -22.1 -31.7 At first, there is contemplated from Tables 9 and 10 the influence on voltage ratio (voltage nonlinearity), limit voltage ratio characteristic and surge current resistance characteristic by a Y 2 O 3 content contained in a lead borosilicate-type glass frit in an electrode material for a zinc oxide varistor. As compared with the lead borosilicate glass of the conventional example containing no Y 2 O 3 , the composition systems having a Y 2 O 3 content of 0.1% by weight or more are improved in voltage ratio (voltage nonlinearity) but those having a Y 2 O 3 content in excess of 30.0% by weight will be deteriorated in surge current resistance. Accordingly, it is a necessary condition that lead borosilicate-type glass in an electrode material for zinc oxide varistor is a composition system containing at least 0.1 - 30.0% by weight of Y 2 O 3 . On the other hand, since the limit voltage ratio characteristic (V 25A /V 1mA ) and surge current resistance characteristic are affected by contents of PbO, B 2 O 3 and SiO 2 in addition a Y 2 O 3 content, these compositions are required to be considered. Therefore, influence on the limit voltage ratio and the surge current resistance characteristic by constituents of lead borosilicate-type glass contained in an electrode material for zinc oxide varistor will be considered on the basis of Tables 9 and 10. Glass of a composition system having a PbO content less than 40.0% by weight has a higher glass transition point and too small fluidity of glass, which result in a deterioration of solder-wetness of glass. Contrarily, glass of a composition system having PbO content of more than 80.0% by weight has a lower glass transition point Tg and too great a fluidity of glass, which result in a lower adhesion strength of an electrode. This lacks reliability. In a composition system having a B 2 O 3 content of less than 5.0% by weight, surge current resistance characteristic becomes largely inferior. On the other hand, in a composition system having a B 2 O 3 content of more than 30.0% by weight, surge current resistance characteristic is also deteriorated. In a composition system having a SiO 2 content of less than 5.0% by weight, limit voltage ratio and surge current resistance characteristic are also deteriorated. In a composition system having a SiO 2 content of more than 30.0% by weight, surge current resistance characteristic will also become deteriorated. From the above results, it is understandable that composition of glass components of electrode material for zinc oxide varistor is optimum to be in a range of 40.0 - 80.0% by weight of PbO, 5.0 - 30.0% by weight of B 2 O 3 , 5.0 - 30.0% by weight of SiO 2 and 0.1 - 30.0% by weight of Y 2 O 3 . Although lead oxide, boron oxide, silicon oxide and antimony oxide were used, as material of lead borosilicate-type glass, in the forms of PbO, B 2 O 3 , SiO 2 and Sb 2 O 3 , respectively in the present working example, it was confirmed that similar characteristics could have also been obtained by using the other oxide forms. Further, the present working example refers only to the case in which a lead borosilicate-type glass content in an electrode material for a zinc oxide varistor was 5.0% by weight. However, so far as said content is within 1.0 - 30.0% by weight, no change is seen in the effect of the present invention. Furthermore, a zinc oxide varistor of a system consisting of ZnO, Bi 2 O 3 , Co 3 O 4 , MnO 2 , NiO, Sb 2 O 3 , Cr 2 O 3 and Al 2 O 3 was produced into a sintered-body and then used for evaluation. However, even when the electrode material for a zinc oxide varistor according to the present invention is applied to a zinc oxide varistor containing Pr 6 O 11 , Cao, BaO, Sb 2 O 3 , K 2 O, SiO 2 , etc., no change is seen in effect. (Working Example 6) According to the composition list of the following Table 11, PbO, B 2 O 3 , SiO 2 , Co 2 O 3 and Al 2 O 3 each was weighed in a given amount and then glass was produced by a procedure similar to that of the above Working Example 1, characteristics of the obtained glass are shown in Table 11. Then, this glass was used to produce an electrode material for a zinc oxide varistor as in the above Working Example 1, and further said material was applied to the zinc oxide varistor element 1 used in the above Working Example 1 to obtain electrode 2. With respect to the thus-obtained samples, voltage ratio (V 1mA /V 10µA ), limit voltage ratio (V 50A /V 1mA ) and surge current resistance characteristic are shown in the following Table 12. Herein, the voltage ratio and limit voltage ratio were obtained through determination using a direct current constant current electric source. Further, the surge current resistance characteristic was obtained by determining a variation ratio of varistor voltage (V 1mA ) occurring when an impact current of 8/20 µS standard waveform and 2500 A crest value was applied two times in the same direction. The number of Samples was 10 per lot. Designation of glass Component ratio (wt.%) Tg ( o C) PbO B 2 O 3 SiO 2 Co 3 O 4 Al 2 O 3 A 70 15.0 15.0 0 0 405 B 69.9 15.0 15.0 0.1 0 405 C 69.8999 15.0 15.0 0.1 0.0001 406 D 59.99 15.0 15.0 10.0 0.01 420 E 50.0 15.0 15.0 20.0 0 453 F 49.9 15.0 15.0 20.0 0.1 455 G 49.0 15.0 15.0 20.0 1.0 458 H 48.5 15.0 15.0 20.0 1.5 463 I 40.0 15.0 15.0 30.0 0 475 J 40.0 14.9 15.0 30.0 0.1 476 K 35.0 14.9 15.0 35.0 0.1 488 L 30.0 34.9 35.0 0.1 0 545 M 30.0 34.8 35.0 0.1 0.1 549 N 40.0 29.9 30.0 0.1 0 520 O 40.0 29.8 30.0 0.1 0.1 526 P 84.8 5.0 10.0 0.1 0.1 336 Q 64.9 0 15.0 20.0 0.1 437 R 59.9 5.0 15.0 20.0 0.1 448 S 49.9 30.0 15.0 5.0 0.1 481 T 49.0 30.0 15.0 5.0 1.0 485 U 44.9 35.0 15.0 5.0 0.1 496 V 59.9 15.0 0 25.0 0.1 443 W 54.9 15.0 5.0 25.0 0.1 445 X 49.9 15.0 30.0 5.0 0.1 497 Y 49.0 15.0 30.0 5.0 1.0 506 Z 44.9 15.0 35.0 5.0 0.1 510 Sample No. Designation of glass V 1mA /V 10µA V 50A /V 1mA Surge current resistance characteristic ΔV 1mA (%) Direction same as that of current Direction reverse to that of current 1 A 1.83 2.78 -22.3 -28.9 2 B 1.52 2.56 -10.9 -18.0 3 C 1.53 2.24 -10.8 -18.3 4 D 1.38 1.96 -9.6 -14.4 5 E 1.31 2.48 -4.9 -12.1 6 F 1.33 1.86 -5.0 -8.4 7 G 1.36 1.87 -9.4 -12.3 8 H 1.42 1.88 -12.6 -15.7 9 I 1.32 2.33 -8.8 -11.9 10 J 1.37 2.26 -10.5 -12.5 11 K 1.70 2.24 -20.9 -28.0 12 L 1.51 2.31 -16.2 -23.5 13 M 1.53 2.14 -15.8 -34.6 14 N 1.54 2.12 -12.8 -35.6 15 O 1.52 1.95 -10.3 -13.4 16 P 1.73 2.00 -18.2 -32.3 17 Q 1.41 2.21 -20.3 -26.1 18 R 1.39 2.19 -10.8 -15.4 19 S 1.40 2.31 -9.8 -21.7 20 T 1.47 2.25 -11.6 -20.2 21 U 1.43 2.18 -20.3 -22.6 22 V 1.38 2.24 -26.3 -30.1 23 W 1.42 1.96 -12.1 -16.8 24 X 1.38 2.11 -10.9 -18.0 25 Y 1.46 2.02 -11.8 -20.3 26 Z 1.51 2.38 -21.5 -29.6 At first, there is contemplated from Tables 11 and 12 the influence on voltage ratio (voltage nonlinearity), limit voltage ratio characteristic and surge current resistance characteristic by Co 3 O 4 and Al 2 O 3 contents contained in a lead borosilicate-type glass frit in an electrode material for a zinc oxide varistor. A composition system having a Co 3 O 4 content of 0.1% by weight or more is improved in voltage ratio (voltage nonlinearity) but those having a Co 3 O 4 content of more than 30.0% by weight will be deteriorated both in voltage ratio (voltage nonlinearity) and surge current resistance. Further, in a composition system having an Al 2 O 3 content of 1.0 x 10 -4 % by weight or more, limit voltage ratio characteristic is improved but in a composition system having an Al 2 O 3 content of more than 1.0% by weight, voltage ratio (voltage nonlinearity) and surge current resistance will become deteriorated. Accordingly, it is a necessary condition that lead borosilicate glass in an electrode material for a zinc oxide varistor is a composition system containing 0.1 - 30.0% by weight of Co 3 O 4 and 1.0 x 10 -4 - 1.0% by weight of Al 2 O 3 . On the other hand, surge current resistance characteristic and voltage ratio (voltage nonlinearity) are affected by contents of PbO, B 2 O 3 and SiO 2 in addition to Co 3 O 4 and Al 2 O 3 contents. However, for similar reasons in the above working examples, it is understandable that composition of glass components of electrode material for zinc oxide varistor is optimum in a range of 40.0 - 80.0% by weight of PbO, 5.0 - 30.0% by weight of B 2 O 3 , 5.0 - 30.0% by weight of SiO 2 and 0.1 - 30.0% by weight of Co 3 O 4 , in addition to 1.0 x 10 -4 - 1.0% by weight of Al 2 O 3 . Although aluminium oxide (Al 2 O 3 ) was used in the present working example, it was confirmed that the similar results could have also been obtained by using at least one of indium oxide (In 2 O 3 ), gallium oxide (Ga 2 O 3 ) and germanium oxide (GeO 2 ) in an amount of 1.0 x 10 -4 - 1.0% by weight, in place of aluminium oxide. Also, it was confirmed that when combination of these oxides was used, a similar effect could have been obtained. (Working Example 7) According to the composition list of the following Table 13, PbO, B 2 O 3 , SiO 2 , MgO and Al 2 O 3 were each weighed in a given amount, and then glass was produced by a procedure similar to that of the above working examples. Characteristics of the obtained glass are shown in Table 13. Then, this glass was used to produce an electrode material for a zinc oxide varistor in a similar manner to that of the above working examples, and further, said material was applied to the varistor element 1 used in the above working example, which was followed by estimation by a similar method. The results are shown in Table 14. Designation of glass Component ratio (wt.