ARMSTRONG, OPERATING FEATURES OF THE AVDION 



233 



when it is used as an electron relaij.^ When there is an appreciable 

 amount of gas in the tube in tJie ionized state, disturbances of an entirely 

 different character occur. 



Audio Fekquej^cy Tuisting 



One of the very important advantages of the receiver when used for 

 continuous waves is that the alternating current produced in the tele- 

 phones is almost a pure sine wave. Only when the audio frequency is 

 simple harmonic can selectivity be obtained by tuning the telephone cir- 

 cuit. A distorted wave such as that produced by spark signals possesses 

 many harmonics, and as each may be picked out by the tuned telephone 

 circuit there is little chance of separating two spark signals l^y audio 



3^ ^^ 



Pig. 26 



frequency tuning. With continuous waves, however, the pure wave pro- 

 duced by the beat method of reception makes it possible to obtain selec- 

 tivity by the audio frequency tuning, resonance being fully as sharp as in 

 radio frequency circuits. Two methods of audio frequency tuning are 

 shown in Figs. 36 and 37. In Fig. 36, the telephone is inductively con- 

 nected to the wing circuit of the audion by means of a transformer; the 

 secondary of which includes besides the telephone a tuning condenser. 

 In this connection, the telephone, with a resistance of many thousand 

 ohms, is placed directly in ttie tuned audio frequency circuit, and hence 



^Electrical World, December 12, 1914; and also discussiou in London Electrician, be- 

 tween Reisz and de Forest on the difference between electron and gas relays. (February 

 6, 1914, page 726; March 10. 1914, page 9.56; .Tune 12. 1914, page 402; .Tuly .3, 1914, 

 page 538 ; July 31, 1914, page 702.) 



