160 



Adams — Electromagnetic Effects of 



nicated directly to the magnetic system. But when the speed 

 exceeded 60 revolutions per second the needle began to vibrate 

 sufficiently to make observations difficult. 



The following is the method used in taking a series of read- 

 ings. The motor was started, and the time of 2500 revolutions 

 of the axle determined. The spheres were then charged ; two 

 elongations of the needle on one side of the zero-position were 

 read, and one on the other side; the electrification was then 

 reversed, and similar readings taken. Sometimes the readings 

 were not taken until the needle had come nearly to rest, and 

 its new equilibrium position estimated. This was repeated 

 until about ten reversals had been made. The speed was 

 then again determined. The average of the deflections, and 

 the average of the two values of the speed were taken as repre- 

 senting the series. The needle constant was determined before 

 and after a number of series. 



An attempt will be made in the following to compare the 

 results obtained with the results expected from theory. From 

 reasons which will appear later this comparison can be regarded 

 only as approximate, and is given merely to show that the 

 observed results are of the right order of magnitude. Row- 

 land's method of using the experiment to determine V, the 

 ratio of the units, will be used. 



The magnetic force produced by a moving charge, q, travel- 

 ing at velocity i\ is 



.■2S- 



H: 



qv sin € 



where p is the radius vector drawn from the charge to the point 

 at which H is measured, and e is the angle between p and the 



direction of motion. This expres- 

 sion holds only in case the veloc- 

 ity v is small compared with the 

 velocity of light. This may also 

 be taken as the magnetic force 

 produced by a moving charged 

 sphere, the charge being supposed 

 concentrated at its center. The 

 force acts in a direction perpen- 

 dicular to p and to the direction 

 of motion. 



The magnetic force at either the 

 upper or lower needle, due to one 

 of the spheres at any point in its 

 path, is found as follows : 



The two sets of spheres revolve 



* J. J. Thomson, Phil. Mag., xi, p. 236, 1881; Heaviside, Electrical Papers, 

 vol. ii, p. 505. 



