1920] Thermionic Valve in Wireless Telegraphy & Telephony 179 



arrangements on the same principle have telephoned from Chelmsford 

 to Rome. 



For aircraft radio-telephony it is usual to provide a small high- 

 tension dynamo driven by a wind-screw, which is fixed to some 

 external part of the fuselage, and this gives the requisite direct high 

 plate voltage for the generating valve. The filament-heating 

 currents for the generating, the control and the receiving valves art- 

 provided from small closed storage cells. The aerial Avire is a long 

 trailing wire about 250 feet in length, which is unwound when 

 required from a drum. 



The principles of operation are as above described, and the 

 engineers of the Marconi Company have worked out an ingenious 

 arrangement by which the actual valve apparatus may be placed at 



Fig. 15. — Circuit Connections of a Marconi Wireless 

 Telephone Transmitter. 

 0,, the generating valve. 2 , the control valve. 

 E, the rectifying valve. 



any convenient place in the aeroplane body and yet be controlled by 

 the pilot or observer from their seats. The mere act of taking hold 

 of the microphone transmitter closes a switch which lights up the 

 valves and throws over the aerial wire into connection with the 

 transmitting valve. 



The pilot has also in front of him a small ammeter which shows 

 at a glance if the proper high-frequency oscillations are being- 

 generated in the aerial wire. Such aircraft radio-telephones will 

 operate over a distance of 50 miles or more. So sensitive are these 

 cascaded valve detectors that it is not even necessary to use a long 

 aerial wire at all. A very few turns of insulated wire wound on a 

 wooden frame, called a frame aerial, connected to the receiver suffice 

 to collect and detect the electric wave signals. 



Experiments were conducted in March, 1919, by the Marconi 

 Companv to ascertain the minimum power required by these valve 



N 2 