%) Tg (°C) PbO B 2 O 3 SiO 2 MgO Al 2 O 3 A 70 15.0 15.0 0 0 405 B 69.9 15.0 15.0 0.1 0 405 C 69.8999 15.0 15.0 0.1 0.0001 406 D 59.99 15.0 15.0 10.0 0.01 420 E 50.0 15.0 15.0 20.0 0 410 F 49.9 15.0 15.0 20.0 0.1 416 G 49.0 15.0 15.0 20.0 1.0 422 H 48.5 15.0 15.0 20.0 1.5 430 I 40.0 15.0 15.0 30.0 0 420 J 40.0 14.9 15.0 30.0 0.1 426 K 35.0 14.9 15.0 35.0 0.1 445 L 30.0 34.9 35.0 0.1 0 545 M 30.0 34.8 35.0 0.1 0.1 552 N 40.0 29.9 30.0 0.1 0 520 O 40.0 29.8 30.0 0.1 0.1 526 P 84.8 5.0 10.0 0.1 0.1 336 Q 64.9 0 15.0 20.0 0.1 405 R 59.9 5.0 15.0 20.0 0.1 410 S 49.9 30.0 15.0 5.0 0.1 471 T 49.0 30.0 15.0 5.0 1.0 480 U 44.9 35.0 15.0 5.0 0.1 493 V 59.9 15.0 0 25.0 0.1 420 W 54.9 15.0 5.0 25.0 0.1 435 X 49.9 15.0 30.0 5.0 0.1 496 Y 49.0 15.0 30.0 5.0 1.0 502 Z 44.9 15.0 35.0 5.0 0.1 506 Sample No. Designation of glass V 1mA /V 10µA V 50A /V 1mA Surge current resistance characteristic ΔV 1mA (%) Direction same as that of current Direction reverse to that of current 1 A 1.83 2.78 -22.3 -28.9 2 B 1.50 2.48 -11.2 -18.3 3 C 1.49 2.16 -10.7 -18.8 4 D 1.36 1.93 -5.9 -8.7 5 E 1.24 1.88 -5.3 -7.8 6 F 1.29 1.80 -4.0 -7.2 7 G 1.33 1.86 -8.1 -11.4 8 H 1.41 1.89 -13.2 -16.0 9 I 1.35 2.44 -7.4 -11.7 10 J 1.38 2.19 -9.6 -13.2 11 K 1.69 2.32 -19.1 -30.6 12 L 1.51 2.46 -17.8 -24.1 13 M 1.55 2.08 -15.3 -33.7 14 N 1.45 2.49 -11.4 -28.4 15 O 1.55 1.92 -10.5 -14.2 16 P 1.71 2.02 -18.0 -27.7 17 Q 1.40 2.30 -13.9 -31.4 18 R 1.35 2.13 -11.6 -12.7 19 S 1.37 2.24 -12.1 -13.8 20 T 1.41 2.20 -12.5 -19.1 21 U 1.43 2.08 -19.4 -28.5 22 V 1.41 2.12 -25.5 -30.6 23 W 1.40 1.93 -11.3 -17.3 24 X 1.37 2.09 -9.4 -17.7 25 Y 1.44 1.97 -10.9 -18.9 26 Z 1.53 2.21 -20.6 -30.1 At first, there is contemplated from Tables 13 and 14 the influence on voltage ratio (voltage nonlinearity), limit voltage ratio characteristic and surge current resistance characteristic by MgO and Al 2 O 3 contents contained in a lead borosilicate-type glass frit in an electrode material for a zinc oxide varistor. A composition system having a MgO content of 0.1% by weight or more is improved in voltage ratio (voltage nonlinearity) but that having a MgO content of more than 30.0% by weight will be deteriorated in surge current resistance characteristic. Further, a composition system having an Al 2 O 3 content of 1.0 x 10 -4 % by weight or more is improved in limit voltage ratio characteristic but a composition system having an Al 2 O 3 content in excess of 1.0% by weight will become deteriorated in surge current resistance characteristic. Accordingly, it is a necessary condition that lead borosilicate glass in an electrode material for zinc oxide varistor is a composition system containing 0.1 - 30.0% by weight of MgO and 1.0 x 10 -4 - 1.0% by weight of Al 2 O 3 . On the other hand, surge current resistance characteristic and voltage ratio (voltage nonlinearity) are affected by contents of PbO, B 2 O 3 and SiO 2 in addition to MgO and Al 2 O 3 contents. By similar reasons in the above working examples, it is understandable that composition of glass components of electrode material for a zinc oxide varistor is optimum in a range of 40.0 - 80.0% by weight of PbO, 5.0 - 30.0% by weight of B 2 O 3 , 5.0 - 30.0% by weight of SiO 2 , 0.1 - 30.0% by weight of MgO and 1.0 x 10 -4 - 1.0% by weight of at least one chemical element selected from Al 2 O 3 , In 2 O 3 , Ga 2 O 3 and GeO 2 . Aluminium oxide (Al 2 O 3 ) was used in the present working example, it was confirmed that similar results could have also been obtained even when indium oxide (In 2 O 3 ), gallium oxide (Ga 2 O 3 ) and germanium oxide (GeO 2 ) were used in place of aluminium oxide. Also, it was confirmed that when a combination of these oxides was used, similar results could have been obtained. (Working Example 8) Hereinunder, detailed explanation is made for the 8th working example of the present invention. According to composition list of the following Table 15, PbO, B 2 O 3 , SiO 2 , Y 2 O 3 and Al 2 O 3 were each weighed each in a given amount, and then glass was produced by a procedure similar to that of the above working examples. Characteristics of the obtained glass are shown in Table 15. Then, this glass was used to produce an electrode material for zinc oxide varistor in a similar manner to that of the above working examples, and further, said material was applied to the varistor element 1 used in the above working example to form an electrode, which was followed by evaluation by a similar method. The results are shown in Table 16. Designation of glass Component ratio (wt.%) Tg ( o C) PbO B 2 O 3 SiO 2 Y 2 O 3 Al 2 O 3 A 70 15.0 15.0 0 0 405 B 69.9 15.0 15.0 0.1 0 405 C 69.8999 15.0 15.0 0.1 0.0001 406 D 59.99 15.0 15.0 10.0 0.01 427 E 50.0 15.0 15.0 20.0 0 460 F 49.9 15.0 15.0 20.0 0.1 465 G 49.0 15.0 15.0 20.0 1.0 467 H 48.5 15.0 15.0 20.0 1.5 473 I 40.0 15.0 15.0 30.0 0 490 J 40.0 14.9 15.0 30.0 0.1 496 K 35.0 14.9 15.0 35.0 0.1 526 L 30.0 34.9 35.0 0.1 0 545 M 30.0 34.8 35.0 0.1 0.1 544 N 40.0 29.9 30.0 0.1 0 520 O 40.0 29.8 30.0 0.1 0.1 523 P 84.8 5.0 10.0 0.1 0.1 330 Q 64.9 0 15.0 20.0 0.1 453 R 59.9 5.0 15.0 20.0 0.1 459 S 49.9 30.0 15.0 5.0 0.1 478 T 49.0 30.0 15.0 5.0 1.0 487 U 44.9 35.0 15.0 5.0 0.1 493 V 59.9 15.0 0 25.0 0.1 463 W 54.9 15.0 5.0 25.0 0.1 478 X 49.9 15.0 30.0 5.0 0.1 510 Y 49.0 15.0 30.0 5.0 1.0 517 Z 44.9 15.0 35.0 5.0 0.1 524 Sample No. Designation of glass V 1mA /V 10µA V 50A /V 1mA Surge current resistance characteristic ΔV 1mA (%) Direction same as that of current Direction reverse to that of current 1 A 1.83 2.78 -22.3 -28.9 2 B 1.52 2.57 -10.8 -18.3 3 C 1.49 2.32 -11.4 -18.6 4 D 1.40 2.01 -8.9 -15.4 5 E 1.33 2.51 -3.8 -7.2 6 F 1.36 1.92 -6.7 -7.5 7 G 1.40 1.91 -8.9 -13.6 8 H 1.39 1.94 -11.3 -14.2 9 I 1.40 2.38 -9.2 -12.5 10 J 1.35 2.22 -11.6 -13.3 11 K 1.66 2.19 -10.3 -27.9 12 L 1.52 2.33 -15.6 -28.3 13 M 1.49 2.17 -15.8 -31.5 14 N 1.53 2.09 -18.2 -34.2 15 O 1.48 2.10 -11.3 -12.9 16 P 1.74 2.13 -20.3 -29.8 17 Q 1.43 2.24 -21.1 -26.7 18 R 1.40 2.18 -9.3 -11.5 19 S 1.41 2.29 -7.8 -18.4 20 T 1.46 2.24 -10.3 -19.8 21 U 1.40 2.12 -19.7 -24.3 22 V 1.37 2.30 -25.8 -31.0 23 W 1.46 1.82 -11.8 -17.1 24 X 1.39 2.16 -10.2 -17.3 25 Y 1.45 1.99 -10.9 -19.5 26 Z 1.49 2.33 -20.4 -28.1 At first, there is contemplated from Tables 15 and 16 the influence on voltage ratio (voltage nonlinearity), limit voltage ratio characteristic and surge current resistance characteristic by Y 2 O 3 and Al 2 O 3 contents contained in a lead borosilicate-type glass frit in an electrode material for a zinc oxide varistor. A composition system having a Y 2 O 3 content of 0.1% by weight or more are improved in voltage ratio (voltage nonlinearity) and surge current resistance characteristic but that having a Y 2 O 3 content of more than 30.0% by weight will be deteriorated in both voltage ratio (voltage nonlinearity) as well as surge current resistance characteristic. Further, a composition system having an Al 2 O 3 content of 1.0 x 10 -4 % by weight or more is improved in limit voltage ratio characteristic but a composition system having an Al 2 O 3 content in excess of 1.0% by weight will become deteriorated in surge current resistance characteristic. Accordingly, it is a necessary condition that lead borosilicate glass in an electrode material for zinc oxide varistor is a composition system containing 0.1 - 30.0% by weight of Y 2 O 3 and 1.0 x 10 -4 - 1.0% by weight of Al 2 O 3 . On the other hand, surge current resistance characteristic and voltage ratio (voltage nonlinearity) are affected by contents of PbO, B 2 O 3 and SiO 2 in addition to the Y 2 O 3 and Al 2 O 3 contents. For similar reasons in the above working examples, it is understandable that composition of glass components of electrode material for zinc oxide varistor is optimum to be in a range of 40.0 - 80.0% by weight of PbO, 5.0 - 30.0% by weight of B 2 O 3 , 5.0 - 30.0% by weight of SiO 2 , 0.1 - 30.0% by weight of Y 2 O 