Two lout produced in Gases l>y Hontgea Radiation, 123 



Suppose now a current of gas to move through the plates 

 from A to B, having in all of its parts a uniform velocity of 

 H centimetres per second. 



The resulting velocity for the positive ions will be H — u— , 



and for the negative ones H + v-7. 



By properly changing the value of U the positive ions can 

 be made to move from A. to B, from B to A, or to have a zero 

 resultant velocity. 



In the last case H = w -y. 

 a 



By reversing the poles of the battery C the negative ions 



can in turn be forced to move against the stream, and a 



potential-difference V be found such that now 



Hence it follows that 



a 





In practice, however, it is impossible to obtain a perfectly 

 uniform stream of gas, or a uniform potential gradient between 

 the plates. The potential gradient is changed by the presence 

 of free charges due to the separation of the two kinds of ions 

 (§ 11); and the electric force thus varies for different points 

 between the plates, and it cannot be assumed to be propor- 

 tional at any point to the voltages used on the plates. This 

 necessitates the experimental determination of the potential 

 gradient between the plates, as from it is obtained the actual 

 electric force which acts to bring the ions to either plate. 



Owing to the irregularities in the gas-stream, some ions reach 

 the plate A while their velocity due to the electric force 

 is less than the average velocity of the opposing blast ; so 

 that it is not possible to find very accurately when the two 

 opposed velocities are equal by simply increasing gradually 

 the electric force, and observing the charges that reach the 

 plate. 



But the irregularities that exist in the gas-stream are quite 

 uniform when averaged over a period of time, so that for 

 different determinations the gas-blast may be considered as 

 an unknown constant quantity. And for obtaining the ratio 

 of the two ionic velocities, instead of finding under what 

 electric forces the two ionic velocities are separately just 

 equal to that of the blast, it is sufficient to find what electric 



