riMflTY. 



U.KCTUICITY, COMMON. 



M gilt Ull of light wood mupended by silk, or with a dink of gilt 

 paper attached to .tick of reun and oolled proof ,Ja,, it will take 

 away from the charged jar quantity of electricity of the name tension 

 as that of the charge, and by thi* means the tension-can be measured 

 by a delicate electrometer. Now, in Faraday'* first experiment, the 

 interval i in both cases, contained air, and the outer upbore was uninsu- 

 lated. The small knob o being electrified, the inner sphere became 

 charged, and the tendon of the free electricity on ita surface wag 

 moimirml by touching c with a proof plane and then trying it with a 

 torsion electrometer. In thu way a tonion of 250* wiu> obtained, 

 upmojIiiM the tension on the sphere B. On bringing the ball o into 

 contact with the corresponding bull of the second uncharged apparatus, 

 the tension on the inner sphere of each apparatus became 125 ; or, in 

 other word*, the electricity had become equally distributed in the two 

 instruments, as waa to be expected, seeing that they were identical 

 in make, and both contained air in the interval i. On-lining up the 

 lower half of this interval with a non-conducting body, such as a 

 thick hemispherical cup of shell-lac, charging, the other apparatus in 

 which thu interval, i, contained air as before, and measuring the 

 tension on B, the number 290 was obtained. On dividing the charge 

 between the two instruments there was no longer on equal division of 



the chorge'as before, such as = 145, but the apparatus in which 



the interval contained air, gave a tension of 114 only, and the other 

 apparatus with the shell-lac a tension of 113. Taking these numbers 

 as equal, they differ considerably from 145, or half the charge, had 

 both instruments contained air, and it appeared that whilst in the 

 division the induction through the air hod lost 176 of force, that 

 through the shell-lac had only gained 113*. If this difference be 

 assumed to depend entirely on the greater facility possessed by shell- 

 lac, of allowing or causing inductive action through its substance than 

 that possessed by air, then this capacity for electric induction would be 

 inversely as the respective loss and gain indicated above, and assuming 

 the capacity of the air apparatus as 1, that of the shell-lac apparatus 

 would be {ft, or T55. Corresponding effect*, in the reverse order, 

 were obtained when induction through shell-lac was converted into 

 induction through air, the force or tension of the whole being 

 increased. As one-half of the interval contained air, this number 

 would be too low to represent the specific inductive capacity of shell- 

 lac. Other substances were tried in a similar manner, and they gave 

 different values. One remarkable result was, that gases have the some 

 inductive capacity, and that this is not modified either by their tem- 

 perature, or by their pressure or their hygrometric state. Expressing, 

 therefore, the inductive capacity of air by 1, the following numbers 

 (obtained by Snow Harris by a totally different mode of experiment) 

 nt the different substances named: resin, 177; pitch, 1'80; 

 r wax, 1-86; glass, 1-90; sulphur, 1'93; shell-lac, I'Slf.. Faraday 

 i proposed to call these isolating bodies dielectrics, in contradistinc- 

 tion to conductors, which have no inductive ]x>wer, that is, induction 

 is not possible through conductors in communication with the 

 ground. 



It has been urged as an objection to Faraday's theory that induction 

 take* place in vacuo ; but unless it can be shown that the must perfect 

 vacuum which can be produced contains absolutely no material 

 particles, the objection falls to the ground. Indeed it is a consequence 

 of Faraday's theory that the fewer the particles of the dielectric the 

 stronger is the charge or ultimate result. Nor does there appear to 

 be any limit to the distance of the action. We have seen Faraday 

 trace it from an electrified insulated ball placed in the middle of the 

 theatre of the Koyal Institution to the walls of the room. 



Seeing then that inductive action takes place through the particles 

 of intervening matter, Faraday supposed that such influence was 

 exerted, not in the direction of straight lines only as had been pre- 

 viously assumed, but also in curved lines. To 

 test this opinion a cylinder of shell-lac, about an 

 inch in diameter and seven inches long, was fixed 

 in a wooden support ; it had a concavity at the 

 upper extremity for supporting a brags balL 

 The uppi-r half of the cylinder was excited nega- 

 tively, the brass ball was placed on the top, and 

 the arrangement examined by means of the 

 carrier ball and the torsion electrometer. When 

 the carrier was made to touch the ball the two 

 were uninsulated by touching them with the 

 hand, then insulated aa before, separated, and 

 the charge of the carrier examined. In this 

 way the force was found to vary in different 

 positions. When the carrier was placed at d the 

 effect produced was! 612, at c 270, at i 149"; 

 when not touching the brass ball at all, as at a 

 and r, it was electrified also +, as in the other 

 rases, and at a gave the highest result, <>r <>v,r 

 1000*. The charges obtained at a, c and d might 

 arise from an inductive action in straight lines, 

 but that at 6 was clearly in a curved line, since 

 I'I.T l-all r.iiiM not, from the size of the brass ball, bo con- 

 nected by a Rtraight line with any port of the excited shell-lac, and the 

 up. rinu nt U rendered still more decisive by placing the boll at a 



distance, and obtaining a discharge of positive electricity after uuiu- 

 sulating and then insulating it. If, instead of the ball, a brass plate 

 be placed on the shell-lac, uninsulated, the carrier ball will show signs 

 of electricity at various distances at the side and above the plate. 



