MATTER, SPACE, AND TIME 225 



The continuous or undamped waves may be 

 produced by means of what is called a thermionic 

 valve. A hot wire of tungsten, such as is used 

 in electric light bulbs, as explained on page 157, 

 is found to emit negative corpuscles or electrons. 

 Ordinarily, the wire is thus left positively elec- 

 trified, and, owing to electric attraction, a state of 

 equilibrium is reached and no more electrons are 

 emitted. But if the hot wire be connected with 

 the negative terminal of a battery, and a metal 

 plate within the bulb with the positive terminal, 

 a large continuous negative current will pass from 

 the hot wire to the plate, carried by the continually 

 escaping electrons. On the other hand, if the 

 battery terminals be reversed, no appreciable 

 current will flow, since the electrons now tend to 

 be driven back into the hot tungsten. 



Thus the first use of the thermionic valve is 

 as a rectifier. It will allow to pass that half of an 

 alternating current which flows in one direction, 

 and will stop the half which tries to flow in 

 the other. 



Next, let a grid, made of a piece of wire gauze, 

 be put between the hot wire and the plate. When 

 the grid is positively electrified, it will help the 

 emission of electrons and increase the thermionic 

 current ; when it is negative it will decrease the 

 current. Hence, if it alternate in potential, it will 

 cause the valve of the current to oscillate — in 

 effect superposing an alternating current on a 

 direct or one-directional current. If the primary 

 coil of an induction coil be inserted in the plate 

 circuit, oscillations will be set up in it when the 

 grid potential alternates. These oscillations will 

 be reproduced in the potential at the ends of the 



Q 



