Hood — Experiments on High Electrical Resistance, 287 



the plate D with an undesirable twist. D is a plate of sheet 

 brass faced with platinum foil, and receives the charge from 

 the aluminum leaf when its potential has been sufficiently 

 raised. The distance of this plate from the leaf can be varied 

 by a micrometer screw, M, and thus the sensitiveness of the 

 apparatus regulated. When a Leyden jar was used as the 

 source of the electricity, the plate D was connected with the 



A 



R 



N 



D 



M 



earth ; when the street current 

 was employed, D was connected 

 with one of the electric wires, 

 a resistance of two or three thou- 

 sand ohms being interposed, and 

 as only thousands of megohms 

 were dealt with, this amounted 

 to nothing. An arrangement by 

 which a slight vibration could be 

 communicated to D was added, 

 and by a single motion D was 

 vibrated and the electrometer 

 discharged by contact with A. 

 When the electrometer gives a 

 stroke the leaf usually remains 

 attached to the plate D, which is 

 the reason of the above men- 

 tioned contrivance. At A, vari- 

 ous adjuncts can be placed, bind- 

 ing screws, etc. The whole ar- 

 rangement amounts to a kind of 

 unit jar which discharges itself when charged to a certain 

 potential. Over it was placed a glass case to avoid air currents, 

 but the case was in contact only with the base of the apparatus. 

 Two snch electrometers were made by me, one rather large for 

 Leyden jar experiments ; the other was smaller, especially in 

 capacity, and was used only with currents of 110 to 112 volts. 

 Some small electroscopes on the above mentioned plan were 

 also made ; they could be set to indicate certain potentials in 

 bodies either in contact with them or at fixed distances from 

 them. Their capacities were quite small. I also made a 

 Bohnenberger electroscope with its dry pile, the delicacy of 

 which could be varied at will by moving the brass plates con- 

 nected with the terminals of the pile. It was used only in 

 the study of good, insulators. The source of electricity was 

 sometimes a Leyden jar charged to a potential of 12,000 volts, 

 but in the measurements given in megohms a current of only 

 110 to 112 volts was used. This same current was employed 

 in the preparation of standard resistances. It is to be under- 

 stood that the experiments were performed in dry winter 



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