228 MR. HARRIS ON SOME ELEMENTARY LAWS OF ELECTRICITY. 



circuit than with a short one ; and when it consisted of imperfect conductors, such as 

 wood, or water contained in a glass tube, the instrument was scarcely at all affected. 



41. When long circuits of metallic wire were employed, the effect varied in an in- 

 verse ratio of the length. Thus, with an insulated circuit of 300 feet of thick copper 

 wire, the transmission of a given quantity through the electrometer, N, elevated the 

 fluid ten degrees ; with a circuit of 600 feet, the resistance was such that it only rose 

 between five and six degrees ; with 900 feet, rather more than three degrees. 



42. This law was not so fully apparent on circuits of 50 or 100 feet, circum- 

 stances, necessarily involved in the experiment, being such as greatly to interfere 

 with an exact result on short lengths of metal ; thus the final equalization of the elec- 

 tricity through the metallic coatings of the battery, as also through the connecting 

 rods and the like, seemed of little consequence when great lengths of circuit were 

 compared, but interfered considerably in small ones, the resistance of each compa- 

 rative circuit being increased by this constant. 



43. These experiments on the resistance of Conducting substances to the transmis- 

 sion of electricity through them, will enable us better to appreciate the kind of resist- 

 ance arising from a non-conducting medium, such as air, as in the following cases. 



The electro-thermometer N, fig. 16, (Plate IV.) was placed in connexion with the 

 opposed spheres c d in the receiver R ; the spheres were separable to a greater or less 

 extent by mccins of a brass rod sliding through an airtight collar on the glass plate p, 

 the distance being regulated by a micrometer-screw and index at p : the receiver was 

 connected with a good air-pump at r, furnished with a long mercurial gage, g, and 

 had within it a thermometer, R, to indicate the temperature of the contained air. The 

 temperature could be raised considerably, when required, by means of a metallic en- 

 velop, fig. 18, and a powerful lamp at D : this envelop was so contrived as to be 

 easily removed at the time of experiment, without disturbing the fixed pieces q q\ 

 fig. 16, and cross-bars of glass q y, by which the expansive effect of the heated air on 

 the plates p p was effectually resisted. 



{].) A given quantity being accumulated in the jar E, it was discharged between 

 the balls c c, placed at different distances apart, within the extreme limit of the 

 distance at which the accumulated electricity could of itself escape. In order to 

 effect this, the jar was discharged by the drop-ball f^ which was allowed to fall with 

 force on a small plate of varnished glass a, placed on the opposed ball a in connexion 

 with the positive coating : by this the transmission of the electricity became impeded 

 up to the point of fracture of the glass, as appeared by the retention of the charge, 

 when the ball f rested on a. The results of thirty successive experiments gave an 

 invariable effect on the wire c d, at whatever intermediate distance the balls c d were 

 placed within the limits of the whole discharging distance. 



By diminishing the density of the air, the discharging distance could be extremely 

 increased : the effect, however, on the instrument remained the same. 



44. The ball c being connected with the positive coating, and c' with the negative 



