198 Prof. E. Rutherford on the Charge 



When once the magnetic field had attained a certain value 

 a considerable increase in its strength did not affect the 

 magnitude of the current observed. This showed that the 

 field was strong enough to bend back all the electrons and to 



o © 



prevent them reaching the opposite plate. The field was not 

 sufficiently strong to appreciably deflect the a particles 

 themselves. 



The magnitude of the charge carried by the a particles can 

 readily be deduced. Let i Q be the current due to the ioni- 

 zation of the residual gas between the plates. A very small 

 E.M.F. is required to produce saturation at such a low 

 pressure, and this current should be equal in magnitude but 

 opposite in sign when the potential is reversed. Let i Y be the 

 charge per second communicated to the upper electrode when 

 the lower plate is charged positively, and i 2 the value when 

 charged negatively. Let n be the number of « particles, 

 carrying a charge e, projected per second into the upper 

 plate. Then 



h = i + ne 



u= — /n + ne. 



Adding 



ii + u 

 ne = „ 



Now in the third column of the table above, it is seen that 



the values of ^^- 2 are '395, -41, '435 for 2, 4, and 8 volts 



z 



respectively. The numbers are thus in fairly good agree- 

 ment. The charge communicated to the upper plate when 

 the lower plate is earthed, is, as we should expect, slightly 

 less than that calculated from the mean of the two currents, 

 since the ionization of the residual gas tends to dissipate it 

 as soon as the potential of the upper plates commences 

 to rise. 



It will be observed that the current between the plates 

 before a magnetic field was applied was greater when the 

 lower plate was negative, and for the potential difference of 

 4 volts was about 9 times the current due to the a particles 

 alone. 



A very different result, however, was observed when the 

 radium-coated aluminium plate was used. In this case, the 

 film of radium was not covered with aluminium-foil. 



With magnetic lield. 



Voltage. 



Without magnetic field 



-f 8 volts. 



-8 „ 



(W) 

 14-0 



'3}« 



