THE CATHODE RAY OSCILLOGRAPH 



25 



transatlantic radio channel, the tube plotting the amplitude of the 

 received signal at a number of modulating frequencies. In Fig. 36 

 are plotted the magnitude and direction of static crashes as they were 

 shown on the screen of the oscillograph tube. 



For demonstration purposes the tube may be used to determine the 

 value of the ratio of charge to mass of the electron, designated by e/m. 

 The classical method is obvious from the derivation of the sensitivity 

 of the tube given above. Especially is this so if we can assume the 



two fundamental relations ^mv^ ^ eV 



ellR. The method is 



Fig. 35 — Transmission characteristic of transatlantic short-wave radio channel.^^ 



subject to some error because the extent of the deflecting fields cannot 

 be exactly determined, and because of the space charges in the tube. 

 A more accurate method of arriving at the value of e/m is due to 

 H. Busch.2* A long solenoid carrying constant current creates a uni- 

 form magnetic field in the tube parallel to its axis. The electrons 

 travel in spirals in this field in such a way that when the field has one 

 of a series of values the electrons are focused on the screen. These 

 critical values of the magnetic field are given by the equation 



H 



_ 2ir jlm. 



V; 



" Potter, R. K., Lnst. Radio Eng., 18, p. 581, 1930. 

 28 Busch, H., Phys. Zeits., 23, p. 438, 1922. 



