RADIO COMMUNICATION 273 



Small models of such a continuous wave generator are used 

 as the source of the local continuous wave employed in connec- 

 tion with the receiving circuit to produce the "beat" effect 

 required in receiving continuous wave signals. When so used 

 this circuit is known as the heterodyne. 



Practically the same system as shown in Figure 126, and 

 explained above, may be used for producing the continuous wave 

 sent out by transmitting stations. The vacuum tubes used in 

 such stations are necessarily much larger than the small tube. 

 Since, in transmitting, considerable energy must be supplied 

 to the antenna circuit, it is necessary to withstand heavy voltage 

 on the plate. The vacuum in the power tube must be extremely 

 high; otherwise the effect of this high plate potential will be to 

 produce a blue glow in the tube and impair its action. Com- 

 paratively large plate currents, due to this high plate potential, 

 cause the transmitting tubes to become very hot. To prevent 

 this excessive heating, the power tube is supplied with cooling 

 devices such as heat radiating fins on the plate connection outside 

 the tube. Devices are also now being employed which make 

 use of circulating systems of water to carry away the excess heat. 



In order to supply the high-voltage plate current, direct- 

 current dynamos are installed as part of the transmitting equip- 

 ment. Such a dynamo usually has two commutators so that the 

 current for lighting the filaments of the power tube may be taken 

 from the same dynamo that supplies the plate with the high- 

 voltage current. 



Because of the high vacuum required and the necessity for 

 getting rid of the heat, the size of these tubes is limited. For 

 large-power output several tubes are connected in parallel, so 

 that it is possible to radiate considerable energy from the antenna 

 of the continuous- wave transmitting station. 



Since it would be quite impractical to break the dynamo cur- 

 rent supplying the tubes, in order that dots and dashes could be 

 sent from the antenna, some other means must be employed for 

 modifying this antenna current to produce the desired signals. 