3 and 1.0 x 10 -4 - 1.0% by weight of at least one chemical element selected from Al 2 O 3 , In 2 O 3 , Ga 2 O 3 and GeO 2 . Aluminium oxide (Al 2 O 3 ) was used in the present working example, but it was confirmed that the similar results could have also been obtained even when indium oxide (In 2 O 3 ), gallium oxide (Ga 2 O 3 ) and germanium oxide (GeO 2 ) were used in place of aluminium oxide. Also, it was confirmed that when a combination of these oxides was used, similar results could have been obtained. (Working Example 9) Hereinunder, detailed explanation is made for the 9th working example of the present invention. According to the composition list of the following Table 17, PbO, B 2 O 3 , SiO 2 , Sb 2 O 3 and Al 2 O 3 were each weighed in a given amount, and then glass was produced by the procedure similar to that of the above working examples. Characteristics of the obtained glass are shown in Table 17. Then, this glass was used to produce an electrode material for a zinc oxide varistor in a similar manner to that of the above working examples, and further, said material was applied to the varistor element 1 used in the above working examples to form electrodes 2, which was followed by evaluation in a similar method. The results are shown in Table 18. Designation of glass Component ratio (wt.%) Tg ( o C) PbO B 2 O 3 SiO 2 Sb 2 O 3 Al 2 O 3 A 70 15.0 15.0 0 0 405 B 69.9 15.0 15.0 0.1 0 405 C 69.8999 15.0 15.0 0.1 0.0001 407 D 59.99 15.0 15.0 10.0 0.01 438 E 50.0 15.0 15.0 20.0 0 460 F 49.9 15.0 15.0 20.0 0.1 463 G 49.0 15.0 15.0 20.0 1.0 468 H 48.5 15.0 15.0 20.0 1.5 471 I 40.0 15.0 15.0 30.0 0 480 J 40.0 14.9 15.0 30.0 0.1 487 K 35.0 14.9 15.0 35.0 0.1 520 L 30.0 34.9 35.0 0.1 0 545 M 30.0 34.8 35.0 0.1 0.1 550 N 40.0 29.9 30.0 0.1 0 520 O 40.0 29.8 30.0 0.1 0.1 526 P 84.8 5.0 10.0 0.1 0.1 339 Q 64.9 0 15.0 20.0 0.1 452 R 59.9 5.0 15.0 20.0 0.1 457 S 49.9 30.0 15.0 5.0 0.1 498 T 49.0 30.0 15.0 5.0 1.0 522 U 44.9 35.0 15.0 5.0 0.1 535 V 59.9 15.0 0 25.0 0.1 451 W 54.9 15.0 5.0 25.0 0.1 464 X 49.9 15.0 30.0 5.0 0.1 526 Y 49.0 15.0 30.0 5.0 1.0 531 Z 44.9 15.0 35.0 5.0 0.1 540 Sample No. Designation of glass V 1mA /V 10µA V 50A /V 1mA Surge current resistance characteristic ΔV 1mA (%) Direction same as that of current Direction reverse to that of current 1 A 1.83 2.78 -22.3 -28.9 2 B 1.61 2.52 -11.0 -18.3 3 C 1.55 2.36 -10.5 -17.9 4 D 1.38 2.12 -9.3 -14.2 5 E 1.35 2.23 -6.8 -9.2 6 F 1.36 1.92 -7.7 -8.3 7 G 1.39 1.87 -10.9 -12.4 8 H 1.37 1.89 -13.3 -15.2 9 I 1.41 2.34 -9.6 -12.9 10 J 1.35 2.15 -10.8 -13.4 11 K 1.45 2.29 -14.3 -29.9 12 L 1.54 2.31 -15.8 -28.5 13 M 1.48 2.18 -16.1 -32.0 14 N 1.53 2.16 -17.2 -34.7 15 O 1.45 2.13 -12.3 -13.6 16 P 1.69 2.10 -20.7 -30.4 17 Q 1.41 2.41 -21.5 -27.1 18 R 1.43 2.28 -9.7 -12.0 19 S 1.43 2.39 -10.9 -17.4 20 T 1.45 2.24 -11.3 -18.7 21 U 1.46 2.31 -20.3 -25.9 22 V 1.40 2.29 -26.7 -32.8 23 W 1.45 2.02 -12.8 -16.8 24 X 1.42 2.21 -12.1 -17.2 25 Y 1.46 1.96 -11.2 -18.3 26 Z 1.47 2.27 -21.4 -27.5 At first, there is contemplated from Tables 17 and 18 the influence on voltage ratio (voltage nonlinearity), limit voltage ratio characteristic and surge current resistance characteristic by Sb 2 O 3 and Al 2 O 3 contents contained in a lead borosilicate-type glass frit in an electrode material for a zinc oxide varistor. A composition system having an Sb 2 O 3 content of 0.1% by weight or more is improved in voltage ratio (voltage nonlinearity) and surge current resistance characteristic but that having a Sb 2 O 3 content of more than 30.0% by weight will be deteriorated in surge current resistance characteristic. Further, a composition system having an Al 2 O 3 content of 1.0 x 10 -4 % by weight or more is improved in limit voltage ratio characteristic but a composition system having an Al 2 O 3 content in excess of 1.0% by weight will become deteriorated in surge current resistance characteristic. Accordingly, it is a necessary condition that lead borosilicate glass in an electrode material for a zinc oxide varistor is a composition system containing 0.1 - 30.0% by weight of Sb 2 O 3 and 1.0 x 10 -4 - 1.0% by weight of Al 2 O 3 . On the other hand, surge current resistance characteristic and voltage ratio (voltage nonlinearity) are affected by contents of PbO, B 2 O 3 and SiO 2 in addition to Sb 2 O 3 and Al 2 O 3 contents. For similar reasons as in the above working examples, it is understandable that composition of glass components of electrode material for a zinc oxide varistor is optimum in a range of 40.0 - 80.0% by weight of PbO, 5.0 - 30.0% by weight of B 2 O 3 , 5.0 - 30.0% by weight of SiO 2 , 0.1 - 30.0% by weight of Sb 2 O 3 and 1.0 x 10 -4 - 1.0% by weight of at least one chemical element selected from Al 2 O 3 , In 2 O 3 , Ga 2 O 3 and GeO 2 . Aluminium oxide (Al 2 O 3 ) was used in the present working example, it was confirmed that similar results could also have been obtained even when indium oxide (In 2 O 3 ), gallium oxide (Ga 2 O 3 ) and germanium oxide (GeO 2 ) were used in place of aluminium oxide. Also, it was confirmed that when a combination of these oxides was used, the similar results could have been obtained. (Working Example 10) Hereinunder, detailed explanation is made for the 10th working example of the present invention. According to the composition list of the following Table 19, PbO, B 2 O 3 , SiO 2 , MnO 2 and Al 2 O 3 were each weighed in a given amount, and then glass was produced by a procedure similar to that of the above working examples. Characteristics of the obtained glass are shown in Table 19. Then, this glass was used to produce an electrode material for zinc oxide varistor in a similar manner to that of the above working examples, and further, said material was applied to the varistor element 1 used in the above working examples to form electrodes 2, which was followed by evaluation by a similar method. The results are shown in Table 20. Designation of glass Component ratio (wt.%) Tg ( o C) PbO B 2 O 3 SiO 2 MnO 2 Al 2 O 3 A 70 15.0 15 0 0 0 405 B 69.9 15.0 15.0 0.1 0 405 C 69.8999 15.0 15.0 0.1 0.0001 405 D 59.99 15.0 15.0 10.0 0.01 431 E 50.0 15.0 15.0 20.0 0 470 F 49.9 15.0 15.0 20.0 0.1 473 G 49.0 15.0 15.0 20.0 1.0 480 H 48.5 15.0 15.0 20.0 1.5 485 I 40.0 15.0 15.0 30.0 0 495 J 40.0 14.9 15.0 30.0 0.1 502 K 35.0 14.9 15.0 35.0 0.1 533 L 30.0 34.9 35.0 0.1 0 545 M 30.0 34.8 35.0 0.1 0.1 551 N 40.0 29.9 30.0 0.1 0 520 O 40.0 29.8 30.0 0.1 0.1 525 P 84.8 5.0 10.0 0.1 0.1 327 Q 64.9 0 15.0 20.0 0.1 458 R 59.9 5.0 15.0 20.0 0.1 466 S 49.9 30.0 15.0 5.0 0.1 490 T 49.0 30.0 15.0 5.0 1.0 500 U 44.9 35.0 15.0 5.0 0.1 515 V 59.9 15.0 0 25.0 0.1 457 W 54.9 15.0 5.0 25.0 0.1 460 X 49.9 15.0 30.0 5.0 0.1 519 Y 49.0 15.0 30.0 5.0 1.0 528 Z 44.9 15.0 35.0 5.0 0.1 536 Sample No. Designation of glass V 1mA /V 10µA V 50A /V 1mA Surge current resistance characteristic ΔV 1mA (%) Direction same as that of current Direction reverse to that of current 1 A 1.83 2.78 -22.3 -28.9 2 B 1.53 2.56 -11.1 -17.8 3 C 1.49 2.36 -9.9 -12.4 4 D 1.38 1.89 -5.1 -8.7 5 E 1.32 2.39 -7.8 -13.6 6 F 1.37 1.92 -12.7 -14.9 7 G 1.41 1.89 -9.5 -13.0 8 H 1.45 1.91 -12.3 -16.3 9 I 1.39 2.20 -9.7 -12.6 10 J 1.44 2.18 -11.6 -13.4 11 K 1.58 2.07 -18.9 -29.2 12 L 1.52 2.29 -16.3 -24.1 13 M 1.49 2.21 -14.9 -35.5 14 N 1.50 2.20 -12.6 -33.1 15 O 1.48 1.88 -11.6 -14.2 16 P 1.69 1.93 -16.9 -30.3 17 Q 1.43 2.23 -19.7 -28.9 18 R 1.38 2.12 -11.4 -14.7 19 S 1.42 2.29 -10.2 -23.1 20 T 1.48 2.24 -10.9 -20.5 21 U 1.45 2.33 -21.5 -23.3 22 V 1.39 2.27 -25.8 -31.4 23 W 1.40 1.95 -12.3 -15.9 24 X 1.39 2.16 -11.7 -17.4 25 Y 1.45 1.98 -10.9 -19.1 26 Z 1.50 2.30 -20.8 -30.2 At first, there is contemplated from Tables 19 and 20 the influence on voltage ratio (voltage nonlinearity), limit voltage ratio characteristic and surge current resistance characteristic by MnO 2 and Al 2 O 3 contents contained in a lead borosilicate-type glass frit in an electrode material for zinc oxide varistor. A composition system having a MnO 2 content of 0.1% by weight or more is improved in voltage ratio (voltage nonlinearity) and surge current resistance characteristic but that having a MnO 2 content of more than 30.0% by weight will be deteriorated in both voltage ratio (voltage nonlinearity) and surge current resistance characteristic. Further, a composition system having an Al 2 O 3 content of 1.0 x 10 -4 % by weight or more is improved in limit voltage ratio characteristic but a composition system having an Al 2 O 3 content in excess of 1.0% by weight will become deteriorated in surge current resistance characteristic. Accordingly, it is a necessary condition that lead borosilicate glass in an electrode material for a zinc oxide varistor is a composition system containing 0.1 - 30.0% by weight