1920] Thermionic Valve in Wireless Telegraphy & Telephony 175 



Fig. 11) in the plate circuit. The action resembles that in the well- 

 known experiment called the singing telephone. If we place an 

 ordinary carbon microphone transmitter in series with a battery and 

 receiving telephone, both having diaphragms of equal size, and hold 

 the receiver mouth towards the transmitter mouthpiece, the receiver 

 will emit a shrill musical note. 



The reason is not far to seek. Small noises in the room make 

 little vibrations in the diaphragm of the transmitter. These vary 

 the carbon resistance, and therefore the current flowing through the 

 receiver. The latter then emits a sound, and as the natural rate of 

 vibration of the diaphragm of the transmitter is the same, this sound 

 maintains the transmitter in vibration. The two act and re-act on 



Fig. 11. — Connections for Generator Valve. 



each other, and the energy required to produce the sound is drawn 

 from the battery. 



Another illustration which assists in explaining the action of the 

 oscillating valve is that of the steam engine. The steam is admitted 

 first at one end of the cylinder and then at the other to cause a 

 reciprocating motion in the piston. The agent for effecting this 

 distribution of the steam is the slide valve, which admits the steam 

 alternately above or below the piston. But the piston itself moves 

 the slide valve by the intermediation of the eccentric or in some 

 other appliance, and so the engine becomes self-acting and the steady 

 pressure of the steam maintains a to-and-fro motion in the piston rod. 



The potential of the grid of the valve answers to the slide valve, 

 the plate current to the piston, and the steady E.M.F. of the plate 

 battery to the steam pressure, whilst the alternating current set up 

 in the plate circuit corresponds to the reciprocating motion of the 

 piston. 



