﻿142 Prof. E. Rutherford on the Retardation of the 



reversal of the magnetic field. The actual photograph 

 obtained in this way is shown in fig. 3 (PL II.) 



[Since the layer of radium was of sensible thickness, the 

 a particles escaped from the radium over a wide range of 

 velocity, and the rays were unequally deflected by the mag- 

 netic field. The inner surface of the deflected bands was, 

 however, produced by the rays of maximum velocity from the 

 radium, which it is our object to determine. The distance, 

 for example, from the right edge of the photographic trace, 

 obtained without a magnetic field, to the inside of the deflected 

 band on the left represents the deflexion of the rays of maximum 

 velocity expelled from the radium. The deflexion was com- 

 pared with that due to the rays from a bare wire coated with 

 radium C. In this way, it was found that the initial velocity 

 of the cc particles expelled from radium is *76Y — a value 

 which agrees closely with the value '75 V calculated from 

 the formula. By observation of the outside limit of visible 

 photographic effect, the lowest velocity of the a rays which just 

 produced visible photographic action was found to be '5lVo« 



Magnetic Deflexion of the a Particles from Radium A. 



Since the activity of radium A decreases to half value in 

 3 minutes, an accurate measurement of the magnetic de- 

 flexion of its rays by the photographic method was a matter 

 of some difficulty. The radium A was collected on a thin 

 wire by exposing it for 3 minutes in the presence of a large 

 quantity of emanation. Before the introduction of the wire, 

 the emanation had been exposed to a strong electric field so 

 as to remove as much as possible of the radium C which has 

 a tendency to collect in the emanation vessel. During the 

 time of exposure, 3 minutes, the wire collects one half of the 

 equilibrium amount of radium A upon it and very little 

 radium C. A photographic plate was first placed in position 

 in the apparatus shown in fig. 1. The active wire was then 

 quickly removed, placed in position, and the cover fixed on. 

 Not more than half a minute elapsed after the removal of the 

 wire before the apparatus was in position between the poles 

 of the magnet, and the vessel exhausted to a pressure of a few 

 millimetres ot mercury. The field was reversed every half 

 minute for six minutes, when the plate was removed and 

 developed. 



After intensification, the deflected bands were sufficiently 

 clearly defined to be measured. The amount of magnetic 

 deflexion was 1*14 times as great as for a bare wire coated 

 with radium C exposed under the same conditions. This 



