72 



Mr. F. S. Spiers on Contact Electricity. 



Figr. 1. 



nected through the brass spindle S, with which they can 

 revolve, there being a small magnet, N, fixed in the spindle 

 which can be pulled round by an external horseshoe- 

 magnet revolving on a turntable. C and D are two inductors 

 of platinum rigidly fixed, top and bottom, 

 in glass supports. In consequence of the 

 Yolta force, A and B are at different 

 potentials, real or apparent. If now C 

 and D be connected to the respective ter- 

 minals of a quadrant-electrometer, brought 

 to the same potential, and then one of them 

 insulated and A-B turned through 180°, 

 there will be a P.D. induced between C 

 and D which, cceteris paribus, is propor- 

 tional to the contact P.D. between A and B. 

 (See Ayrton and Perry's papers quoted 

 above.) The tube could be connected to 

 a sprengel-pump by means of an india- 

 rubber covered joint in the cup E, sealed 

 in mercury. 



The results obtained were not very satis- 

 factory. The apparatus had been lying 

 about in dusty cupboards for many years 

 with the opening unclosed, so that the 

 insulating-power of the glass supports was 

 anything but high. In consequence of 

 the small capacity of the system, quite a 

 small amount of leakage caused a large 

 change in the potential of the insulated 

 inductor; and it is quite possible that the 

 paraffin-wax in which the terminals G, H 

 were sealed was not a sufficiently good 

 insulator for this purpose. It was not 

 possible to measure the absolute value of 

 the Yolta P.D. between A and B, as the 

 two metals were in permanent metallic 

 contact inside the tube; one could only 

 measure any relative changes that took 

 place in that value. 



Several readings were taken in very high 

 vacua, from about 50V0 mm - 0I> mercury 

 pressure and upwards (as measured on a 

 McLeod gauge), and a decrease of about 2 or 3 per cent, was 

 observed in the deflexion when, the tube w r as exhausted, the 

 original value returning in each case when air was readmitted 

 into the apparatus. In consequence of the vagueness of the 



