Experiments on Positive Rays. 



215 



example, the case when the magnetic force extends beyond 

 the plates used to produce the electric field and when the 

 magnetic force gets weaker as we approach the cathode. 

 The curve will start from the vertical line at some distance 

 above the origin, and the curve will get flatter and flatter as 

 we travel upwards f rom P ; in this respect it will resemble a 

 parabola. This secondary curve must ultimately cut the 

 primary curve corresponding to its particle at a point 

 on the primary produced by a particle moving with the 

 velocity v. 



Considerable variations in the shape of the secondary 

 curves may be produced by variations in the distribution of 

 the electric and magnetic fields. Thus, suppose that the 

 delivery-tube for the positive rays extends up to one end of 

 the plates so that at this end there is no place where a 

 charged particle is acted on by magnetic and not by electric 

 forces. Then, if the electric and magnetic fields were co- 

 terminous at the other end, the shape of the secondary might 

 be somewhat like that shown in fig. 6', the thick line repre- 

 senting the primary curve. 



6. 



If, however, the magnetic field at the end next the camera 

 overlapped to a considerable extent the electric field,, then 

 the secondary curve might resemble that shown in fig. 7, the 

 secondary curve being now above the primary instead of 

 below it as in fig. 6. 



Thus, if the secondary curves were not completed on the 

 plate, but stopped, for example, at AB (figs. 6 and 7), we 

 might easily mistake the secondary line for a primary due 

 to a particle which in fig. 7 had a value of e/m greater 

 and in fig. 6 less than the primary to which it really corre- 

 sponds. This is a point which requires careful attention 

 when we use the curves produced by the positive rays to 

 analyse the gases in the tube. 



