ELECTRICITY. 



(80.) The following experiment of 

 Biot's contains also a striking practical 

 illustration of the same truth. Let a 

 (fig. 17.) represent a section of any 

 spheroid of conducting matter, suspend- 

 ed by a thread which perfectly insulates 

 it. Let c c be two caps formed of gilt- 



Fig. 17. 



paper, tin-foil, or any conductor, and 

 such that, when united, they accurately 

 iit the surface of the spheroids ; and let 

 them be also furnished with insulating 

 handles of gum-lac. Let there be com- 

 municated to the ball, a, any degree of 

 electricity ; and then let the two caps, 

 held by their insulating handles, be care- 

 fully applied to its surface. Upon the 

 removal of these caps, it will be found 

 that the whole of the electricity has been 

 Abstracted from the spheroid, so that it 

 will no longer affect the most delicate 

 electrometer ; whilst the two caps will 

 be found, upon accurate trial, to have 

 acquired precisely the same quantity of 

 electricity which had at first resided in 

 the body a. 



We may conclude, both from theory 

 and experiment, therefore, that al- 

 though, strictly speaking, the electricity 

 must reside within the substance of 

 conducting bodies, it extends, in fact, 

 to a depth so small as to be inappreci- 

 able by any known methods of observa- 

 tion, 



(81.) The effect of an expansion of 

 surface in lessening the intensity of 

 electricity, while its absolute quantity 

 remains the same, is well illustrated by 

 the following experiment mentioned by 

 Biot. Fig. 18 represents an insulated 

 cylinder, a b, moveable round a hori- 

 zontal axis, and capable of being turned 

 by an insulating handle h. Around the 

 -cylinder is coiled a thin lamina of any 

 metal, c, the end of which is semicir- 

 cular, and has attached to it a silk 

 thread /. The whole apparatus com- 

 municates with an electroscope e, formed 

 of two_ linen threads, each terminating 

 m a pith ball. On" communicating a 



charge of electricity to the cylinder, the 

 threads and balls of the electroscope 

 diverge. Upon taking hold of the silk 



Fig. 18. 



thread, and unrolling the metallic lamina 

 from the cylinder, the balls gradually 

 collapse ; thus indicating a diminution 

 in the intensity of electrical repulsion. 

 If the lamina be sufficiently long, the 

 electrical charge may be spread over so 

 great an extent of surface, as to allow 

 the balls to hang perpendicularly and 

 come in contact. But on winding up 

 the lamina, the intensity of the electricity 

 is restored, and the balls diverge to the 

 same extent as before, allowance being 

 made for the small dissipation of elec- 

 tricity which may have occurred from 

 the contact of the air during the experi- 

 ment. 



(82.) In the case of a long and slender 

 lamina of conducting matter, charged 

 with electricity, Coulomb found that its 

 intensity continued nearly uniform from 

 the middle of the lamina to within a 

 short distance from the ends ; at that 

 part it rapidly increased ; and at the 

 very extremity it became twice as much 

 as at the middle part. In a circular 

 plate, the electricity is accumulated in 

 much greater quantities at the circum- 

 ference than about the centre ; the in- 

 tensities being in the proportion of 2.9 

 to 1 : that is, the intensity at the centre 

 is nearly one- third of that at the circum- 

 ference. 



(83.) If the body be an oblong sphe- 

 roid, arising from the revolution of an 

 ellipse on its smaller axis, the thickness 

 of the strata of electricity, or, in other 

 words, its intensity, at the extremities 

 of the two axes, is exactly in the pro- 

 portion of the respective axes them- 

 selves. It thus appears, that if the 

 ellipsoid be much elongated, the inten- 

 sity must be very feeble at the equator, 

 but very great at the poles. A still 

 more rapid augmentation of the relative 



