founded on Induction and Convection. 



71 



a metal plate at a considerable distance from it), it gives by or- 

 dinary tests, as is well known, indications of positive electrifica- 

 tion towards the one end P, and of negative electrification 

 towards the other end N. The wheel in the arrangement now 

 described is kept turning at a rapid rate ; and the effect of the 

 carrier is to produce in the springs T A, T' A' the same potentials, 

 approximately, as those which would exist in the air at the points 

 T, T' if the wheel and springs were removed. The springs 

 being connected with the electrodes of the divided-ring qua- 

 drant electrometer, the spot of light is deflected to the right, let 

 us say. After continuing the application of heat for some 

 time the hot plate is removed, and a little later the spot of 

 light goes to zero and passes to the left, remaining there for a 

 long time, and indicating a difference of potentials between the 

 springs, in the direction A' T' positive and A T negative. The 

 electrometer being of such sensibility as to give a deflection of 

 about 100 scale-divisions to the right or left when tested by a 

 single galvanic cell, and having a range of 300 scale-divisions on 

 each side, it is necessary to place the tourmaline at a distance of 

 several inches from the disk to keep the amount of the deflection 

 within the limits of the scale. 



Another application of this instrument is for the experimental 

 investigation of the voltaic theory, acording to the general prin- 

 ciple described in the communication to the Royal Society 

 already referred to*. In it two inductors are placed as repre- 

 sented in fig. 4. The inner surface of each of these is of smooth 



Fig. 4. 



brass ; and one of them is lined wholly, or partially, with sheet 

 zinc, copper, silver, or other metal to be tested. Thus, to expe- 

 riment upon the contact difference of potentials between zinc and 

 copper, one of the inductors is wholly lined with sheet zinc 

 or with sheet copper, and the two inductors are placed in me- 

 tallic communication with one another. The springs are each 

 in metallic communication with the electrodes of the quadrant 

 mirror electrometer, and the wheel is kept turning. The spot 

 of light is observed to take positions differing, according as the 

 lining is zinc or copper, by 72J per cent, of the difference pro- 

 * Proceedings of the Royal Society, May 1867. 



