RADIO COMMUNICATION 277 



nected through a small transformer (tr) into the grid circuit of 

 the modulator tube (m). The double commutator dynamo is 

 shown at (d). This dynamo supplies both the filament current 

 and the plate potential to both tubes. Inductances (/ 2 ) and 

 (7 3 ) are placed in the oscillating circuits. In actual operation 

 several modulator tubes are connected in parallel to increase the 

 strength of the speech-input current. There are also several 

 generator tubes connected in parallel in order to increase the 

 strength of the outgoing or carrier wave. 



Very briefly the action may be explained as follows. When 

 words are spoken into the transmitter, or microphone, a speech 

 wave of audio frequency is impressed on the grid circuit of the 

 modulator tube. This change in potential of the grid will pro- 

 duce corresponding changes in the plate current of the modulator. 

 This oscillation of the plate current of the modulator causes this 

 tube to build up or absorb energy from the antenna. This build- 

 ing up and reducing process corresponds to the vibrations of the 

 sound taken in by the microphone. The carrier wave, then, is 

 oscillating at regular radio frequency during the whole time the 

 station is sending. At the same time the current strength of the 

 antenna circuit, or the amplitude of the carrier wave, is vibrating 

 at audio frequency. This audio-frequency vibration reproduces 

 exactly all the sounds that strike the diaphragm of the microphone. 



It will be recalled that a receiving circuit employing a simple 

 crystal detector is used to pick up signals from discontinuous- wave 

 sending stations. This result was explained as possible because 

 the wave-trains were at audio frequency. Now, when such a 

 receiving circuit is tuned to the frequency of the carrier wave 

 from a radio telephone transmitting station, the frequency of the 

 carrier wave is too fast to actuate the diaphragm of the telephone 

 receiver. The result will be that no sound is produced by the 

 carrier wave itself. The current intensity of the carrier wave is 

 vibrating at audio frequency, corresponding to the sounds strik- 

 ing the diaphragm of the microphone at the sending station. 

 This fluctuation in current strength will cause the diaphragm of 