In good conductors the polarisation of the particles must, according 

 to this theory, take place, but the discharge from |article to particle 

 occurs so instantaneously, that they can only exhibit the polarised 

 state as a whole, as in the case of an insulated sphere. Now it is 

 evident that this result is brought iiU.ut w lu-ther the body be solid or 

 hollow, since the charge will be found only on the surface uf conductors, 

 for it is there only that the surrounding air or other resisting dielectric 

 medium exists, and on this medium does the power of mainlining 

 induction and consequently charge on the part of the insulated elec- 

 trical body depend. The early electricians were not a little puzzled at 

 finding no traces of electricity within a hollow insulated electrified con- 

 ductor. Thus Priestley (' History of Electricity,' p. 688), says, " I 

 electrified a tin quart vessel standing upon a stool of baked wood, and 

 observed that a pair of pith lolls insulated by being fastened to the 

 end of a stick of glass, and hanging entirely within the cup, so that no 

 port of the threads were above the mouth of it, remained just where 

 they were placed without being in the least affected by the electricity." 

 Many of Faraday's experiments beautifully illustrate the fact of the 

 accumulation of the charge on the outside. For example, if an ice- 

 jail bu insulated and a brass ball also insulated be electrified and 

 duccd into the vessel so as to touch it, and then bo withdrawn, it will 

 be found to have given up the whole of its charge to the pail. If now 

 we introduce the proof plane, or the carrier ball into the vessel, no 

 electricity will be carried away, but on touching the outside a full 

 charge will be obtained. There con evidently be no charge within the 

 vessel because there is no object towards which induction can take 

 place. If, however, on uninsulated brass ball be held within the pail 

 but without touching it, the particles of air will immediately become 

 polarised, since the brass ball con assume an electrical condition oppo- 

 site to that of the pail : the^proof plane can then take away a charge 

 from the interior as well as from the exterior of the vessel. If the 

 pail be + the ball will be , for its -I- electricity will pass to the ground 

 along the conducting wire. 



According to the new theory, the action of the condenser is thus 

 explained: If an insulated metal ball be charged and placed in the 

 centre of a hollow sphere, induction will diminish in every dii 

 as the square of the distance. If an uninsulated conducting surface 

 be introduced, the induction will be diverted from distant objects, and 

 the more so the nearer the conducting surface U brought to the charged 

 ball. The surface will be electrified in an opposite state to that nt the 

 ball, and the fewer the intervening particles of the dielectric air, 

 the stronger will be the polarity in them, and the more complctrly 

 will induction be drawn off from distant objects. The tension of the 

 interposed air may become so great that it ceases to insulate, and a 

 discharge takes place between the two surfaces, manifested by a spark : 

 for example, let a metal disk A insulated on a glass pillar, be connected 



by a wire with a gold-leaf electroscope, and a small charge of + elec- 

 tricity be given to A, so as to cause the leaves to diverge ; if now a 

 second metal disc B, uninsulated, be mode to approach A, the gold- 

 leaves will gradually collapse, but will open out again if B be \vith- 

 drawn. This will take place a number of times, an effect which could 

 not bo explained by the old theory of a charge being retained by the 

 pressure of the air. Now the action of the charged disc A is to decom- 

 pose the electricity of B, to retain upon it an equivalent portion of 

 electricity, while the + electricity of B passes off to the earth. In 

 consequence of this latter action the disc A may receive a considerable 

 charge by repeated contact with a body very feebly charged, the ex- 

 citation being renewed after every contact with A : during this time 

 the plate B must be very near to A, and if B be suddenly withdrawn, 

 the gold leaves will diverge, because the whole induction is now 

 directed upon surrounding objects, instead of being limited by B. 



The opening out of the gold leaves in the above experiment by what 

 is called electrical repulsion, is due to induction, for no electricity r.m 

 be detected on the inner surfaces of the leaves, but only on the mini 

 surfaces, as may be proved by substituting slips of gilt paper for the 

 gold leaf, and when diverging under a charge, bringing a proof 

 into contact with them, and testing it by means of a torsion electro- 

 meter. The leaves diverge because induction takes place from tlu-ir 

 outer surfaces towards surrounding objects, which attract the 

 and canst: them to separate from each other. If, however, an unin- 

 sulated body bo placed IK t\\. . n the leaves, induction takes place from 

 the inner surfaces towards it, and they collapse. Every case of attrac- 