of MnO 2 and 1.0 x 10 -4 - 1.0% by weight of Al 2 O 3 . On the other hand, surge current resistance characteristic and voltage ratio (voltage nonlinearity) are affected by contents of PbO, B 2 O 3 and SiO 2 in addition to MnO 2 and Al 2 O 3 contents. For similar reasons in the above working examples, it is understandable that composition of glass components of electrode material for a zinc oxide varistor is optimum to be in a range of 40.0 - 80.0% by weight of PbO, 5.0 - 30.0% by weight of B 2 O 3 , 5.0 - 30.0% by weight of SiO 2 , 0.1 - 30.0% by weight of MnO 2 and 1.0 x 10 -4 - 1.0% by weight of at least one chemical element selected from Al 2 O 3 , In 2 O 3 , Ga 2 O 3 and GeO 2 . Aluminium oxide (Al 2 O 3 ) was used in the present working example, it was confirmed that the similar results could have also been obtained even when indium oxide (In 2 O 3 ), gallium oxide (Ga 2 O 3 ) and germanium oxide (GeO 2 ) were used in place of aluminium oxide. Also, it was confirmed that when a combination of these oxides was used, similar results could have been obtained. Further, lead oxide, boron oxide, silicon oxide, manganese oxide, aluminium oxide and indium oxide were used, as material of lead borosilicate-type glass, in the forms of PbO, B 2 O 3 , SiO 2 , MnO 2 , Al 2 O 3 and In 2 O 3 , respectively in the present working examples 6 - 10. However, it was confirmed that the similar physical properties could have also been obtained by using the other oxide forms. Further, the present working examples 6 - 10 referred only to the case in which lead borosilicate-type glass content in electrode material for a zinc oxide varistor was 5.0% by weight, but so far as said content is within 1.0 - 30.0% by weight, no change is seen in the effect of the present invention. Furthermore, zinc oxide varistors of systems consisting of ZnO, Bi 2 O 3 , Co 2 O 3 , MnO 2 , NiO, TiO 2 , Sb 2 O 3 , Cr 2 O 3 and Al 2 O 3 were used as a sintered-body (varistor element 1) for evaluation. However, even when the electrode material for zinc oxide varistor according to the present invention is applied to a zinc oxide varistor containing Pr 6 O 11 , CaO, BaO, MgO, K 2 O, SiO 2 , etc., no change is seen in effect. (Working Example 11) Hereinunder, detailed explanation is made for the 11th working example of the present invention. At first, the description refers to formulation of glass frit to be incorporated to electrode material for a zinc oxide varistor. According to the composition list of the following Table 21, PbO, B 2 O 3 , SiO 2 and TeO 2 each weighed in a given amount were mixed and simultaneously ground in a ball mill, and then fused under a temperature condition of 1000 o C - 1500 o C in a Pt-crucible, which was followed by quenched to be glassified. The thus-obtained glass was roughly crushed and then finely milled in a ball mill to obtain lead borosilicate-type glass frit. Also, glass powder composed of 70.0% by weight of PbO, 15.0% by weight of B 2 O 3 and 15.0% by weight of SiO 2 was prepared in a similar procedure, as a conventional example of lead borosilicate glass. The glass transition point (Tg) of the thus-obtained glass is shown in the following Table 21. Herein, the glass transition point (Tg) was determined using a thermal analysis apparatus. Then, the lead borosilicate-type glass frit was weighed in a given amount (5.0% by weight), which was followed by milling in the above-mentioned Ag paste (65% by weight of Ag powder was dissolved into 30% by weight of a vehicle, in which ethyl cellulose is dissolved into butyl carbitol) to produce an electrode material for a zinc oxide varistor. In order to evaluate the electrode material for a zinc oxide varistor, which was produced as above, a zinc oxide varistor sintered-body (varistor element 1) (a disk-shape being 13 mm in diameter and 1.5 mm in thickness) was provided, said sintered-body consisting of bismuth oxide (Bi 2 O 3 ), cobalt oxide (Co 3 O 4 ), manganese oxide (MnO 2 ), nickel oxide (NiO), antimony oxide (Sb 2 O 3 ) and chromium oxide (Cr 2 O 3 ) respectively in 0.5 mole%, and 0.005 mole% of Al 2 O 3 , the rest being zinc oxide (ZnO). On both surfaces of said sintered-body, an electrode material for zinc oxide varistor was screen-printed to be 10 mm in diameter, and then baked at 750 o C for 10 min. to form electrodes 2, which was followed by soldering lead wires 3 thereon and subsequently molding with insulative resin 4 to obtain a sample. With respect to the thus-obtained samples, voltage ratio (voltage nonlinearity) (V 1mA /V 10µA ), limit voltage ratio characteristic (V 50A /V 1mA ) and, surge current resistance characteristic are shown in the following Table 22. Herein, the voltage ratio (V 1mA /V 10µA ) and limit voltage ratio (V 50A /V 1mA ) was obtained through determination using a direct current constant current electric source. Further, the surge current resistance characteristic was obtained by determining a variation ratio of varistor voltage (V 1mA ) occurring when an impact current of 8/20 µS standard waveform and 5000 A crest value was applied two times in the same direction. The number of samples was 10 per lot. Designation of glass Component ratio (wt.%) Tg ( o C) PbO B 2 O 3 SiO 2 TeO 2 A 70.0 15.0 15.0 0 405 B 69.9 15.0 15.0 0.1 405 C 60.0 15.0 15.0 10.0 400 D 50.0 15.0 15.0 20.0 405 E 40.0 15.0 15.0 30.0 420 F 40.0 10.0 15.0 35.0 425 G 30.0 30.0 30.0 10.0 580 H 79.9 10.0 10.0 0.1 360 I 84.9 10.0 5.0 0.1 345 J 70.0 0 20.0 10.0 470 K 65.0 5.0 20.0 10.0 485 L 50.0 5.0 35.0 10.0 560 M 70.0 20.0 0 10.0 460 N 50.0 35.0 5.0 10.0 545 Sample No. Designation of glass V 1mA /V 10µA V 50A /V 1mA Surge current resistance characteristic ΔV 1mA (%) Direction same as that of current Direction reverse to that of current 1 A 1.42 1.67 -18.4 -27.5 2 B 1.25 1.53 -16.4 -24.8 3 C 1.06 1.48 -4.2 -7.3 4 D 1.20 1.47 -5.1 -8.9 5 E 1.23 1.47 -7.5 -11.6 6 F 1.35 1.68 -19.3 -26.9 7 G 1.37 1.57 -18.4 -27.1 8 H 1.26 1.48 -8.9 -10.2 9 I 1.29 1.51 -12.8 -21.7 10 J 1.36 1.49 -10.3 -18.5 11 K 1.22 1.45 -9.7 -18.0 12 L 1.33 1.46 -22.2 -34.5 13 M 1.25 1.47 -17.0 -23.8 14 N 1.22 1.50 -19.6 -41.3 At first, there is contemplated from Tables 21 and 22 the influence on voltage ratio (voltage nonlinearity), limit voltage ratio characteristic and surge current resistance characteristic by a TeO 2 content contained in a lead borosilicate-type glass in an electrode material for a zinc oxide varistor. As shown in Sample No. 6 in Table 22, a composition system having a TeO 2 content of 0.1% by weight or more are improved in voltage ratio (voltage nonlinearity) but that having a TeO 2 content of more than 30.0% by weight will be deteriorated in limit voltage ratio characteristic and surge current resistance characteristic. Accordingly, it is a necessary condition that lead borosilicate-type glass in an electrode material for zinc oxide varistor is a composition system containing at least 0.1 - 30.0% by weight of TeO 2 . On the other hand, since surge current resistance characteristic is affected by contents of PbO, B 2 O 3 and SiO 2 in addition to the TeO 2 content, these compositions are required to be considered. Therefore, influence on limit voltage ratio characteristic and surge current resistance characteristic by constituents of a lead borosilicate type glass contained in an electrode material will be considered on the basis of Tables 21 and 22. Glass of a composition system having PbO content less than 40.0% by weight such as Glass G in Table 21 has a higher glass transition point Tg and too low a fluidity of glass, which result in a deteriorated solder-wetness of the glass. Contrarily, glass of a composition system having a PbO content in excess of 80.0% by weight, such as Glass I in Table 21 has a lower glass transition point Tg and too great a fluidity of the glass, which result in a lower adhesion strength of electrode. Therefore, this lacks reliability. In a composition system having a B 2 O 3 content of less than 5.0% by weight, as shown in Sample No. 10 in Table 22, voltage ratio (voltage nonlinearity) is deteriorated. On the other hand, in a composition system having a B 2 O 3 content in excess of 30.0% by weight, as shown in Sample No. 14 in Table 22, surge current resistance characteristic is also deteriorated. In a composition system having SiO 2 content of less than 5.0% by weight, as shown in Sample No. 13 in Table 22, surge current resistance characteristic is also deteriorated. In a composition system having a SiO 2 content in excess of 30.0% by weight, as shown in Sample No. 12 in Table 22, surge current resistance characteristic will also become inferior. From the above results, it is understandable that composition of glass components of an electrode material for a zinc oxide varistor is optimum to be in a range of 40.0 - 80.0% by weight of PbO, 5.0 - 30.0% by weight of B 2 O 3 , 5.0 - 30.0% by weight of SiO 2 and 0.1 - 30.0% by weight of TeO 2 . (Working Example 12) Hereinunder, detailed explanation is made for the 12th working example of the present invention. According to the composition list of the following Table 23, PbO, B 2 O 3 , SiO 2 , TeO 2 , Al 2 O 3 , In 2 O 3 , Ga 2 O 3 and GeO 2 were each weighed in a given amount, and then glass was produced in the similar procedure as in the above working examples. The characteristics of said glass are shown in Table 23. Then, this glass was used to produce an electrode material for a zinc oxide varistor in a similar manner to those of the above working examples. Said material was applied onto the varistor element 1 used in the above working examples to form electrodes 2. Evaluation was made in a similar manner. The results are shown in Table 24. Designation of glass Component ratio (wt.