LAWS. — COEFFICIENTS OF SELF-INDUCTION. 



265 



set in sulphur; mechanically, the arrangement leaves nothing to he 

 desired. It has been run continuously at the rate of 3,000 revolutions 

 per minute, without perceptible jarring. 



The machine was at first supplied with a mechanical counter, which 

 could be thrown on or off by means of an electro-magnet operated 

 by a key. We found, however, that the arrangement was uusatisfac- 



Fj&,3 Field Mac.net and Armature 



tory, as variations of speed were introduced by its friction and inertia. 

 Moreover, such an arrangement gives only the average speed during 

 a given time ; we therefore abandoned this counter, and adopted a 

 stroboscopic device. A ray of light is reflected upon the revolving 

 segmented disk from a mirror attached to an electrically driven 

 tuning-fork of known rate. With such an arrangement the desired 

 speed may be attained, and variations from it measured by noting 

 the apparent motion of the disk. 



