286 Mr. E. P. Adams on the Electromagnetic 



rentier further investigation desirable, and the present paper 

 contains a description of an experiment with this end in 

 view. 



All previous experiments have been made with rotating 

 disks. With one exception, the direct effect of a charged 

 rotating disk upon a magnetic needle has been examined. 

 The exception referred to is the method employed by 

 Cremieu in one of his experiments, where he sought to 

 observe the inductive effect of charging and discharging a 

 rotating disk upon a neighbouring coil of wire in circuit with 

 a sensitive galvanometer. 



The use of charged spheres was suggested by Professor 

 J. J. Thomson* in 1881, and he gives a calculation of the 

 magnetic force which would be produced by the motion of a 

 sphere charged to the highest possible potential. In many 

 respects this seems the most natural method of procedure and 

 was adopted in this experiment. The description of the 

 apparatus employed follows. 



A hollow brass shaft, A A (figs. 1 & 2), is separated into 

 two portions by the hard-wood bar B. The shaft turns in 

 fibre bearings, and the pulley and belt at D communicate 

 power from the countershaft F F. The spheres which carry 

 the electric charges are spun out of sheet copper into hemi- 

 spheres, soldered together. There are two sets of spheres, 

 sixteen in each set. Brass rods pass through the hollow 

 spheres, and are soldered to them. These brass rods are 

 screwed into collars carried on the axle. The two sets of 

 spheres are thus insulated from each other by the hard-wood 

 bar, 8 cms. in length. The electricity is communicated to 

 the two portions of the axle by the copper brushes C C, 

 and the spheres, being connected with the axle, thus become 

 charged themselves. 



The speed-counter, E, is directly attached to one end of 

 the axle. The gearing is so proportioned that one revolution 

 of the crown-gear and dial 1 corresponds to 500 revolutions 

 of the axle. 



The magnetic system upon which the direct effect of the 

 moving charged spheres is observed, is inclosed in the brass 

 tube H, closed on the bottom by a glass plate coated with 

 tinfoil. This tinfoil is cut into strips parallel to the axle 

 for the purpose of preventing conduction -currents flowing 

 in it in a direction in which they could cause a deflexion of 

 the needle. The needle-system is carried on a piece of 

 mica 7*5 cms. long and 1*25 cms. wide. Pieces of well- 

 hardened magnetized watch-spring are cemented at the lower 

 * Phil. Mag. vol. xi. p. 236 (1881). 