%) Tg ( o C) PbO B 2 O 3 SiO 2 TeO 2 Al 2 O 3 In 2 O 3 Ga 2 O 3 GeO 2 C 60.0 15.0 15.0 10.0 0 0 0 0 400 O 59.9999 15.0 15.0 10.0 0.0001 0 0 0 400 P 59.9 15.0 15.0 10.0 0.1 0 0 0 395 Q 59.9 15.0 15.0 10.0 0.05 0.05 0 0 395 R 59.9 15.0 15.0 10.0 0 0.1 0 0 390 S 59.9 15.0 15.0 10.0 0 0 0.1 0 400 T 59.9 15.0 15.0 10.0 0 0 0 0.1 395 U 58.5 15.0 15.0 10.0 1.5 0 0 0 400 V 58.5 15.0 15.0 10.0 0.05 0.05 0.05 0 395 Sample No. Designation of glass V 1mA /V 10µA V 50A /V 1mA Surge current resistance characteristic ΔV 1mA (%) Direction same as that of current Direction reverse to that of current 3 C 1.06 1.48 -4.2 -7.3 15 O 1.06 1.40 -4.0 -7.5 16 P 1.07 1.34 -4.5 -8.2 17 Q 1.07 1.35 -5.3 -8.7 18 R 1.10 1.33 -6.8 -10.0 19 S 1.08 1.36 -5.9 -11.8 20 T 1.09 1.35 -3.7 -7.1 21 U 1.37 1.38 -16.3 -24.9 22 V 1.41 1.37 -17.2 -30.3 At first, there is contemplated from Tables 23 and 24 the influence on voltage ratio (voltage nonlinearity), limit voltage ratio characteristic and surge current resistance characteristic by Al 2 O 3 , In 2 O 3 , Ga 2 O 3 and GeO 2 contents contained in a lead borosilicate-type glass frit in an electrode material for zinc oxide varistor. As shown in Sample Nos. 15 - 20 in Table 24, a composition system containing 1.0 x 10 -4 % by weight of at least one chemical element selected out of Al 2 O 3 , In 2 O 3 , Ga 2 O 3 and GeO 2 is improved in limit voltage ratio characteristic. However, as in Sample Nos. 21 and 22 in Table 24, a composition system in which amounts to be added of the above chemical elements exceed 1.0% by weight in the total becomes deteriorated in voltage ratio (voltage nonlinearity) and surge current resistance characteristic. Accordingly, it is a necessary condition that lead borosilicate glass in an electrode material for zinc oxide varistor is a composition system containing 1.0 x 10 -4 - 1.0% by weight of at least one chemical element selected out of Al 2 O 3 , In 2 O 3 , Ga 2 O 3 and GeO 2 . On the other hand, surge current resistance characteristic is affected by contents of PbO, B 2 O 3 , SiO 2 and TeO 2 in addition to contents of Al 2 O 3 , In 2 O 3 , Ga 2 O 3 and GeO 2 . For similar reasons in the above working examples, it is understandable that composition of glass components of electrode material for zinc oxide varistor is optimum in a range of 40.0 - 80.0% by weight of PbO, 5.0 - 30.0% by weight of B 2 O 3 , 5.0 - 30.0% by weight of SiO 2 , 0.1 - 30.0% by weight of TeO 2 and 1.0 x 10 -4 - 1.0% by weight of at least one chemical element selected from Al 2 O 3 , In 2 O 3 , Ga 2 O 3 and GeO 2 . Further, as shown in Sample No. 17 in Table 17, it was confirmed that even when a combination of the oxides such as Al 2 O 3 , In 2 O 3 , Ga 2 O 3 , GeO 2 and the like, such results as above could have been obtained. Although lead oxide, boron oxide, silicon oxide tellurium oxide, aluminium oxide and indium oxide were used, as material of lead borosilicate-type glass, in the forms of PbO, B 2 O 3 , SiO 2 , TeO 2 , Al 2 O 3 and In 2 O 3 , respectively in the present working example, it was confirmed that the use of other oxide forms could have also acquired equal physical properties. Further, the present working example referred only to the case in which lead borosilicate-type glass content in electrode material for zinc oxide varistor was 5.0% by weight. However, so far as said content is within 1.0 - 30.0% by weight, no change is seen in the effect of the present invention. Furthermore, a zinc oxide varistor of a system consisting of ZnO, Bi 2 O 3 , Co 3 O 4 , MnO 2 , NiO, Sb 2 O 3 , Cr 2 O 3 and Al 2 O 3 was used as a sintered-body (varistor element 1) for evaluation. However, even when the electrode material for zinc oxide varistor according to the present invention is applied to a zinc oxide varistor containing Pr 6 O 11 , CaO, BaO, MgO, K 2 O, SiO 2 , etc., no change is seen in effect. Next, a lead borosilicate-type glass containing lanthanoid-series oxides was fritted in the same manner as in the above working examples. This glass frit was milled into the Ag paste same as in the above working examples, which was followed by applying onto a fired varistor element 1 to form electrodes 2. Hereinunder explanation is given thereon. The lead borosilicate-type glass in this case contains lanthanoid-series oxide (0.1 - 30.0% by weight), boron oxide (5.0 - 30.0% by weight), silicon oxide (5.0 - 30.0% by weight) and lead oxide (40.0 - 80.0% by weight). The following Tables 25 and 26 concern those having used lanthanum oxide (LaO 3 ), in which its content of 0.1% by weight or more will become better in voltage ratio (voltage nonlinearity). Further, when such a content is more than 30% by weight, glass transition point Tg becomes higher and the diffusion into varistor element 1 becomes difficult, thereby rendering surge current resistance characteristic to be deteriorated. Further, when an amount of boron oxide is less than 5.0% by weight, voltage ratio (voltage nonlinearity) will become inferior, and when it is more than 30%, surge current resistance characteristic will become deteriorated. Furthermore, when silicon oxide content is less than 5.0% by weight, surge current resistance characteristic will become inferior, and when it is more than 30.0% by weight, voltage ratio (voltage nonlinearity) and surge current resistance characteristic will become deteriorated. Designation of glass Component ratio (wt.%) Tg ( o C) PbO B 2 O 3 SiO 2 La 2 O 3 A 70.0 15.0 15.0 0 405 B 69.9 15.0 15.0 0.1 405 C 67.5 15.0 15.0 2.5 415 D 65.0 15.0 15.0 5.0 420 E 55.0 15.0 20.0 10.0 460 F 40.0 10.0 20.0 30.0 518 G 32.5 15.0 20.0 32.5 545 H 72.0 3.0 20.0 5.0 415 I 70.0 5.0 20.0 5.0 420 J 57.5 30.0 10.0 2.5 440 K 52.5 35.0 10.0 2.5 453 L 69.5 25.0 3.0 2.5 420 M 72.5 20.0 5.0 2.5 422 N 52.5 15.0 30.0 2.5 460 O 50.0 15.0 32.5 2.5 465 Sample No. Designation of glass V 1mA /V 10µA Limit voltage ratio V 50A /V 1mA Surge current resistance characteristic ΔV 1mA (%) Direction same as that of current Direction reverse to that of current 1 A 1.33 1.57 -18.4 -27.5 2 B 1.20 1.57 -18.0 -25.1 3 C 1.08 1.47 -5.1 -10.6 4 D 1.06 1.47 -7.3 -12.4 5 E 1.07 1.46 -8.9 -17.9 6 F 1.10 1.50 -10.4 -22.5 7 G 1.27 1.55 -18.9 -36.2 8 H 1.33 1.50 -15.5 -18.6 9 I 1.15 1.52 -11.2 -19.7 10 J 1.10 1.50 -10.9 -23.6 11 K 1.11 1.53 -21.4 -32.8 12 L 1.15 1.50 -19.8 -38.3 13 M 1.17 1.51 -10.7 -23.7 14 N 1.22 1.50 -16.6 -24.0 15 O 1.25 1.50 -24.8 -41.6 Next, characteristics are shown with respect to the cases having used therein the other oxides, in place of lanthanum oxide: cerium oxide in Tables 27 and 28, praseodymium oxide also in Tables 29 and 30, neodymium oxide further in Tables 31 and 32, sammarium oxide in Tables 33 and 34, europium oxide in tables 35 and 36, gadolinium oxide in Tables 37 and 38, terbium oxide in Tables 39 and 40, dysprosium oxide in Tables 41 and 42, holmium oxide in Tables 43 and 44, erbium oxide in Tables 45 and 46, thulium oxide in Tables 47 and 48, yitterbium oxide in Tables 49 and 50, and lutetium oxide in Tables 51 and 52. In all the above cases, voltage ratio (voltage nonlinearity) becomes better, if each lanthanoid-series oxide is contained in an amount of 0.1% by weight or more. Further, if it is more than 30% by weight, surge current resistance characteristic will be deteriorated. Designation of glass Component ratio (wt.