INDUCTANCE AND CAPACITY 



353 



The dielectric strength of air condensers may be greatly 

 increased if the dielectric be dry compressed air, at a pressure 

 of 60 Ib. per square inch or greater. 7 This eliminates brush 

 discharges and energy losses at high voltages. For example, 

 with an air pressure of 175 Ib. per square inch, a condenser with 

 plates 2.1 mm. apart showed no appreciable energy loss at 27,500 

 volts. It broke down at 28,500 volts. 



In another case with the plates 3.2 mm. apart the break- 

 down voltage at atmospheric pressure was 6,000 volts; .when the 



FIG. 209. Compressed gas condenser for use in radio- telegraphy. 



air pressure was raised to 260 Ib. per square inch the break- 

 down voltage became 30,000. 



It is necessary to use a drying material in the condenser cases. 

 The use of compressed air, of course, necessitates a strong and, 

 therefore, a very heavy metal case. A practical difficulty arises 

 in the introduction of the lead to the insulated set of plates; 

 it must be perfectly insulated and all joints must be air-tight 

 as well. Practically, the casing cannot be made absolutely 



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