176 



Professor Sir J. J. Thomson 



[June 8, 



waves is now very general, and hundreds of thousands of such valves 

 are now in use for this purpose. 



In these valves a stream of electrons is liberated from a cathode 

 of tungsten wire heated by an electric current to incandescence. The 

 principle by which these valves act as detectors of electric waves, in 

 which the electric force is first in one direction and then in the 

 opposite, will be understood if we consider the relation between the 

 current through sucli a tube and the potential difference between the 

 terminals. This relation is shown by the graphs in Fig. 1, which 



20 40 60 Vo\H 



Fig. 1. 



represent the result of experiments made years ago in the Cavendish 

 Laboratory. The current at first increases with the voltage, but 

 then bent over so that for a certam range the current was independent 

 of the voltage. Suppose now we apply to the tube a voltage just 

 above that corresponding to the beginning of the "knee," then a 

 diminution in the voltage will produce a great diminution in the 

 current, while an increase in the voltage would not affect the current. 

 Thus when electric waves fell on such a valve the positive part would 

 not increase the current, while the negative would diminish it ; thus 

 every time an electric wave fell on the valve, the current would be 



