ELECTRONIC MAGNITUDES 305 



direction when a transverse electrical field is impressed 

 upon it. 



The force deflecting the stream, when it is subjected 

 to a magnetic field of intensity H, is obtained from equa- 

 tion (3) of page 207 as F = qvH if q represents the total 

 quantity of electricity transferred along a length L 

 of the path in a time t (and hence with a velocity of 

 v = L/t). Similarly if the electrical field intensity is 

 E the force acting on the quantity q which is con- 

 tained by this length of beam is F = Eq. The velocity 

 of the particles was measured by J. J. Thomson by 

 opposing the actions of these two fields and adjust- 

 ing their values until there was no deflection, hi which 

 case Hvq equals Eq and v is E/H. 



Having determined hi this way the velocity of the 

 electrons in a particular cathode ray it was possible 

 to find the ratio of the charge, e, on each electron to 

 its mass, m, by the deflection of the ray under the ac- 

 tion of an electrical field only. The case is exactly 

 analogous to that of a bullet shot in a horizontal line 

 with a velocity of v t except that the medium in this 

 case is ether and offers no friction. If the accelera- 

 tion at right angles to this motion is a, then in a time 

 t the bullet will travel downward the distance s = at 2 /2 

 and horizontally the distance L = vt, following a para- 

 bolic path. Now a is always F/m, and this was Ee/m 

 in the experiment and hence s was 



~ 



Of these terms s and L are directly observable and v 

 is known from the preceding experiment; hence e/m 



