50 DETERMINATION OF SPEED [CH. v 



u = . 

 //xi c; 



Hence 



m 







This method, when applicable, appears to give fairly 

 accurate results ; and the outcome of the measure- 

 ments is, that when H or C0 2 or Air is in the tube, 



u 2 or 3 x 1 9 centimetres per second, 

 and = from 1*1 to 1*5 x 10~ 7 c.g.s. units. 



e 



The chief difficulty about this mode of experimenting 

 is caused by the fact that the ionisation of residual 

 air in the tube causes it to become a temporary 

 conductor, thereby screening the flying particles from 

 most of the electrical influence. There is no 

 guarantee that they feel the full effect of the electric 

 field which is ostensibly being applied ; indeed it 

 is not easy to let them feel any of the effect. It 

 used to be thought that they were not susceptible 

 to electrostatic action at all, and this was often 

 adduced as an obvious argument against their 

 being electrically charged particles ; but fortunately 

 Thomson soon surmised the cause of this masking 

 of the simple effect to be expected, and succeeded 

 in showing that with high enough vacua, and other 



Erecautions, the screening ionised atmosphere could 

 e removed, and the electrostatic deflexion metrically 

 observed. 



Measuring Velocity by combined Electric and 

 Magnetic Deflexion Method. 



Now that it is possible to apply the electrostatic 

 deflexion method to curve the path of flying charged 



