1907.] 



on 



Rays of Positive Electricity. 



581 



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 rav. 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. 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 



Fig. 3. 



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 opening through 

 which the rays emerge as the screen on which the phosphorescence had 

 been observed, was placed just behind the wire, and was read by a 

 reading microscope with a micrometer eyepiece. When the magnetic 

 field was put on, the wire was deflected : and if T is the tension of 

 the wire, p the radius or curvatm-e into which it is bent, / the current 



through the wire, 



H 



or, if y^ is the vertical displacement of the wire, 



^1 



