Rays of Positive Electricity. 565 



If y is the vertical displacement of the particle, we have 



1 d 2 // . , n 



— = _£ approximately, 

 p «~- 



where c is measured along the path of the ray Hence 



clhj 



2 = — H ; 

 dzr mv 



y = 



Jo «/ J 



(1) 



In these strong fields there are considerable variations of 

 H along the path, so that to calculate the integrals we should 

 have to map out the value of H along the path of the ray. 

 This would be a very laborious process, and it was rendered 

 unnecessary by the following simple method, which, while not 

 involving anything like the labour of the direct method, gives 

 much more accurate results. The method is shown in fig. 3. 



Fig. 3. 



The part of the tube through which the rays pass was cut off, 

 and a metal rod placed so that its tip Z coincided with the 

 aperture of the narrow tube through which the positive 

 rays had emerged. A very fine wire soldered to the end of 

 this tube passed over a light pulley, and carried a weight at 

 the free end. The pulley was supported by a screw by 

 means of which it could be raised or lowered ; a known 

 current passed through the wire, entering it at Z and leaving 

 it through the pulley. The pulley was first placed so that 

 the path of the stretched wire when undeflected by a magnetic 

 field coincided with the path of the undeflected rays. A 

 vertical scale whose edge was at the same distance from the 



