the Galvanometer used in Electrical Discharges, 171 



lims. The mirror was above the frame which was surrounded 

 by the wire ; and the whole was protected by a bell-jar. When 

 the magnetic needle was removed and replaced by a brass needle 

 of the same size, and the mirror with its affixed needle was sus- 

 pended by two cocoon-threads, by which the moveable system 

 obtained a definite position of equilibrium, it was observed that 

 this position of equilibrium was altered when a strong discharge 

 was passed through the galvanometer-wire. The alteration in 

 the position of equilibrium could not be due to a change in the 

 magnetism of the needle ; for there was no magnetic needle in the 

 apparatus. When the glass globe was carefully removed and the 

 mirror investigated, it was found to be electrical. This alteration 

 in the position of equilibrium was thus due to the fact that in 

 the discharge electrical induction was produced in the moveable 

 parts of the instrument, which acted electroscopically on the 

 fixed parts and produced an altered position of equilibrium. 



Hence the moveable parts had to be constructed in such a 

 manner that the electrical action between them and the fixed 

 parts should be unable to turn the moveable system about its 

 own axis. It is clear that if the moveable body suspended by a 

 cocoon-thread were bounded by a surface of rotation the axis of 

 which were the prolongation of the cocoon-thread, and if the sur- 

 face were made of a conducting material, the electroscopic action 

 between this body and the fixed parts of the instrument could 

 not effect any rotation about the axis in question. If electricity 

 of either kind has collected upon any place (for instance on the 

 gutta-percha-covered wire), this induces electricity in the body 

 in question : the electricity of the opposite kind collects in 

 the point nearest to the fixed attracting point, and the other elec- 

 tricity is driven to the furthest. But if the body is bounded 

 by a surface of the kind mentioned, it is readily seen that the 

 line of junction between the fixed point of action and the two 

 corresponding points upon the moveable body will go through 

 the axis of rotation, and there can thus be no rotation. All 

 that could possibly happen is, that the system would be attracted 

 a little on one side, so that the axis of rotation would no longer 

 be vertical ; but no rotation can be thereby produced, provided 

 the centre of gravity of the system lies in the axis. But since 

 a plane mirror is necessary for reading off, the moveable system 

 cannot have the form in question. I have accordingly endea- 

 voured to obtain this object in the following manner : — 



The glass mirror which I previously used was exchanged for a 

 round plane-polished silver mirror, the diameter of which was 

 30 millims. The object of this was to remove the non-conduct- 

 ing glass. Both above and below this mirror, and in direct con- 

 tact with it, a horizontal circular disk of thin metal foil was placed. 



N2 



