276 



BELL SYSTEM TECHNICAL JOURNAL 



After fritting, the resultant glass was sized and suspended in a water 

 suspension of white lead and light calcined magnesia in a tank provided 

 with mechanical agitation. This method of suspension aided in in- 

 creasing the fired viscosity without the formation of blisters and pin 

 holes. Smooth glassy resistances were obtained with this enamel in a 

 ten minute firing at 1150° F. without appreciable bubbling or flowing 

 of the coating. This eliminated the necessity of three or four thin 

 fired coats and resultant greater variations in resistance values after 

 firing. 



Close Tolerance Ceramic Barriers and Insulators 



In the design of the handset type of telephone transmitter, it was 

 found desirable to use a thin washer type insulator, shown in the lower 

 portion of Fig. 8, as a barrier to control the path of the current between 



/ 



Fig. 8 — Close tolerance insulators. 



electrodes. This necessitated very close dimensional tolerances, un- 

 usual freedom from surface and edge defects, and reasonable strength 

 to withstand the clamping force used in assembly. 



Various materials such as fiber, lava and metal coated with vitreous 

 enamel, were tried and lava gave the most promising results. In view 

 of the cost of lava parts, experiments were made using the usual process 

 of dry pressing a porcelain body in which the clay content furnished the 

 raw strength. The difficulties inherent with this process were the 

 fragility of the raw part and the variable dimensions resulting from 

 uneven drying and firing shrinkages. Because of the fragility of the 

 parts, it was necessary to mold them 0.050 inch thick and then lap the 



