■ CONDUCTION THROUGH GASES 143 



by a magnetic field also, and this effect too can 

 be measured in the same apparatus. Both these 

 deflections are to be expected if the rays consist 

 of moving electrified particles ; and the directions 

 of the deflections are such that the electrification 

 must be that to which is conventionally given the 

 negative sign. 



The conclusions drawn from these experi- 

 ments are of extreme importance. In analysing 

 the deflections of the particles three things are 

 involved: (i) the velocity; (2) the mass; and 

 (3) the electric charge. For both deflections, the 

 electric and magnetic, the two last quantities 

 appear as the ratio ejin — that is, the charge 

 divided by the mass. If we treat this ratio as 

 a single quantity, we find ourselves with two 

 unknown values to be determined by the two 

 experiments, the one on the magnetic, and the 

 other on the electric deflections. Both the un- 

 known quantities — to wit, the velocity and the 

 ratio ejm — can therefore be found from the results 

 of the experiments. 



When a magnetic force is applied, the spot 

 of phosphorescent light in the tube of Fig. 30 

 is drawn out into a band of appreciable length. 

 This result is a consequence of a difference in 

 velocity of the rays : in any one discharge, rays 

 are found with a considerable range of velocity, 

 and therefore these rays are deflected, according 

 to their velocities, through a series of different 

 angles. 



The following table gives some of the results 

 of Sir J. J. Thomson's experiments, and shows 

 the mean values of the velocity, Vy in centimetres 

 per second, and of the ratio nije for cathode rays, 



