.«nr{^ l^* oj the Induction Coil. '^ .>*i^ 379 



racter of frictional and voltaic electricity, and I believe that I 

 have now succeeded in indicating a path through which this 

 important object may be obtained. In order to examine the 

 thermal eiFects of the Ley den jar, I had recourse to a modifica- 

 tion of Sir W. Snow Harrises thermo-electrometer, which I intro- 

 duced about twenty-six years since, in order to adapt the mstru- 

 ment for voltaic as well as frictional electricity, and for which, 

 in 1844, I received the first bronze medal of the Devonport and 

 Stonehouse Polytechnic Exhibition. 



4. My apparatus differs from that of Sir W. Snow Harris only 

 in the construction and arrangement of the air-vessel which 

 contains the wire. In Sir W. Snow Harrises instrument (Phil. 

 Mag. No. 73. May 1856, p. 343, fig. 5), the wire passes hori- 

 zontally and diametrically through a glass globe, having external 

 brass caps on the ends of the wire for the purpose of making 

 connexions. The air-chamber which I employ is a cylinder 

 about 3- inches diameter, with two necks like a small electrical 

 cylinder placed vertically. The necks are capped with brass ; the 

 lower one screws upon the reservoir of the bent indicating tube, 

 ^nd the upper one is furnished with a stuffing-box, through 

 which passes vertically a sliding wire surmounted with a binding 

 screw, and formed at the lower and inner extremity into a for- 

 ceps. The test wire hangs vertically in the axis of the cylinder, 

 the upper end being attached to the sliding forceps, whilst from 

 the lower end hangs a short detached forceps, the tail of which 

 can be lowered into a brass cup containing mercury. This cup 

 is in connexion with the brass screw of the reservoir upon which 

 the cylinder is screwed, and to which is also attached a second 

 binding screw for completing the circuit. There is also an ad- 

 justing valve in the cap of the reservoir. The advantages of this 

 arrangement are, that it affords the greatest facility for changing 

 the wire, the stuffing-box being made to unscrew from the cap of 

 the cylinder, whereby the wire with its lower forceps can be lifted 

 out of the mercury and other wire substituted, or the length of 

 the same wire varied in a few moments. The instrument also 

 affords facilities for examining the effects of the voltaic arc, &c. 

 The principle, however, is the same as that originally contrived 

 by Sir W. Snow Harris, who has the merit of having first applied 

 it as a measure of electrical forces ; and with the exception of 

 the air-chamber and its appliances just described, is in all other 

 respects similar in construction to his. 



5. I was anxious to know whether the discharge from the 

 Ley den jar, when charged by the induced current, possessed the 

 same character as that obtained from the charge of an electrical 

 machine ; for I could scarcely bring myself to believe, that, in 

 the rapid succession of discharges which I could procure from 

 my coil, amounting sometimes to more than 200 per second 



2 C 2 



