238 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



electrical field is to deflect the a-particles in the direction of the 

 field and the /3-particles in the opposite direction, while the 7-rays 

 are not affected at all. The amount of deflection in each case may 



A. photographic plate Jt 



S-pole of 

 .magnet 



/. ff-pole of 



lead block . / 



V lead block 



^active material 



Fig. 168. 



a-particles deflected towards the reader. 

 /?-particles deflected away from the reader, 

 y-waves not deflected at all. 



be determined by developing the photographic plate upon which 

 the rays impinge. The effect of the magnetic field upon the rays 

 from a radio-active substance is shown in Fig. 168 in which the 

 fine horizontal lines represent the lines of force of a magnetic field 

 between the two large magnet poles. 



The determination of the velocity of the a- and /8-particles 

 is somewhat analogous to the following method for determining 



Fig. 169. 



the velocity of a cannon ball. The curved line in Fig. 169 

 represents the orbit of a cannon .ball, D being the horizontal 

 distance traveled by the ball in a given time and d being the 



