Moving Charged Spheres. 



157 



The magnetic system upon which the direct effect of the 

 moving charged spheres is observed, is enclosed in the brass 

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

 foil. This tin-foil 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 deflection of the needle. 

 The needle-system is carried on a piece of mica, ?*5 cm long 

 and l'25 cm wide. Pieces of well-hardened magnetized watch- 



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spring are cemented at the lower edge and near the upper edge 

 of the mica so as to form an astatic system. The needles are 

 placed perpendicularly to the axle. The mirror is also 

 cemented to the mica, a little above its center, and is observed 

 through an opening in the brass tube at S, covered with a thin 

 glass plate. The whole needle-system is suspended by a quartz 

 fiber, 32 cm in length. 



The magnetometer tube H is carried in a brass collar P, 

 which is screwed to the brass plate M. This plate is provided 

 with levelling screws, and rests on the board shelf N, sup- 

 ported at its ends on two brick piers. Underneath the level- 

 ling screws are placed pieces of felt, to take up any mechanical 



