Electric Absorption of Crystals. 417 



of the handle, one forward and the other back, the crystal 

 condenser could be successively charged from the Leyden 

 battery, discharged, the guard-ring raised, the upper plate, 

 a, again insulated, and the connexion 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 k. By 

 the up-and-down motion of the rod the ball came into contact 

 with the ball (I) connected with the earth, or the ball (on) con- 

 nected with the battery. When the cranks were in the posi- 

 tion 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, 5, and connected with the quadrant- 

 electrometer by a very fine wire. It could also turn around 

 a pivot at£; so that when the ring u rested upon it, it fell on 

 the upper condenser-plate a, and connected it with the elec- 

 trometer : when the weight u was raised by the crank v, 

 the rod rested against /, and so connected the electrometer to 

 the earth, to which the other quadrants were already con- 

 nected. 



At the beginning of an experiment, the insulating plate to 

 be tested having been placed between the condenser-plates a 

 and b, 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 / were such that o for this position of 

 the handle did not touch to, and so h remained in connexion 

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

 and thus also with b. On now turning the handle further, 

 the ball h descended to the ball m, and thus charged the 

 condenser for any time desired. On now reversing the 

 motion, the following operations took place: — 



First, the ball h rose and discharged the condenser. 



Second, the guard-ring d ascended. 



Third, the rod q, which had been previously in contact with 

 p, thus bringing the quadrant- electrometer to zero, now moved 

 down and rested on the upper condenser-plate a. Thus any 

 return charge quickly showed itself on the electrometer. 

 The amount of deflection of the instrument depends upon the 

 character of the dielectric, its thickness, the charge of the 

 battery, the time of contact with the battery, and upon the 

 length of time of discharging. 



