A DIRECT COMPARISON OF ELECTROSTATIC 



ultl exist at its sides or back, as these were at the same potential with the 

 surrounding surfaces. 



The large disk was mounted on a slide worked by a micrometer-screw. 

 The qmftll disk was suspended on one arm of a torsion-balance so that in its 

 position of equilibrium its surface and that of the guard-ring were in one plane. 



If E is the difference of potential between the two disks in electromag- 

 netic measure, the attraction between them is 





where a is the radius of the small disk, b its distance from the large one, 

 and v is the velocity representing the ratio of the electromagnetic to the elec- 

 trostatic unit of electricity. 



The electromagnetic force observed was the repulsion between two circular 

 coils, of which one was attached to the back of the suspended disk, and the 

 other was placed behind the large disk, being separated from it by a plate of 

 glass and a layer of Hooper's compound. A current was made to pass through 

 these coils in opposite directions, so as to produce a repulsion 



9 A 

 ' 



(2), 

 where n and ' are the number of windings of each coil, y is the current, and 



* 



a, and a, are the mean radii of the coils, and &' the mean distance of their 

 planes, and E e and F t are the complete elliptic functions for modulus c = siny. 



2 A <2d' 



When b' is small compared with a', -jr- becomes very nearly -p- 



If we take into account the fact that the section of each coil is of sensible 

 area, this formula would require correction; but in these coils the depth v;i* 

 equal to the breadth of the section, whence it follows, by the differential 



