THE VAt"UUM there's SOMETHING IN IT WHITNEY 201 



EDISON EFFECT 



Another experiment shows the historic Edison effect and its rela- 

 tionship to pure thermionic currents. Where it was once thought that 

 nearly visible particles of the filament were shot across the space 

 between filament legs in vacua, now we recognize, in very high 

 vacua, only the unidirectional motion of negative electrons. 



RECTIFIERS 



In vacuum tubes like the kenotron these electrons pass from the 

 hot filament to an electrode commonly called the " plate." This pure 

 emission current is the basis for the so-called rectifiers because only 

 when the filament is negative does any current flow across the space. 

 "Wlien gases are present greater currents may be carried, because by 

 the ionization of the gases the moving electrons produce new con- 

 ductors from the gas molecules. Thus the tungar rectifiers, contain- 

 ing a little argon, and the older mercury vapor rectifiers involve the 

 same principle. Without some gas present the negative electrons, 

 by their vei-y concentration, constitute a space charge which limits 

 the current. This space charge is removed by the ions produced 

 within the gas when present. 



RADIOTRONS 



When we interpose a grid or wire screen between the hot filament 

 and the plate of the two-electrode tube or rectifier, we have what we 

 now so commonly use in radio for receiving, for amplifying, and for 

 production of high-frequency currents. The discovery of the con- 

 trolling or triggering action of the third or intermediate electrode 

 was made by De Forest. A negative charge applied to this electrode 

 or grid interrupts or modifies the electron stream, the current, from 

 the hot filament to the plate. As it takes almost no energy to charge 

 this grid (little more than a "token" of energy, or voltage) the 

 slight power from a radio antenna in its fluctuation may be used to 

 control or to trigger or to let through corresponding jolts of greater, 

 energy, which are in turn supplied by some local battery. In the 

 experiment illustrated in Plate 2, Figure 1, an ordinary incandes- 

 cent lamp is lighted by current which is passing through a three- 

 electrode tube from hot filament to plate. Its light indicates this 

 current. The grid, or antenna wire, is sticking up from the tube so 

 that it can " pick up " electric charges from space. A small negative 

 charge, produced on a rod of insulating material by merely rubbing 

 it with a piece of paper, causes the lamp to go out or to light up as 

 the charged rod is brought near to or removed from the exposed 

 end of the grid wire. 



