286 



BELL SYSTEM TECHNICAL JOURNAL 



taken to keep the spot to be coated free from all contaminations before 

 sputtering. 



The unit developed for commercial sputtering is shown in operation 

 in Fig. 3. A two-gallon bell jar fitted upon a heavy ground pyrex 



GRANULAR CARBON 



DIAPHRAGM 



PAPER 



ymmmm 



OUTPUT 



Fig. 2 — Schematic diagram of conventional electrical connections of the microphone 



in a push-pull circuit. 



glass plate contains six glass-covered aluminum cathodes, the exposed 

 parts of which are htted with renewable gold discs (Fig. 4). An ad- 

 justable aluminum stand to hold the diaphragm fixture and shield is 

 placed upon the plate directly below the cathodes. The shield covers 



