270 OUR PHYSICAL WORLD 



As long as the grid is neutral, the plate current is steady and 

 direct. When the incoming signals set the receiving circuit into 

 electrical vibration, the potential of the grid will change from 

 positive to negative very rapidly as each wave-train passes. 

 When the grid is negative it will repel the negative particles of 

 electricity and so stop the flow of the plate current. The effect 

 will be a pulsating current of audio frequency through the tele- 

 phone receivers each time a wave- train affects the grid. Since 

 slight changes in potential of the grid produce large changes in 

 the current through the plate circuit, the vacuum tube is said 

 to act as an electrical valve, allowing current to flow through the 

 plate circuit in one direction only. 



As was stated above, the vacuum tube is also used to pro- 

 duce continuous waves. Larger power tubes, of course, are used 

 in the large continuous-wave transmitting stations. The tubes 

 for this use bear the names of pliotrons, oscillions, or other 

 names derived from characteristic features in their construction 

 (Fig. 125). It has been shown that slight variations in the grid 

 circuit of a tube produce large variations in the plate current. 

 This action is made use of by causing the plate current to flow 

 through an inductance placed close to a similar inductance in the 

 grid circuit. When oscillations are started in the grid circuit 

 they produce oscillations in the plate circuit which are "fed back " 

 into the grid circuit through this inductive coupling of the grid 

 and plate circuits. These inductances can be so adjusted that 

 the oscillations will be sustained, and a continuous wave will be 

 produced in the antenna circuit. 



The reception of continuous wave signals cannot be accom- 

 plished with the ordinary rectifying detector. Although the 

 incoming wave may be rectified and caused to pass through the 

 telephone receivers, its frequency is so great that the diaphragm 

 of the telephone receiver will not respond to it, and so some 

 means must be introduced to produce a frequency of audible 

 range in the telephone receiver. This production of audio- 

 frequency vibrations in the telephone receiver is accomplished 