%) Tg ( o C) PbO B 2 O 3 SiO 2 CeO 2 A 70.0 15.0 15.0 0 405 B 69.9 15.0 15.0 0.1 405 C 67.5 15.0 15.0 2.5 415 D 65.0 15.0 15.0 5.0 420 E 55.0 15.0 20.0 10.0 465 F 40.0 10.0 20.0 30.0 515 G 32.5 15.0 20.0 32.5 540 H 72.0 3.0 20.0 5.0 412 I 70.0 5.0 20.0 5.0 417 J 57.5 30.0 10.0 2.5 435 K 52.5 35.0 10.0 2.5 455 L 69.5 25.0 3.0 2.5 420 M 72.5 20.0 5.0 2.5 425 N 52.5 15.0 30.0 2.5 460 O 50.0 15.0 32.5 2.5 467 Sample No. Designation of glass V 1mA /V 10µA Limit voltage ratio V 50A /V 1mA Surge current resistance characteristic ΔV 1mA (%) Direction same as that of current Direction reverse to that of current 1 A 1.33 1.57 -18.4 -27.5 2 B 1.21 1.56 -17.9 -24.8 3 C 1.08 1.46 -4.8 -9.2 4 D 1.05 1.47 -6.9 -11.0 5 E 1.08 1.47 -8.8 -17.4 6 F 1.11 1.49 -9.7 -21.7 7 G 1.27 1.53 -20.3 -36.0 8 H 1.32 1.50 -14.8 -20.7 9 I 1.14 1.52 -11.3 -18.5 10 J 1.11 1.50 -10.4 -21.1 11 K 1.10 1.51 -19.7 -32.6 12 L 1.16 1.50 -19.3 -36.3 13 M 1.17 1.50 -10.9 -20.8 14 N 1.23 1.51 -15.1 -21.3 15 O 1.25 1.49 -25.1 -42.1 Designation of glass Component ratio (wt.%) Tg ( o C) PbO B 2 O 3 SiO 2 Pr 6 O 11 A 70.0 15.0 15.0 0 405 B 69.9 15.0 15.0 0.1 405 C 67.5 15.0 15.0 2.5 417 D 65.0 15.0 15.0 5.0 422 E 55.0 15.0 20.0 10.0 460 F 40.0 10.0 20.0 30.0 515 G 32.5 15.0 20.0 32.5 547 H 72.0 3.0 20.0 5.0 420 I 70.0 5.0 20.0 5.0 418 J 57.5 30.0 10.0 2.5 440 K 52.5 35.0 10.0 2.5 445 L 69.5 25.0 3.0 2.5 425 M 72.5 20.0 5.0 2.5 427 N 52.5 15.0 30.0 2.5 460 O 50.0 15.0 32.5 2.5 465 Sample No. Designation of glass V 1mA /V 10µA Limit voltage ratio V 50A /V 1mA Surge current resistance characteristic ΔV 1mA (%) Direction same as that of current Direction reverse to that of current 1 A 1.33 1.57 -18.4 -27.5 2 B 1.22 1.59 -18.0 -26.2 3 C 1.09 1.47 -5.6 -10.8 4 D 1.07 1.46 -7.8 -12.7 5 E 1.10 1.46 -9.5 -18.5 6 F 1.12 1.48 -11.2 -21.9 7 G 1.26 1.51 -20.4 -37.0 8 H 1.35 1.49 -16.8 -19.2 9 I 1.16 1.50 -11.3 -20.2 10 J 1.12 1.50 -11.0 -24.8 11 K 1.11 1.52 -21.1 -33.1 12 L 1.15 1.51 -19.6 -40.3 13 M 1.16 1.50 -11.0 -24.9 14 N 1.23 1.50 -16.2 -22.6 15 O 1.28 1.51 -25.3 -42.8 Designation of glass Component ratio (wt.%) Tg ( o C) PbO B 2 O 3 SiO 2 Nd 2 O 3 A 70.0 15.0 15.0 0 405 B 69.9 15.0 15.0 0.1 406 C 67.5 15.0 15.0 2.5 417 D 65.0 15.0 15.0 5.0 420 E 55.0 15.0 20.0 10.0 470 F 40.0 10.0 20.0 30.0 520 G 32.5 15.0 20.0 32.5 550 H 72.0 3.0 20.0 5.0 420 I 70.0 5.0 20.0 5.0 415 J 57.5 30.0 10.0 2.5 440 K 52.5 35.0 10.0 2.5 457 L 69.5 25.0 3.0 2.5 423 M 72.5 20.0 5.0 2.5 430 N 52.5 15.0 30.0 2.5 465 O 50.0 15.0 32.5 2.5 470 Sample No. Designation of glass V 1mA /V 10µA Limit voltage ratio V 50A /V 1mA Surge current resistance characteristic ΔV 1mA (%) Direction same as that of current Direction reverse to that of current 1 A 1.33 1.57 -18.4 -27.5 2 B 1.19 1.55 -18.1 -26.4 3 C 1.08 1.46 -6.3 -11.2 4 D 1.06 1.47 -8.0 -12.9 5 E 1.06 1.46 -10.7 -17.1 6 F 1.08 1.50 -12.4 -21.6 7 G 1.29 1.53 -20.3 -37.3 8 H 1.31 1.50 -16.3 -19.2 9 I 1.16 1.51 -11.4 -19.4 10 J 1.10 1.50 -11.8 -23.0 11 K 1.12 1.53 -20.4 -33.7 12 L 1.14 1.49 -19.8 -38.5 13 M 1.17 1.50 -11.2 -22.9 14 N 1.23 1.50 -15.3 -23.8 15 O 1.26 1.50 -25.0 -42.4 Designation of glass Component ratio (wt.%) Tg ( o C) PbO B 2 O 3 SiO 2 Sm 2 O 3 A 70.0 15.0 15.0 0 405 B 69.9 15.0 15.0 0.1 405 C 67.5 15.0 15.0 2.5 415 D 65.0 15.0 15.0 5.0 422 E 55.0 15.0 20.0 10.0 465 F 40.0 10.0 20.0 30.0 525 G 32.5 15.0 20.0 32.5 553 H 72.0 3.0 20.0 5.0 413 I 70.0 5.0 20.0 5.0 415 J 57.5 30.0 10.0 2.5 442 K 52.5 35.0 10.0 2.5 458 L 69.5 25.0 3.0 2.5 425 M 72.5 20.0 5.0 2.5 430 N 52.5 15.0 30.0 2.5 460 O 50.0 15.0 32.5 2.5 465 Sample No. Designation of glass V 1mA /V 10µA Limit voltage ratio V 50A /V 1mA Surge current resistance characteristic ΔV 1mA (%) Direction same as that of current Direction reverse to that of current 1 A 1.33 1.57 -18.4 -27.5 2 B 1.20 1.56 -17.9 -26.1 3 C 1.07 1.47 -5.9 -11.3 4 D 1.05 1.48 -9.4 -13.1 5 E 1.07 1.47 -9.8 -17.8 6 F 1.09 1.50 -12.6 -22.0 7 G 1.28 1.54 -21.0 -38.5 8 H 1.33 1.50 -17.5 -19.9 9 I 1.15 1.52 -10.6 -20.8 10 J 1.09 1.50 -11.9 -25.2 11 K 1.13 1.53 -22.2 -32.3 12 L 1.15 1.50 -20.2 -41.8 13 M 1.15 1.50 -11.1 -23.9 14 N 1.22 1.51 -16.4 -21.8 15 O 1.25 1.49 -25.6 -42.6 Designation of glass Component ratio (wt.%) Tg ( o C) PbO B 2 O 3 SiO 2 Eu 2 O 3 A 70.0 15.0 15.0 0 405 B 69.9 15.0 15.0 0.1 407 C 55.0 15.0 20.0 10.0 470 D 40.0 10.0 20.0 30.0 523 E 32.5 15.0 20.0 32.5 550 Sample No. Designation of glass V 1mA /V 10µA Limit voltage ratio V 50A /V 1mA Surge current resistance characteristic ΔV 1mA (%) Direction same as that of current Direction reverse to that of current 1 A 1.33 1.57 -18.4 -27.5 2 B 1.21 1.57 -18.0 -26.5 3 C 1.08 1.47 -9.7 -18.2 4 D 1.10 1.49 -11.9 -21.8 5 E 1.30 1.52 -20.3 -39.7 Designation of glass Component ratio (wt.%) Tg ( o C) PbO B 2 O 3 SiO 2 Gd 2 O 3 A 70.0 15.0 15.0 0 405 B 69.9 15.0 15.0 0.1 405 C 55.0 15.0 20.0 10.0 475 D 40.0 10.0 20.0 30.0 525 E 32.5 15.0 20.0 32.5 553 Sample No. Designation of glass V 1mA /V 10µA Limit voltage ratio V 50A /V 1mA Surge current resistance characteristic ΔV 1mA (%) Direction same as that of current Direction reverse to that of current 1 A 1.33 1.57 -18.4 -27.5 2 B 1.22 1.56 -17.9 -26.1 3 C 1.08 1.47 -9.3 -18.7 4 D 1.10 1.48 -12.2 -22.0 5 E 1.30 1.51 -20.8 -39.5 Designation of glass Component ratio (wt.%) Tg ( o C) PbO B 2 O 3 SiO 2 Tb 4 O 7 A 70.0 15.0 15.0 0 405 B 69.9 15.0 15.0 0.1 405 C 55.0 15.0 20.0 10.0 475 D 40.0 10.0 20.0 30.0 520 E 32.5 15.0 20.0 32.5 550 Sample No. Designation of glass V 1mA /V 10µA Limit voltage ratio V 50A /V 1mA Surge current resistance characteristic ΔV 1mA (%) Direction same as that of current Direction reverse to that of current 1 A 1.33 1.57 -18.4 -27.5 2 B 1.20 1.55 -18.1 -26.3 3 C 1.09 1.48 -9.9 -19.1 4 D 1.09 1.49 -12.0 -22.6 5 E 1.31 1.50 -21.1 -40.4 Designation of glass Component ratio (wt.%) Tg ( o C) PbO B 2 O 3 SiO 2 Dy 2 O 3 A 70.0 15.0 15.0 0 405 B 69.9 15.0 15.0 0.1 405 C 55.0 15.0 20.0 10.0 472 D 40.0 10.0 20.0 30.0 528 E 32.5 15.0 20.0 32.5 555 Sample No. Designation of glass V 1mA /V 10µA Limit voltage ratio V 50A /V 1mA Surge current resistance characteristic ΔV 1mA (%) Direction same as that of current Direction reverse to that of current 1 A 1.33 1.57 -18.4 -27.5 2 B 1.22 1.57 -17.8 -26.1 3 C 1.09 1.48 -9.2 -19.3 4 D 1.10 1.49 -11.8 -22.5 5 E 1.31 1.50 -20.7 -39.6 Designation of glass Component ratio (wt.%) Tg ( o C) PbO B 2 O 3 SiO 2 Ho 2 O 3 A 70.0 15.0 15.0 0 405 B 69.9 15.0 15.0 0.1 407 C 55.0 15.0 20.0 10.0 475 D 40.0 10.0 20.0 30.0 532 E 32.5 10.0 25.0 32.5 560 Sample No. Designation of glass V 1mA /V 10µA Limit voltage ratio V 50A /V 1mA Surge current resistance characteristic ΔV 1mA (%) Direction same as that of current Direction reverse to that of current 1 A 1.33 1.57 -18.4 -27.5 2 B 1.22 1.57 -18.1 -25.4 3 C 1.09 1.47 -10.3 -19.7 4 D 1.10 1.48 -11.7 -22.9 5 E 1.31 1.51 -19.2 -39.8 Designation of glass Component ratio (wt.%) Tg ( o C) PbO B 2 O 3 SiO 2 Er 2 O 3 A 70.0 15.0 15.0 0 405 B 69.9 15.0 15.0 0.1 408 C 55.0 15.0 20.0 10.0 477 D 40.0 10.0 20.0 30.0 530 E 32.5 10.0 25.0 32.5 558 Sample No. Designation of glass V 1mA /V 10µA Limit voltage ratio V 50A /V 1mA Surge current resistance characteristic ΔV 1mA (%) Direction same as that of current Direction reverse to that of current 1 A 1.33 1.57 -18.4 -27.5 2 B 1.24 1.56 -18.0 -25.7 3 C 1.10 1.50 -11.2 -19.3 4 D 1.15 1.50 -11.8 -22.4 5 E 1.35 1.52 -21.6 -40.6 Designation of glass Component ratio (wt.%) Tg ( o C) PbO B 2 O 3 SiO 2 Tm 2 O 3 A 70.0 15.0 15.0 0 405 B 69.9 15.0 15.0 0.1 405 C 55.0 15.0 20.0 10.0 475 D 40.0 10.0 20.0 30.0 535 E 32.5 10.0 25.0 32.5 565 Sample No. Designation of glass V 1mA /V 10µA Limit voltage ratio V 50A /V 1mA Surge current resistance characteristic ΔV 1mA (%) Direction same as that of current Direction reverse to that of current 1 A 1.33 1.57 -18.4 -27.5 2 B 1.25 1.55 -18.0 -26.4 3 C 1.10 1.49 -9.3 -20.2 4 D 1.13 1.48 -12.8 -23.5 5 E 1.33 1.51 -21.5 -41.1 Designation of glass Component ratio (wt.%) Tg ( o C) PbO B 2 O 3 SiO 2 Yb 2 O 3 A 70.0 15.0 15.0 0 405 B 69.9 15.0 15.0 0.1 405 C 55.0 15.0 20.0 10.0 475 D 40.0 10.0 20.0 30.0 530 E 32.5 10.0 25.0 32.5 558 Sample No. Designation of glass V 1mA /V 10µA Limit voltage ratio V 50A /V 1mA Surge current resistance characteristic ΔV 1mA (%) Direction same as that of current Direction reverse to that of current 1 A 1.33 1.57 -18.4 -27.5 2 B 1.24 1.56 -18.2 -27.1 3 C 1.11 1.50 -10.4 -19.8 4 D 1.12 1.48 -13.0 -24.1 5 E 1.36 1.53 -21.6 -42.5 Designation of glass Component ratio (wt.