208 HENEY A. KOWLAND 



The charging battery consisted of six large Leyden jars of nearly a 

 square foot of coated surface each, charged to a small potential. 

 Although accurate instruments were at hand for measuring the poten- 

 tial in absolute measure, it was considered sufficient to use a Harris 

 unit-jar for giving a definite charge; for very accurate measurements 

 were not desired, and the Harris unit-jar was entirely sufficient for the 

 purpose. The return charge was measured by a Thomson quadrant- 

 electrometer of the original well-known form. 



The apparatus shown in Plate IV performs all the necessary opera- 

 tions by a half turn of the handle e. By two half turns of the handle, 

 one forward and the other back, the crystal condenser could be succes- 

 sively charged from the Leyden battery, discharged, the guard-ring 

 raised, the upper plate, a, again insulated, and the connection made 

 with the quadrant-electrometer. 



The copper ring, d, was suspended by three silk threads from the 

 brass disk, /, which in turn could be raised and lowered by the crank, g. 

 A small wire connected the ring with the rod on which was the ball, h. 

 This rod was insulated by the glass tube i, and could revolve about an 

 axis at fc. By the up-and-down motion of the rod the ball came into 

 contact with the ball (Z) connected with the earth, or the ball (ra) con- 

 nected with the battery. When the cranks were in the position shown 

 in the figure, the heavy ball n caused the ball h to rise and press 

 against I; but when / descended, the piece o pressed on the rod and 

 caused h to fall on m. 



Another rod, q, also more than balanced by a ball, r, was insulated by 

 a glass tube, s, and connected with the quadrant-electrometer by a 

 very fine wire. It could also turn around a pivot at t; so that when 

 the ring u rested upon it, it fell on the upper condenser-plate a, and 

 connected with the electrometer; when the weight u was raised by the 

 crank v, the rod rested against f, and so connected the electrometer to 

 the earth, to which the other quadrants were already connected. 



At the beginning of an experiment, the insulating plate to be tested 

 having been placed between the condenser-plates a and &, the handle 

 was brought into such a position that the ring, d, rested on the plate 

 around a. The lengths of the threads between d and f were such that o 

 for this position of the handle did not touch w, and so li remained in 

 connection with the earth; and so d was also connected with the earth, 

 and thus also with &. On now turning the handle further, the ball li 

 descended to the ball m, and thus charged the condenser for any time 

 desired. On now reversing the motion, the following operations took 

 place : 



