On the other hand, in the samples Nos. 5 to 7 with Al.sub.2 O.sub.3 powders having particle sizes of 1 .mu.m or smaller, particularly in the samples Nos. 6 and 7 withAl.sub.2 O.sub.3 powders having particle sizes of 0.5 .mu.m or smaller, higher electrical resistance was obtained with a smaller amount of Al.sub.2 O.sub.3 powder added.