%) Tg ( o C) PbO B 2 O 3 SiO 2 Lu 2 O 3 A 70.0 15.0 15.0 0 405 B 69.9 15.0 15.0 0.1 407 C 55.0 15.0 20.0 10.0 480 D 40.0 10.0 20.0 30.0 540 E 32.5 10.0 25.0 32.5 565 Sample No. Designation of glass V 1mA /V 10µA Limit voltage ratio V 50A /V 1mA Surge current resistance characteristic ΔV 1mA (%) Direction same as that of current Direction reverse to that of current 1 A 1.33 1.57 -18.4 -27.5 2 B 1.25 1.55 -18.2 -26.8 3 C 1.12 1.51 -10.3 -15.9 4 D 1.14 1.50 -13.7 -23.8 5 E 1.36 1.51 -21.0 -43.5 The above working examples indicated the cases in which a lead borosilicate glass frit is milled into Ag-paste and then applied onto varistor element 1 to form electrodes 2, and upon baking of electrodes 2, chemical elements constituting said lead borosilicate glass frit are diffused into the varistor element 1. However, the present invention is not limited to said procedure. A similar effect concerning voltage ratio (voltage nonlinearity) has been obtained also by the following procedure, wherein prior to the formation of electrodes 2, a paste containing a lead borosilicate-type glass frit is applied onto a surface of a fired varistor element 1 and then the resultant is heated under such a state as it is, thereby allowing the chemical elements composing said lead borosilicate-type glass frit to penetrate into varistor element 1, and thereafter, a Ag-paste containing no lead borosilicate-type glass frit is used to form electrodes 2. Further, an electrode material for forming electrodes 2 is not limited to Ag-paste, which may be replaced with pastes of the other metals such as Pd, etc. INDUSTRIALLY AVAILABLE FIELD As mentioned above, according to the present invention as defined by the claims, there is diffused from a surface of a fired varistor element a lead borosilicate-type glass containing at least one metal oxide selected out of cobalt oxide, magnesium oxide, yttrium oxide, antimony oxide, manganese oxide, tellurium oxide, lanthanum oxide, cerium oxide, praseodymium oxide, neodymium oxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, thulium oxide, ytterbium oxide and lutetium oxide. Thus, when voltage nonlinearity is so improved, energy saving and efficiency improvement can be seen for various kinds of electronic instruments to be used owing to these being less leakage current.

Claims: A zinc oxide varistor characterized by providing a varistor element, whose main component is zinc oxide, with at least two electrodes fitted up on said varistor element, and by diffusing the following lead borosilicate-type glass from a surface of the fired varistor element into said varistor element; said lead borosilicate-type glass containing an amount of boron oxide being 5.0 - 30% by weight in the term of B 2 O 3 , an amount of silicon oxide being 5.0 - 30% by weight in the term of SiO 2 , an amount of lead oxide being 40.0 - 80% by weight in the term of PbO and an amount of at least one metal oxide being 0.1 - 30.0% by weight, and said at least one metal oxide is selected out of cobalt oxide, magnesium oxide, yttrium oxide, antimony oxide, manganese oxide, tellurium oxide, lanthanum oxide, cerium oxide, praseodymium oxide, neodymium oxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, thulium oxide, ytterbium oxide and lutetium oxide. The zinc oxide varistor according to claim 1, containing an amount of at least one further metal oxide being 1.0 x 10 -4 - 1.0% by weight, said at least one further metal oxide is selected out of aluminum oxide, indium oxide, gallium oxide and germanium oxide. The zinc oxide varistor according to claim 2, which is characterized in that said at least one further metal oxide is selected from aluminum oxide in the term of Al 2 O 3 , indium oxide in the term of In 2 O 3 , gallium oxide in the term of Ga 2 O 3 and germanium oxide in the term of GeO 2 , in an amount of 1.0 x 10 -4 - 1.0% by weight. A method for producing the zinc oxide varistor according to any of claims 1 to 3, wherein boron oxide, silicon oxide, lead oxide and cobalt oxide are mixed, and then the mixture is fused and thereafter quenched, said mixture containing an amount of boron oxide being 5.0 - 30% by weight in the term of B 2 O 3 , an amount of silicon oxide being 5.0 - 30% by weight in the term of SiO 2 , an amount of lead oxide being 40.0 - 80% by weight in the term of PbO and an amount of cobalt oxide being 0.1% by weight - 30.0% by weight in the term of Co 3 O 4 . A method for producing the zinc oxide varistor according to any of claims 1 to 3, wherein boron oxide, silicon oxide, lead oxide and cobalt oxide are mixed, and then the mixture is fused and thereafter quenched, said mixture containing an amount of boron oxide being 5.0 - 30% by weight in the term of B 2 O 3 , an amount of silicon oxide being 5.0 - 30% by weight in the term of SiO 2 , an amount of lead oxide being 40.0 - 80% by weight in the term of PbO and an amount of magnesium oxide being 0.1% by weight - 30.0% by weight in the term of MgO. A method for producing the zinc oxide varistor according to any of claims 1 to 3, wherein boron oxide, silicon oxide, lead oxide and cobalt oxide are mixed, and then the mixture is fused and thereafter quenched, said mixture containing an amount of boron oxide being 5.0 - 30% by weight in the term of B 2 O 3 , an amount of silicon oxide being 5.0 - 30% by weight in the term of SiO 2 , an amount of lead oxide being 40.0 - 80% by weight in the term of PbO and an amount of yttrium oxide being 0.1% by weight - 30.0% by weight in the term of Y 2 O 3 . A method for producing the zinc oxide varistor according to any of claims 1 to 3, wherein boron oxide, silicon oxide, lead oxide and cobalt oxide are mixed, and then the mixture is fused and thereafter quenched, said mixture containing an amount of boron oxide being 5.0 - 30% by weight in the term of B 2 O 3 , an amount of silicon oxide being 5.0 - 30% by weight in the term of SiO 2 , an amount of lead oxide being 40.0 - 80% by weight in the term of PbO and an amount of antimony oxide being 0.1% by weight - 30.0% by weight in the term of Sb 2 O 3 . A method for producing the zinc oxide varistor according to any of claims 1 to 3, wherein boron oxide, silicon oxide, lead oxide and cobalt oxide are mixed, and then the mixture is fused and thereafter quenched, said mixture containing an amount of boron oxide being 5.0 - 30% by weight in the term of B 2 O 3 , an amount of silicon oxide being 5.0 - 30% by weight in the term of SiO 2 , an amount of lead oxide being 40.0 - 80% by weight in the term of PbO and an amount of manganese oxide being 0.1% by weight - 30.0% by weight in the term of MnO 2 . A method for producing the zinc oxide varistor according to any of claims 1 to 3, wherein boron oxide, silicon oxide, lead oxide and cobalt oxide are mixed, and then the mixture is fused and thereafter quenched, said mixture containing an amount of boron oxide being 5.0 - 30% by weight in the term of B 2 O 3 , an amount of silicon oxide being 5.0 - 30% by weight in the term of SiO 2 , an amount of lead oxide being 40.0 - 80% by weight in the term of PbO and an amount of tellurium oxide being 0.1% by weight - 30.0% by weight in the term of TeO 2 . A method for producing the zinc oxide varistor according to any of claims 1 to 3, wherein boron oxide, silicon oxide, lead oxide and cobalt oxide are mixed, and then the mixture is fused and thereafter quenched, said mixture containing an amount of boron oxide being 5.0 - 30% by weight in the term of B 2 O 3 , an amount of silicon oxide being 5.0 - 30% by weight in the term of SiO 2 , an amount of lead oxide being 40.0 - 80% by weight in the term of PbO and an amount of lanthanum oxide being 0.1% by weight - 30.0% by weight in the term of La 2 O 3 . A method for producing the zinc oxide varistor according to any of claims 1 to 3, wherein boron oxide, silicon oxide, lead oxide and cobalt oxide are mixed, and then the mixture is fused and thereafter quenched, said mixture containing an amount of boron oxide being 5.0 - 30% by weight in the term of B 2 O 3 , an amount of silicon oxide being 5.0 - 30% by weight in the term of SiO 2 , an amount of lead oxide being 40.0 - 80% by weight in the term of PbO and an amount of cerium oxide being 0.1% by weight - 30.0% by weight in the term of CeO 2 . A method for producing the zinc oxide varistor according to any of claims 1 to 3, wherein boron oxide, silicon oxide, lead oxide and cobalt oxide are mixed, and then the mixture is fused and thereafter quenched, said mixture containing an amount of boron oxide being 5.0 - 30% by weight in the term of B 2 O 3 , an amount of silicon oxide being 5.0 - 30% by weight in the term of SiO 2 , an amount of lead oxide being 40.0 - 80% by weight in the term of PbO and an amount of praseodymium oxide being 0.1% by weight - 30.0% by weight in the term of Pr 6 O 11 . A method for producing the zinc oxide varistor according to any of claims 1 to 3, wherein boron oxide, silicon oxide, lead oxide and cobalt oxide are mixed, and then the mixture is fused and thereafter quenched, said mixture containing an amount of boron oxide being 5.0 - 30% by weight in the term of B 2 O 3 , an amount of silicon oxide being 5.0 - 30% by weight in the term of SiO 2 , an amount of lead oxide being 40.0 - 80% by weight in the term of PbO and an amount of neodymium oxide being 0.1% by weight - 30.0% by weight in the term