Rays of Positive Electricity, 7f>l 



they strike, we can explain why the velocity of these rays 

 should be independent of the potential difference between 

 the electrodes in the tube. For in the first place, the positive 

 and negative charges will not unite unless their relative 

 velocity falls below a certain value which does not depend 

 upon the strength of the electric field, and in the next place 

 if the velocity were less than a limiting value they would 

 not dissociate themselves nor could they dissociate other 

 molecules by collisions when moving through a gas. The 

 first condition gives a superior limit to the velocity, the 

 second an inferior one ; and both are independent of the 

 strength of the electric field. 



I shall now proceed to give the details of the measurements 

 of the values of e\m and v. These constants were determined 

 by measuring the magnetic and electrostatic deflexion of the 

 rays. If y is the deflexion due to the magnetic force, e the 

 charge on the particle and v its velocity, 



y = ^S a {l -* )mx < 



where x is the distance, measured along the undeflected rav, 

 from the end of the tube through which the rays enter the 

 magnetic field, H the magnetic force at the point x, and I 

 the distance of the screen from the end of this tube. The 



value of I (I— x)Hdx was determined by measuring the 



magnetic induction through a triangular coil with its base at 

 the end of the tube and its apex at the screen (see Phil. 

 Mag Nov. 1D09). If n is the number of turns in this coil, 

 d the base and I the perpendicular from the apex on the 

 base, I the magnetic induction through the coil, then 



1 = ^ ( l (l- x )Bdx; 



-W~ 



hence if we know I we can deduce the value of the integral ; 

 the coil was made so narrow that for a given value of x the 

 magnetic force was constant over the coil. 



The induction \^as measured by means of a Grassot 

 fluxmeter, using for the sake of greater accuracy the de- 

 flexions of a beam of light reflected from the back of the 

 instrument instead of the usual index and scale. 



The fluxmeter was standardized,, (1) by measuring by 

 means of it the induction through one of a pair of coaxial 

 solenoids when a known current was broken in the other, 



