797 



ELECTRICITY, COMMON. 



ELECTRICITY, COMMON. 



798 



The expression which we have put for the action of the plane c on p 

 in equation (3) is in reality the action not only of that plane, but also 

 of the side of the prism or cylinder, of which the base is c and altitude 

 c p ; and a similar remark applies to the action of the plane D : there- 

 fore the total action given in that equation is too great by twice the 

 action of the side of the prism or cylinder included between the plates 

 B and c. For the same reasons w.e have a like excess in the equation 

 (4) ; wherefore we have subtracted these equations, when that excess 

 disappears ; whereas, if we had added them, an error would arise, 

 small with respect to x and x', but comparable to E + E'. 



Also, from equation (7), the apparatus would be discharged by 

 making the two plates communicate. 



In the actual case the lower plate communicates with the ground ; 

 therefore E'=O. 



Adding now the two equations, we find 



and subtracting 



n is a very unall quantity and negative, since the attractive force 

 diminishes as the point acted on becomes more remote. Hence x is 

 very great and positive compared with E, and it follows : 



First, that the greater the extent of the plates, the less will be, 

 being zero when that extent is infinite ; therefore the power of the 

 condenser is increased by the extent of the surfaces being enlarged. 



Second, that another source of increase of the condensing power is 

 the diminution of a, the space occupied by the non-conducting medium 

 interposed between the parallel conducting plates. 



These results are perfectly accordant with experience. 



In practice, the conducting plates are generally separated by a plate 

 of glass or a cover of varnish, the latter being used when the electrical 

 charge is feeble ; for the attractive forces of the two opposite elec- 

 tricities x x' would be too powerful for such an obstacle if E were 

 great, and the electricities would penetrate it and unite ; but in 

 chemical operations, where the electricity developed is of weak ten- 

 sion, the diminution of a is of great advantage, the quantity of elec- 

 tricity acquired by the plates becoming very sensible to the electrometer. 

 [ELECTROMETER.] 



The Leyden jar is an instrument founded on these principles. A 

 glass bottle is coated within and without with tin-foil. The conductor 

 of an electrical machine communicates with the foil on the inside by 

 means of a metallic chain, while the outside is in communication with 

 the ground. The opposite electricities are therefore accumulated on 

 the internal and external sides of the glass : hence a flash and a power- 

 ful shock is produced, when the two fluids combine, by touching the 

 outside foil with one hand while the conductor or chain communicating 

 with the inside is touched by the other. 



It was ascertained by Cavendish that the quantity of electricity pro- 

 duced in the Leyden jar, with given surfaces, was inversely proportional 

 to the breadth of the glass; this completely corresponds with the 

 result* which we have above obtained by theoretical considerations. 



There seems little doubt, from the experiments of Wollaston, that 

 much of the electricity produced by the common machine is attri- 

 butable to chemical action ; for the bent amalgam to use with the 

 rubber is that which oxidises most readily, such as tin and zinc ; and 

 scarcely any quantity of electricity is produced if, by the nature of the 

 amalgam, there is no tangible oxidation, or if we envelope the apparatus 

 in a medium which will not communicate oxygen, as carbonic acid gas. 

 As the quantity taken by the conductors is proportional, cceteris partttus, 

 to their surfaces, it is usual to employ several narrow cylindrical con- 

 ductors placed parallel to each other ; the total surface in this case 

 being the same as that of a single cylinder of the same length, and of 

 which the radius would be the sum of all then- radii. 



The electrophorua is founded on a principle nearly similar to that of 

 the condenser ; but in this case it Is the non-isolated body which 

 acquires electricity by the influence of that which is isolated. 



It should be observed, that the non-conducting plates employed in 

 the condenser and Leyden jar have a certain retentive power on the 

 electricity, and which is of the same origin as lie non-conducting 

 faculty : hence it will happen generally in experiments that the whole 

 of the electricity will not be discharged at once, when the opposite 

 electricities of the two plates are made to communicate by a conductor, 

 and frequently not after several repetitions. The same principle of 

 the separation of the neutral electricity of remote bodies by influence 

 is observed in the electrical machines which have been at different times 

 constructed. [ELECTRICAL BATTEBY; ELECTRICAL MACHINE.] 