of Nd 2 O 3 . A method for producing the zinc oxide varistor according to any of claims 1 to 3, wherein boron oxide, silicon oxide, lead oxide and cobalt oxide are mixed, and then the mixture is fused and thereafter quenched, said mixture containing an amount of boron oxide being 5.0 - 30% by weight in the term of B 2 O 3 , an amount of silicon oxide being 5.0 - 30% by weight in the term of SiO 2 , an amount of lead oxide being 40.0 - 80% by weight in the term of PbO and an amount of samarium oxide being 0.1% by weight - 30.0% by weight in the term of Sm 2 O 3 . A method for producing the zinc oxide varistor according to any of claims 1 to 3, wherein boron oxide, silicon oxide, lead oxide and cobalt oxide are mixed, and then the mixture is fused and thereafter quenched, said mixture containing an amount of boron oxide being 5.0 - 30% by weight in the term of B 2 O 3 , an amount of silicon oxide being 5.0 - 30% by weight in the term of SiO 2 , an amount of lead oxide being 40.0 - 80% by weight in the term of PbO and an amount of europium oxide being 0.1% by weight - 30.0% by weight in the term of Eu 2 O 3 . A method for producing the zinc oxide varistor according to any of claims 1 to 3, wherein boron oxide, silicon oxide, lead oxide and cobalt oxide are mixed, and then the mixture is fused and thereafter quenched, said mixture containing an amount of boron oxide being 5.0 - 30% by weight in the term of B 2 O 3 , an amount of silicon oxide being 5.0 - 30% by weight in the term of SiO 2 , an amount of lead oxide being 40.0 - 80% by weight in the term of PbO and an amount of gadolinium oxide being 0.1% by weight - 30.0% by weight in the term of Gd 2 O 3 . A method for producing the zinc oxide varistor according to any of claims 1 to 3, wherein boron oxide, silicon oxide, lead oxide and cobalt oxide are mixed, and then the mixture is fused and thereafter quenched, said mixture containing an amount of boron oxide being 5.0 - 30% by weight in the term of B 2 O 3 , an amount of silicon oxide being 5.0 - 30% by weight in the term of SiO 2 , an amount of lead oxide being 40.0 - 80% by weight in the term of PbO and an amount of terbium oxide being 0.1% by weight - 30.0% by weight in the term of Tb 4 O 7 . A method for producing the zinc oxide varistor according to any of claims 1 to 3, wherein boron oxide, silicon oxide, lead oxide and cobalt oxide are mixed, and then the mixture is fused and thereafter quenched, said mixture containing an amount of boron oxide being 5.0 - 30% by weight in the term of B 2 O 3 , an amount of silicon oxide being 5.0 - 30% by weight in the term of SiO 2 , an amount of lead oxide being 40.0 - 80% by weight in the term of PbO and an amount of dysprosium oxide being 0.1% by weight - 30.0% by weight in the term of Dy 2 O 3 . A method for producing the zinc oxide varistor according to any of claims 1 to 3, wherein boron oxide, silicon oxide, lead oxide and cobalt oxide are mixed, and then the mixture is fused and thereafter quenched, said mixture containing an amount of boron oxide being 5.0 - 30% by weight in the term of B 2 O 3 , an amount of silicon oxide being 5.0 - 30% by weight in the term of SiO 2 , an amount of lead oxide being 40.0 - 80% by weight in the term of PbO and an amount of holmium oxide being 0.1% by weight - 30.0% by weight in the term of Ho 2 O 3 . A method for producing the zinc oxide varistor according to any of claims 1 to 3, wherein boron oxide, silicon oxide, lead oxide and cobalt oxide are mixed, and then the mixture is fused and thereafter quenched, said mixture containing an amount of boron oxide being 5.0 - 30% by weight in the term of B 2 O 3 , an amount of silicon oxide being 5.0 - 30% by weight in the term of SiO 2 , an amount of lead oxide being 40.0 - 80% by weight in the term of PbO and an amount of erbium oxide being 0.1% by weight - 30.0% by weight in the term of Er 2 O 3 . A method for producing the zinc oxide varistor according to any of claims 1 to 3, wherein boron oxide, silicon oxide, lead oxide and cobalt oxide are mixed, and then the mixture is fused and thereafter quenched, said mixture containing an amount of boron oxide being 5.0 - 30% by weight in the term of B 2 O 3 , an amount of silicon oxide being 5.0 - 30% by weight in the term of SiO 2 , an amount of lead oxide being 40.0 - 80% by weight in the term of PbO and an amount of thulium oxide being 0.1% by weight - 30.0% by weight in the term of Tm 2 O 3 . A method for producing the zinc oxide varistor according to any of claims 1 to 3, wherein boron oxide, silicon oxide, lead oxide and cobalt oxide are mixed, and then the mixture is fused and thereafter quenched, said mixture containing an amount of boron oxide being 5.0 - 30% by weight in the term of B 2 O 3 , an amount of silicon oxide being 5.0 - 30% by weight in the term of SiO 2 , an amount of lead oxide being 40.0 - 80% by weight in the term of PbO and an amount of ytterbium oxide being 0.1% by weight - 30.0% by weight in the term of Yb 2 O 3 . A method for producing the zinc oxide varistor according to any of claims 1 to 3, wherein boron oxide, silicon oxide, lead oxide and cobalt oxide are mixed, and then the mixture is fused and thereafter quenched, said mixture containing an amount of boron oxide being 5.0 - 30% by weight in the term of B 2 O 3 , an amount of silicon oxide being 5.0 - 30% by weight in the term of SiO 2 , an amount of lead oxide being 40.0 - 80% by weight in the term of PbO and an amount of lutetium oxide being 0.1% by weight - 30.0% by weight in the term of Lu 2 O 3 . A process for producing a zinc oxide varistor characterized by diffusing the following lead borosilicate-type glass into a zinc oxide varistor element from a surface of the fired zinc oxide varistor element, and thereafter providing said zinc oxide varistor element with at least two electrodes, said lead borosilicate-type glass containing an amount of boron oxide being 5.0 - 30% by weight in the term of B 2 O 3 , an amount of silicon oxide being 5.0 - 30% by weight in the term of SiO 2 , an amount of lead oxide being 40.0 - 80% by weight in the term of PbO and an amount of at least one metal oxide being 0.1 - 30.0% by weight, and said at least one metal oxide is selected out of cobalt oxide, magnesium oxide, yttrium oxide, antimony oxide, manganese oxide, tellurium oxide, lanthanum oxide, cerium oxide, praseodymium oxide, neodymium oxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, thulium oxide, ytterbium oxide and lutetium oxide. The process for producing a zinc oxide varistor according to claim 24, which is characterized by applying a lead borosilicate-type glass onto a surface of a varistor element, and then heating it, thereby allowing said lead borosilicate-type glass to diffuse into the varistor element. The process for producing a zinc oxide varistor according to claim 24, which is characterized by allowing a lead borosilicate-type glass to contain at least one of aluminum oxide, indium oxide, gallium oxide and germanium oxide. The process for producing a zinc oxide varistor according to claim 24, which is characterized by applying a lead borosilicate-type glass onto a surface of a varistor element, and then adding at least one of aluminum oxide, indium oxide, gallium oxide and germanium oxide onto the surface of said lead borosilicate-type glass. A process for producing a zinc oxide varistor characterized by adding the following lead borosilicate-type glass to a paste for electrode, and then applying the resulting paste for electrode onto a surface of a fired zinc oxide varistor element, which is followed by baking it to form an electrode, said lead borosilicate-type glass containing an amount of boron oxide being 5.0 - 30% by weight in the term of B 2 O 3 , an amount of silicon oxide being 5.0 - 30% by weight in the term of SiO 2 , an amount of lead oxide being 40.0 - 80% by weight in the term of PbO and an amount of at least one metal oxide being 0.1 - 30.0% by weight, and said at least one metal oxide is selected out of cobalt oxide, magnesium oxide, yttrium oxide, antimony oxide, manganese oxide, tellurium oxide, lanthanum oxide, cerium oxide, praseodymium oxide, neodymium oxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, thulium oxide, ytterbium oxide and lutetium oxide. The process for producing a zinc oxide varistor according to claim 28, which is characterized by adding at least one chemical element of aluminum, indium, gallium and germanium, into the electrode paste having been therein.

Inventors: KATSUMATA MASAAKI, KOYAMA KAZUSHIGE, MUTOH NAOKI, KATSUMATA, MASAAKI, KOYAMA, KAZUSHIGE, MUTOH, NAOKI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1993
Main classification: H01C7
IPC codes: H01C7