801 



ELECTRICITY, COMMON. 



ELECTRICITY, COMMON. 



802 



tion is preceded by induction, for the opposed surfaces becomi 

 oppositely electrified by polar action, and then attraction is possible 

 This may be shown by one of Harris's beautiful experiments, in whicl 

 a disc of gilt card, A, three inches in diameter, is balanced at the eu< 



of a light rod of shell-lac and suspended by a thread. At the back 01 

 the disc is attached a slip of gold-leaf, and a similar insulated disc, also 

 with a slip of gold-leaf at its back, is charged, say + , and brought up 

 to the disc A, when it will be seen that the gold-leaf on A will open 

 out, and then, and not till then, will the disc A be attracted to B. 



The Leyden jar is an example of a solid dielectric between two con- 

 ducting surfaces. A plate of glass partially coated with tin-foil would 

 serve the purpose, only it is not so convenient a form as a jar. The 

 glass should be thin, and the coating of tin-foil, both within and with- 

 out, should extend to within two or three inches of the neck, the 

 upper part of the jar being left uncovered, in order to insulate the 

 two coatings. Through a wooden cover passes a wire, surmounted by 

 a brass knob, and terminating in a brass chain for conveying the 

 charge to the inner coating. On connecting the knob of the jar with 

 the prime conductor of an electrical machine in action, bright sparks 

 will pass from the conductor to the knob ; the electricity passes down 

 the wire, and by means of the inner metallic coating distributes itself 

 over the interior of the glass, the particles of which become polarised, 

 and thus decompose the electricity of the outside of the jar, leaving it 

 in an opposite state. The electricity of the same name passes from 

 the outer coating to the earth, so that it is necessary for the outer 

 coating to be uninsulated. If the jar be insulated, as at a, only a 

 feeble charge can be introduced into it, but if a conductor be held 

 near the outer coating of the insulated jar, a spark will pass from the 

 outer coating for every spark that strikes upon the knob,p, of the jar. 

 In this way a number of jars, 6, <r, &c., may be charged by connecting 

 the knob of one with the outer coating of the jar nearest to the prime 

 conductor, p, of the machine, and the knob of the third jar c, with the 



coating of the second, 4. In this way each jar may receive a powerful 

 charge, although all be insulated except the last in the series, the 

 outer coating of which must communicate with the earth, or a con- 

 ductor, !, in communication therewith. It will be evident, however, 

 that a charged jar does not contain a larger quantity of either elec- 

 tricity than it does in its neutral condition ; only there is a different 

 distribution of the force : in the uncharged jar the compound electricity 

 is equally distributed within and without the glass ; while in the 

 charged jar, the electricity of one kind is accumulated on the interior, 

 and that of the other kind on the exterior of the glass. That the 

 charge is on the glass, and not on the coatings, may be proved by 

 making the coatings moveable, when the jar can be charged, and the 

 glass be separated from its coatings, and restored to them again with- 

 out disturbing the charge. That the intervening glass or dielectric is 

 molecularly polarised is proved by a good experiment by Matteucci : 

 a number of thin plates of mica were placed one upon the other in a 

 pack, and metallic plates being applied to the other pieces, the whole 

 was charged like a Leyden jar. On separating the plates by means 

 of iniulating handles, each was found to be electrified, one side being 

 positive* and the other negative. 



Advantage was taken of the properties of the Leyden jar by Harris, 

 to construct what he calls the unit jar, for measuring out definite 

 quantities of electricity. The unit jar is a small Leyden phial insu- 

 lated on a rod of glass, with a wire attached to the outer coating, 



ARTS AND 8CI. DIV. VOL. HI. 



terminating in a knob, a, placed at a small distance from a similar 

 knob, b, proceeding from the wire which passes into the interior. The 

 bottom of the jar is also furnished with a knob, which may be con- 

 nected bv means of a wire with the prime conductor of the electrical 

 machine, while the foremost knob of the unit jar may be similarly 

 connected with the knob of a Leyden jar, the charge of which is to bo 

 measured. On turning the electrical machine, the outer surface of the; 

 unit jar becomes positively electrified, and an equal amount of positive 

 electricity passes fronrthe interior of the unit jar into the Leyden jar. 

