414 



Mr Rutherford, The Discharge of 



[Feb. 21, 



plates. The actual velocity of the carrier when the e.m.F. was 

 700 volts was 450 cms. per sec. 



From considerations based on the kinetic theory of gases it 

 can readily be shown that a carrier of molecular dimensions 

 carrying an atomic charge attains a limiting velocity in a uniform 

 field after a very short interval of time and it then moves uni- 

 formly. No correction therefore need be made for the interval 

 that elapses before the carrier reaches its limiting velocity. 



Effect of pressure on the velocity of the carriers. 



The effect of pressure on the velocity was investigated by 

 means of the apparatus shown in Fig. 3. 



The following table shows the results obtained for air : 



These results show that down to a pressure of 34 mm. the 

 velocity of the carrier is inversely proportional to the pressure. 



As Stoletow has shown, it was found that the rate of leak for a 

 given e.m.f. increased with the diminution of pressure so that the 

 results were capable of fair accuracy even at low pressures. 



The plates were never separated by more than 1*5 cm. but the 

 electromotive force employed was much smaller for the lower 

 pressures. In the experiment for the pressure of 34 mm. only 

 jL- of the voltage of the transformer was used. 



If the law that the velocity is inversely proportional to the 

 pressure holds for low pressures at a pressure of 1 mm. with a 

 potential gradient of 1000 volts per cm. the carrier would travel 

 with a velocity of 1*4 x 760 x 1000 = 10 6 cms. per sec. 



