1920] Thermionic Valve in Wireless Telegraphy & Telephony 171 



valve is sensitive to feeble electric oscillations and rectifies them, 

 not by quite suppressing all current in one direction, but because 

 the thermionic current is greater for a given E.M.F. applied in one 

 direction in the thermionic current than when that E.M.F. is applied 

 in the opposite direction, whilst the mean value of the thermionic 

 current throughout the complete cycle is greater than its value when 

 the alternating E.M.F. is not applied. 



We must now turn to consider an improvement which was 

 introduced in 1907 into the thermionic value, for which credit must 

 be given to Dr. Lee de Forest. He placed a grid or zig-zag of wire 



Fig. 6. — Conventional Diagram of a 

 Three-Electrode Valve. 



P, a metal-plate or cylinder in a highly exhausted glass bulb. 

 G, a grid or perforated plate or spiral wire. F, the lamp filament. 

 B 1} the filament heating battery. 



carried on a separate leading-in wire between the plate and the 

 filament of my valve, and thereby made what is now called a three- 

 electrode valve (Fig. 6). 



In modern thermionic devices the grid takes the form either of a 

 spiral wire or else a metallic gauze cylinder, which surrounds the 

 filament without touching it, and is in turn surrounded by the plate 

 or cylinder which does not touch the grid (Fig. 7). This addition 

 enables the valve to act as an amplifier of electric oscillations as 

 follows : — 



Suppose we insert in the external plate circuit a battery B 2 (see 

 Fig. G) giving an E.M.F., say, of 100 volts, and also a current- 

 measuring instrument A. If the battery has its positive terminal 