 As soon as the unit jar is fully charged, a spark passes between the 

 balls, a, b, and equilibrium is for a moment restored; but as the 

 machine is kept in action, the outer surface of the unit jar becomes 

 again charged, an equivalent quantity of positive electricity passes into 

 the Leyden jar, and the unit jar discharges itself as before. In this way 

 by counting the number of sparks which pass between the knobs, 



a and i, a numerical value can be given to the charge of the Leyden 

 jar. The distance between the balls, a and b, can be adjusted at 

 pleasure; but supposing that distance to remain equal, successive 

 charges in the Leyden jar will also be equal, provided the same number 

 of sparks passing between a and b be counted for every charge. 



By means of an apparatus of this kind and a modification of the 

 common balance, Harris was enabled to measure the force of attrac- 

 tion between two charged surfaces. For one of the pans of the 

 balance, a disc of gilt wood was substituted, beneath which was placed 

 a similar disc insulated. Attached to the metal support of the balance 

 beam was a wire connected with the outer coating of a Leyden jar, and 

 the suspended disc was connected by conducting materials with the 

 balance and the support, while the lower insulated disc was connected 

 by a wire with the inner coating of the Leyden jar. The scale pan on 

 the left contained grain weights, and the support underneath was to 

 prevent the too great oscillations of the balance. On measuring off 

 into the Leyden jar, by means of the unit jar, a quantity of electricity 

 sufficient to produce between the discs an attractive force that would 

 raise 4 grains in the opposite scale pan, it was found that a double 

 charge of electricity would raise 4 times the amount, or 16 grains, 

 while a triple charge would raise 9 times the amount or 36 grains, so 

 that the area of charged surface remaining constant the attractive force 

 increases as the square of the quantity. If a single charge consisting 

 of ten sparks or units be divided between two Leyden jars presenting 

 equal amounts of covered surface, the attractive force will be diminished 

 to one-fourth, and with three jars to one-ninth, as compared with a 

 single jar, so that a charge which in one jar would raise 18 grains would 

 if diffused over two jars raise only 4J grains, and if diffused over three 

 jars, it would raise only 2 grains. Hence, the quantity of charge 

 remaining the same, the attractive force is inversely as the squares of 

 the charged surfaces of the jars. 



When the electric equilibrium has been disturbed there are three 

 modes in which it may be restored, namely, by conduction, by disrt'/,- 

 tion, and by convection. When, for example, a charged jar is discharged 

 ay means of a discharging rod, the electricity is conducted quietly 

 ;hrough the wire of the discharger so long as contact is maintained, 

 jut in traversing the air between the knob of the jar and that of the 

 rod, a brilliant spark passes, accompanied by a crackling noise. After 

 ,he jar has been discharged, a second smaller spark, known as tin; 

 residual charge, may be obtained from it. Faraday considers this to 

 irise from the circumstance that all bodies, including even resin and 

 glass, have a certain amount of conducting power, by virtue of which 

 \ portion of the charge penetrates the substance of the dielectric, the 

 .wo surfaces being in opposite states. As soon as the principal dis- 

 charge has been made, and the force which caused this penetration 

 removed, the electricity returns to the surface, re-charges the jar to a 

 small extent, and thus accounts for the residual charge. As no sub- 

 itance insulates perfectly, so no substance conducts perfectly, since 

 Aere is experimental proof that the resistance to discharge offered by 

 ^ long thin wire may be greater than that offered by a stratum of air, 

 TO that the discharge will take place through the insulating air, rather 

 than along the conducting wire. Indeed the resistance offered by cou- 

 liictore may be considered as insulation : increase the resistance, and 

 he conducting power is diminished and the insulating power propor- 



