268 



OUR PHYSICAL WORLD 



oscillate at radio frequency and send out on the antenna a con- 

 tinuous wave. Sometimes the dynamo itself is a radio-frequency 

 alternator generating a current of such a large number of alter- 

 nations or cycles per second that when connected in a circuit with 

 suitable capacity and inductance it can be employed to produce 

 directly oscillations of radio frequency. 



Perhaps the most popular method of producing continuous 

 waves for radio transmission is by means of the three-electrode 

 vacuum valve. Since this piece of apparatus is also very gener- 

 ally used as a detector for radio reception, a very 

 brief treatment of its construction and mode of 

 operation will be given. 



The vacuum tube or three-electrode vacuum 

 valve (Fig. 123) depends upon the emission of a 

 stream of electrons or particles of negative 

 i electricity from a hot wire or filament. In con- 

 Ik $&i struction it is similar to an incandescent electric- 

 light bulb. A wire filament is inclosed in a glass 

 globe from which the air has been exhausted. 

 S* B In addition to the filament, which is counted as 



l^^T one of the three electrodes, there is also placed 



within the tube a metal plate. Between the 

 plate and the filament is supported a grid or rack 

 with many strands of wire stretched across much 

 This plate and grid constitute the other two 

 electrodes. Both ends of the filament, the plate, and the grid 

 lead to terminals outside the tube so that there are four connec- 

 tions to the three-electrode vacuum valve. 



When used as a simple detector of damped waves, the three- 

 electrode vacuum valve is connected into the receiving circuit 

 as shown in the wiring diagram of Figure 124. It will be noted 

 that this receiving circuit is practically the same as for the 

 crystal detector circuit, and likewise consists of primary and 

 secondary circuits. The additional feature is that the oscillat- 

 ing circuit is connected on one side of the variable condenser 



FIG. 1 23. Three- 

 electrode vacuum 

 valve. 



like a fence. 



