192o] Thermionic Valve in Wireless Telegraphy & Telephony 165 



in a curve we obtain a so-called characteristic curve (see Fig. 2). 

 The reason the current does not increase indefinitely is because for 

 each particular temperature of the filament there is a certain 

 maximum possible rate of electronic emission. The electrons are 

 drawn away from the filament at a rate which increases with the 

 potential of the plate up to that point at which the maximum 

 emission rate is reached. The thermionic current then becomes 

 stationary and is said to be saturated. 



There is a definite relation between this saturation current 

 reckoned in number of electrons escaping per square centimetre per 

 second and the absolute temperature of the wire. Professor 

 Richardson has deduced from certain theoretical considerations two 

 formulas which express this connection within certain limits of 



POTENTIAL DIFFERENCE OF CATHODE AND ANODE 



Fig. 2. — Characteristic Curve of a Thermionic Tube, 

 Highly Exhausted. 



temperature, and give nearly the same numerical result when 

 compared with experiment. They are as follows : — 



N = AVT 



N = B T 2 e 



where T is the absolute temperature, e the base of Napierian loga- 

 rithms, viz. 2 "718 . . . , and A, B, b, d are certain constants 

 peculiar to each substance. 



For tungsten, Irving Langmuir found 



A = 1*55 x 10 26 b = 5*25 x 10 4 



Taking, for instance, T = 2500° absolute, we find X = 8 x 10 18 , 

 which is about 1 ampere per square centimetre per second. 



These figures show the enormous number of electrons which may 

 be projected per second from quite a small filament of highly 

 incandescent tungsten in a high vacuum. 



