332 Mr. J. N. Hearder on a new Instrument for Registering 



correct indicator, or otherwise, of the effect of single electrical 

 impulses of great power, such as the discharge of the electrical 

 battery, &c., it appears much better adapted for the examination 

 of the effects of continuous voltaic currents, or of rapidly recur- 

 ring Ley den discharges of small amount*. 



When a single electrical dischai'ge is passed through the wire 

 of the instrument, an effect is produced varying in a known de- 

 finite relation to the quantity of electricity transmitted. This 

 ajjpcars to be as the square of the quantity ; but the effect pro- 

 duced is a transient one, and subsides instantly after the passage 

 of the discharge, and even before its registration is completed on 

 the scale. With the induction coil, however, the effect is differ- 

 ent, since the fluid remains almost permanently raised under the 

 influence of the reiterated discharges ; and although the units 

 of effect of each discharge, however small, may be subject to the 

 same laws in relation to variations of quantity, surface, &c., yet 

 their accumulated effects do not bear the same relation to sur- 

 face, being, within certain extreme limits, in the direct propor- 

 tion of the coated surface. I have determined this not only by 

 Leyden jars of different sizes, but more accurately by coated 

 panes, all of equal thickness but of different areas. 



In attempting to compare the induction coil with the electrical 

 machine, it was necessary to employ conditions which should be 

 precisely similar in each ; that is to say, to compare units of 

 effect with each other, or to multiply these units equally in both 

 cases and then compare their indications. The former of these 

 modes was extremely easy, but the latter only practicable to a 

 very limited extent, from the difficulty of imitating the effects of 

 the coil with the electrical machine ; but the results which I have 

 obtained sufficiently establish the correctness of the principle by 

 which I have been guided, and prove indisputably the identity 

 of the agency from both sources. In order to imitate the effects 

 of the coil with the electrical machine or with high tension elec- 

 tricity of this class, it would appear necessary to excite it in suf- 

 ficient quantities to charge a jar to the same degree and discharge 



* I Relieve I was the first who employed this instrument for voltaic pur- 

 poses, since in 1827 and 1828 I worked out with it a complete scale of the 

 conducting powers of metals for voltaic electricity, and then discovered the 

 curious anomaly, that with certainvoltaicarrangements the best conductors 

 are the most heated, and that, too, in the precise relation of their conduct- 

 ing power. Whilst engaged in some experiments in which I employed 

 wires externally to the bulb, I noticed the fact that the application of heat 

 or cold to the external wire diminished or increased its conducting power, 

 as evidenced by the effects on the temperature of the wire in the bulb. I 

 communicated the fact to Sir W. Snow Harris, who repeated the experi- 

 ments, and alluded to them in the Edinb.' Phil. Trans, for 1832; and they 

 are described more at length in his recent work on galvanism, page 162. 