In the Ijcst conducted experiments there will be a loss of electricity, 

 arising either from the hygrometric state of the atmosphere, or the 

 imperfect insulation of the supporters employed. When, for instance, 

 the moist particle* of vapour floating in the air come in contact with the 

 << inductor of an electrical macliine, they acquire by their own conduct- 

 ing power a small portion of the electricity developed in the conductor; 

 iimlarly electrised they are repelled; and new particles of 

 moisture arising, repeat the same process of exhaustion, each tiny 

 robber carrying away as much electricity, not as it can hold, but aa it 



may hold without being itself held. The quantity thus lost iu a small 

 given time is proportional to the whole charge, and therefore the latter 

 must diminish in a geometrical progression when the time increases in. 

 arithmetical. * 



The reviser of the above masterly article (which was furnished by 

 the late Robert Murphy to the ' Penny Cyclopedia ') has been unwilling 

 to do more thau modernise a few of the terms and erase a few lines on 

 subjects which the present state of electrical science required to be 

 treated of at greater length under separate heads. It is now proposed 

 to add some brief particulars of the admirable labours of Faraday in 

 this branch of science, referring those who desire further information 

 to his numerous papers published in the ' Philosophical Transactions ' 

 during the last twenty years, but collected into three volumes entitled 

 'Electrical Researches,' 18381858. 



The theory of electrisation by influence did not take into account 

 the medium which separated the excited body from the one acted on. 

 It was said, for example, that an electrified cloud would throw a corre- 

 sponding portion of the earth's surface into a negative state inde- 

 pendently of, or at any rate without reference to, the interposed air, 

 which was supposed to remain passive during the action of the force, 

 and then to oppose by its non-conductibility the union of the opposite 

 electricities. The experiments of Faraday, however, have led to the 

 conclusion that it is only by means of the interposed air or other non- 

 conducting medium that electrisation by influence is possible ; that 

 there is no electric action at a distance greater than the interval 

 between two adjacent molecules of the medium ; that there takes place 

 in this medium a true polarisation of the particles, or a separation of 

 their opposite electricities, whereby they become arranged into an alter- 

 nate series or succession of positive and negative points or poles ; and 

 that it is by means of this polarisation of the particles that the force is 

 transferred to a distance. So long as the particles of the medium 

 retain this state of polarisation, insulation continues ; but if the forces 

 communicate or discharge one into the other, an equalisation or com- 

 bination of the opposite electricities takes place throughout the whole 

 series of particles. Faraday assumes that all the particles of matter 

 are more or less conductors ; that in their quiescent state they are not 

 polarised, but become so by the influence of contiguous charged par- 

 ticles, in which case they assume a forced condition, and tend to return 

 by a powerful tension to their original normal condition. Such 

 particles being regarded as more or less conductors, they communicate 

 their charge either bodily or by polarity, and contiguous particles 

 impart their forces more or less readily to each other : when less 

 readily, the polarised state becomes more exalted, and insulation is the 

 consequence ; when more readily, conduction results. Induction may 

 therefore be defined as the action of a charged body upon insulating 

 matter, the particles of which communicate the electrical forces to 

 each other in an extremely minute degree, whereby they become 

 polarised and propagate an equal amount of the opposite force to a 

 distance. According to this theory, therefore, a charge is not retained 

 on an insulated conductor by the pressure of the air, as was formerly 

 supposed, but in virtue of the inductive action of surrounding surfaces 

 through the polarisation of the interposed medium. It is a question 

 of action and re-action : the insulated ball charged + produces an 

 opposing but equal amount of electricity, just as when we pull 

 against a fixed point, that point offers an equal amount of resistance in 

 the contrary direction. 



The property possessed by the particles of matter to transmit the 

 electric influence is termed inductive force, or specific inductive capacity ; 

 and this is found to be different in different 

 insulating media. In order to compare them, 

 the apparatus here represented in section was 

 contrived. It consists of two metallic con- 

 centric spheres (A B), the space between the 

 two (i) being destined for the reception of the 

 substance whose inductive capacity was to be 

 measured. The inner sphere is suspended by 

 means of a metal rod, terminating at the top 

 in a brass knob (c), and the rod is coated with 

 a thick mass of shell-lac (a) to insulate it and 

 separate it from the outer sphere. This is 

 formed in two halves, like Madgeburg hemi- 

 spheres [AiR], while a stop-cock (t) and a 

 tube allow the air of the large sphere to be 

 exhausted, and different kinds of gas to be let 

 up into the interval : when solid media were 

 introduced they were cut to the required 

 shape. Two sets of apparatus were made, 

 identically alike, for the sake of comparison. 

 The apparatus was in fact a kind of Leyden 

 jar, in which the two conducting coatings 

 could be separated at pleasure by means of 

 any required non-conducting medium. Now 

 we know, in the case of a charged Leyden 

 jar, that if it be made to communicate by its 

 knob with the knob of an uncharged jar of 

 precisely the same size, the latter will take 

 from the former exactly one-half of the charge. Moreover, if we 

 touch the knob of a charged jar with a small insulated conductor, such 



I 



