878 Prof. J. S. Townsend and Mr. V. A. Bailey on 



the ratio Z/Z 1? where Z : is the force corresponding to R as 

 given by the normal distribution curve. 



With a stream of electrons the ratio R for a fixed force 

 Z diminishes as the pressure of the gas is reduced, and in 

 this respect differs from a stream of ions. 



6. The velocity W of the electrons in the direction of the 

 electric force is determined by deflecting the stream with a 

 transverse magnetic force. The magnetic force was produced 

 by a current in two large circular coils, outside the air-tight 

 case containing the apparatus shown in fig. 1. The coils 

 are fixed in positions to give a uniform magnetic field in the 

 space between the plate B and the electrodes E. The two 

 electrodes E 2 and E 3 were connected together and the 

 current in the coils adjusted so that the charge acquired by 

 E 2 and E 3 is equal to that acquired by E 1# The centre of 

 the stream is thus deflected to the centre of the narrow gap 

 between E x and E 2 , a distance of 2*5 millimetres, 



If B. be the magnetic force required to produce this 

 deflexion, a the distance from B to the electrodes E, 6 the 

 distance between the centres of the two air gaps on either 

 side of E 2 , the velocity W of the electrons in the direction 

 of the electric force Z is given by the equation* 



HW_ b_ l_ 



Z ~ 2a 16 ' 



There was a small difference between the values of the 

 magnetic force required to deflect the centre of the stream 

 to the centres of the two gaps, which showed that when 

 there was no magnetic force the centre of the stream fell 

 on a line one tenth of a millimetre from the centre of the 

 electrode E 2 . This was no doubt due to a slight inaccuracy 

 in the construction of the apparatus. 



The value of H used in obtaining W from the above 

 formula was the mean of the two forces required to deflect 

 the stream to the centres of the two gaps. 



7. It is necessary to show from the results of the experi- 

 ments, how to distinguish between a stream consisting 

 entirely of free electrons and a stream consisting partly of 

 free electrons and partly of ions formed by electrons becoming 

 permanently associated with molecules of the gas. 



In a stream of free electrons moving in a gas the final 

 velocity W in the direction of the electric force, and the 

 final value of the factor k by which the energy of agitation 

 exceeds the normal, must depend only on the ratio of the 



* Proc. Roy. Soc. A, lxxxvi. p. 571 (1912). 



