288 



BELL SYSTEM TECHNICAL JOURNAL 



bleeder valve which is located near the pump. This practice was found 

 necessary in order to overcome variations in pressure due to the early 

 evolution of gases and the later cleanup usually accompanying elec- 

 trical discharges in vacuo. The valve is of rugged construction as 

 shown in Fig. 5 and consists of a standard No. taper pin about 2| 

 inches long, very closely lapped into a bronze bushing. A pressure of 

 .100 ± .005 mm. is readily maintained by this method. After a new 



Fig. 4 — The sputtering chamber, showing multiple electrodes, diaphragm holder 



and shield. 



charge has been placed in the bell jar, the bleeder valve is temporarily 

 cut off by closing a stop cock so that the required vacuum can be more 

 quickly obtained. By this means, sputtering can be started in about 

 four minutes after the bell jar has been placed in position. 



The discharge is produced by means of a 1/2 kva transformer which 

 steps up the voltage from 110 to 10,000 volts and is regulated by means 

 of a rheostat in the primary. Safety for the operator is insured by 



