Rood — Experiments on High Electrical Resistance. 93 



pose. R is a stiff brass wire attached to the same axis with 

 Y, and serves to discharge the apparatus ; it has an arm bent 

 at a right angle which can be brought into contact with the 

 binding post B. A single motion discharges the electrometer 

 and sets the gold leaf free. The axis carrying R and Y is con- 

 nected with the ground. The micrometer, M, is provided 

 with a millimeter screw and carries the plate P. In all of my 

 recent experiments it was connected with the ground. The 

 height of the electrometer from the table, to the top of the 

 binding post, is 27 cm . It is covered by a glass case to prevent 

 air currents, the case being in contact only with the ebonite 

 base. When in actual use the apparatus is covered by a paste- 

 board box coated with tin-foil, through the sides of which 

 suitable openings are made for observation, and for the intro- 

 duction of the two wires that are attached to the binding 

 post. The electrometer as thus described, with a striking dis- 

 tance of a half or of one millimeter, is suitable only for meas- 

 uring resistances of a million megohms and upwards; lower 

 resistances bring about a stroke in a very few seconds, and to 

 measure these, its capacity must be increased. 



Condensers. — A series of these have been employed, the 

 greatest being a large Leyden jar with a capacity of -00507 of 

 a microfarad ; four smaller jars with diminishing capacities 

 and three small mica-condensers have been used. When the 

 electrometer is once made, and a few suitable resistances, such 

 a set of condensers can easily be produced. The mica-con- 

 densers were supported by silk threads that had been heated 

 up in a bath of wax and rosin. A long wooden rod above the 

 apparatus carried the mica-condensers and the units, when 

 more than three or four of them were used in a series, and in 

 this way excellent insulation was readily obtained. 



Table of Capacities of Condensers. 



Capacity in 

 / microfarads. 



Leyden jar, No. L -0050 



" " 2 _.. -0025 



" " 3 -0013 



" " 4 -0008 



" " 5 -00044 



Mica cond., No. 1 -00024 



" " 2 ... -00014 



" " 3 -000037 



Electrometer ... '000014 



It may be remarked that the apparatus described in this 

 paper can easily be used to determine the capacities of quite 

 small condensers, and of conductors or semiconductors sus- 



