CHAMBERS'S INFORMATION FOR THE PEOPLE. 



Fig. 1 6. 



to i of that at 2 inches', and to i at 3 inches' 

 distance. 



(2.) This force depends also on the product of 

 the intensities of the electricities that are acting on 

 each other. So that, if one ball had its electricity 

 raised to three times its first amount, and another 

 to twice, the resulting action between them would 

 be six times as strong as at first. 



The construction of delicate electrometers has 

 of late years occupied the attention of our greatest 

 authorities on this subject. Their aim has been 

 to render electricity a strictly accurate and mathe- 

 matical science ; and the labours of Sir William 

 Thomson and others have done more to effect 

 this than any have ever done before. Sir W. 

 Thomson has himself invented several forms of 

 electrometers from time to time, each more accu- 

 rate than the former ; and his Absolute Electro- \ 

 meter and his Quadrant Electrometer seem as 

 perfect as is reasonably to be expected. The latter 

 is out of sight the most delicate instrument of the 

 kind. A description of these is beyond the scope 

 of this article, but may be found in the various 

 works on electrical science or apparatus. 



CONDENSED ELECTRICITY. 



There is a limit to the accumulation of elec- 

 tricity on any surface. After it has reached a 

 certain degree of intensity or tension, it discharges 

 into the air as fast as it is produced. But if we 

 bring another conducting surface near to the 

 charged one, the latter may be much more highly 

 electrified than before. This is the principle on 

 which the Leyden jar depends, and a description 

 of it will serve to explain the nature of condensed 

 electricity. 



The Leyden jar is simply a wide-mouthed glass 

 bottle with two coatings of tinfoil, 

 pasted one outside, and the other 

 inside (fig. 16). The coatings 

 reach only about four-fifths of the 

 height of the jar, and the bottom 

 is also covered inside and outside. 

 A brass rod ending in a knob 

 passes through a wooden plug in 

 the mouth of the jar, and com- 

 municates with the inside coating 

 by a piece of chain. If, now, we 

 set such a jar on an insulated 

 support, so that sparks can pass 

 from the prime conductor of a 

 machine to its knob, a few sparks will pass, 

 and then it seems charged, and will take no 

 more. But if we hold a knuckle near the outer 

 coating, sparks will again pass freely to the knob ; 

 and for every spark to the knob, a similar one 

 passes to the knuckle from the outer coating. 

 This continues for some time, 

 till again the jar becomes fully 

 ' charged or saturated. So 

 long as it remains on the in- 

 sulated support, we may touch 

 either the outside coating or 

 the knob. But if one hand 

 touch the outside, and the 

 other be brought near the 

 knob, a bright spark passes 

 to it, and a very violent shock 

 is experienced. If the same 

 j be done again, a feebler spark and shock 



266 



Fig. 17. 



thint 



ensue, and then the jar is quite discharged. This- 

 experiment had better be made, however, with a 

 pair of discharging tongs (fig. 17), as it is neither 

 pleasant nor safe to do it with the body. They are 

 simply two brass arms ending in balls, and 

 movable about a hinge by means of glass handles. 

 One ball is made to touch the outer coating, 

 while the other is brought near the knob. The 

 usual way of charging the jar is to hold it in the 

 hand, and present its knob to the prime conductor 

 of a machine. 



Its theoretical explanation is just a case of the 

 general principle, that the charge which a body 

 can receive is greater in proportion to its facility 

 for induction. Now, when the inner coating is 

 positively charged from the machine, it acts by 

 induction on all neighbouring matter, and the 

 nearer and larger this matter is, the greater will be 

 the charge it can take. If, then, the jar be unin- 

 sulated, we have really the surface of the earth 

 brought within the thickness of the glass from/ the 

 inside coating. The glass, too, which intervenes 

 is a very good dielectric, much better than air ; at 

 the same time its tenacity can resist the strain of 

 a far higher polarisation of its particles than air 

 would do. Thus we have every facility for a 

 high charge of the interior. It would, of course, 

 be greater, the thinner the glass ; but thin glass is 

 easily ruptured, and would be of no use. If we 

 could have a jar of indefinite thinness, and yet of 

 great strength, the charge which it could take 

 would be unlimited. As this is not the case, 

 other bodies round about compete, for induction, 

 with the outer coating. Ultimately, the facility of 

 induction through the air on these becomes greater, 

 and there is an end of the charge. 



If the inside is positively charged, the outer 

 coating is negatively next it, and the two are 

 bound together by the action of the glass, which 

 is such a good dielectric. The glass also seems 

 to be the chief seat of the charge, and the coatings 

 seem to serve merely to distribute it over the non- 

 conducting glass. This is usually illustrated by 

 having a jar with movable coatings. After it is 

 charged, we may remove each coating, handle 

 them, put them back in their place, and get almost 

 as strong a discharge as ever. The secondary dis- 

 charge, which is got by connecting the coatings 

 after the first discharge, is ascribed to the same 

 cause. The glass does not give up all its charge 

 at once, and sometimes three or four subsequent 

 discharges may be got. 



For great power, large surfaces are necessary. 

 A large jar would give this, but it is preferable to 

 use several moderate-sized ones, and unite them 

 to form what is called an electric battery. The 

 jars are placed on an insulated stool covered with 

 tinfoil, and their outer coatings are thus in com- 

 munication. All the knobs are connected by 

 metal rods, and so all the inner coatings act as 

 one. If, after charging, we connect any knob 

 with an outer coating, the whole battery will be 

 discharged, and with a force which, for the same 

 degree of charge in a single jar, will be propor- 

 tional to the number of jars employed. 



The effects of condensed electricity are very 

 powerful. If the spark be passed through a bad 

 conductor, such as shellac, resin, or glass, it may 

 be shivered into powder. A fine wire of metal 

 will be made red-hot, or even melted, by the dis- 

 charge of a good battery. The physiological 



