THE ELECTRON THEORY. 295 



in the same sort of an orbit as a cannon ball, and that the accelera- 

 tion qe/m corresponds to the acceleration of gravity g in the 

 case of a cannon ball. Therefore, using qe/m for g in equation 

 (i), we have 



'-2E 



or 



Action of the magnetic field on a moving charged particle. 

 Figure 217 represents a charged particle moving upwards through 

 a magnetic field, the lines of force of which are perpendicular to 

 the plane of the figure. The moving particle is equivalent to an 

 electric current, and the side force F is equal to gvh where q 

 is the charge on the particle in abcoulombs, v is its velocity in 

 centimeters per second, and h is the intensity of the magnetic 

 field in gausses. Therefore the acceleration of the particle in the 

 direction of F is gvh/m. The force F is continuously at right 

 angles to v so that the particle describes a circular orbit. But 

 the acceleration of a particle moving in a circular orbit is t^/r, 

 and the relation between the radius of the circle r, the semichord 

 Z), and the versed sine d is 



Therefore we have 



gvh 

 m 

 whence 



SL _ / \ 



m ~i, W 



Determination of velocity of particles. Reduced to the 

 simplest terms, the method of determining the velocity may be 

 described as follows : An electric field e in the plane of the paper, 

 Fig. 216, and a magnetic field h at right angles to the plane of 

 the paper in Fig. 217 are adjusted so that together they produce 



