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BELL SYSTEM TECHNICAL JOURNAL 



the internal ceramic insulators, satisfactory operation could be obtained at 

 voltages as high as 15 kv. Since the 715A was designed primarily for air- 

 borne applications it was found desirable to design the heater to operate 

 directly from the aircraft's storage battery which was a 24 volt battery. 



Fig. 7 — The 715H vacuum tube. 



It was also desirable that the equipment be operable when the charging 

 generator was not ruiming, at which point the voltage might be as low as 22 

 volts, and also when the generator was chargmg and the voltage as high as 

 28.5 volts. This required a compromise in the design of the heater which 



