Machines producing Electrification by Mechanical Work

197*.] In the ordinary frictional electrical machine the work done in over- coming friction is far greater than that done in increasing the electrification. Hence any arrangement by which the electrification may be produced entirely by mechanical work against the electrical forces is of scientific importance if not of practical value.

The first machine of this kind seems to have been Nicholson’s Revolving Doubler, described in the Philosophical Transactions for 1788 as ‘an instrument which by the turning of a Winch produces the two states of Electricity without friction or communication with the Earth.’ 198*.] It was by means of the revolving doubler that Volta succeeded in developing from the electrification of the pile an electrification capable of affecting his electrometer. Instruments on the same principle have been in- vented independently by Mr. C. F. Varley∗ and Sir W. Thomson.

These instruments consist essentially of insulated conductors of various forms, some fixed and others moveable. The moveable conductors are called Carriers, and the fixed ones may be called Inductors, Receivers, and Regener- ators. The inductors and receivers are so formed that when the carriers arrive at certain points in their revolution they are almost completely surrounded by a conducting body. As the inductors and receivers cannot completely sur- round the carrier and at the same time allow it to move freely in and out without a complicated arrangement of moveable pieces, the instrument is not theoretically perfect without a pair of regenerators, which store up the small

Specification of Patent, Jan. 27, 1860, No. 206.THE REVOLVING DOUBLER. 177 amount of electricity which the carriers retain when they emerge from the receivers. For the present, however, we may suppose the inductors and receivers to surround the carrier completely when it is within them, in which case the theory is much simplified.

We shall suppose the machine to consist of two inductors A and C, and of two receivers B and D, with two carriers F and G. Suppose the inductor A to be positively electrified so that its potential is A, and that the carrier F is within it and is at potential F. Then, if Q is the coefficient of induction (taken positive) between A and F, the quantity of electricity on the carrier will be Q(F − A).

If the carrier, while within the inductor, is put in connexion with the earth, then F = 0, and the charge on the carrier will be −QA, a negative quantity. Let the carrier be carried round till it is within the receiver B, and let it then come in contact with a spring so as to be in electrical connexion with B. It will then, as was shewn in Art. 20, become completely discharged, and will communicate its whole negative charge to the receiver B. The carrier will next enter the inductor C, which we shall suppose charged negatively. While within C it is put in connexion with the earth and thus ac- quires a positive charge, which it carries off and communicates to the receiver D, and so on.

In this way, if the potentials of the inductors remain always constant, the receivers B and D receive successive charges, which are the same for every revolution of the carrier, and thus every revolution produces an equal incre- ment of electricity in the receivers. But by putting the inductor A in communication with the receiver D, and the inductor C with the receiver B, the potentials of the inductors will be continually increased, and the quantity of electricity communicated to the re- ceivers in each revolution will continually increase. For instance, let the potential of A and D be U, and that of B and C, V, and when the carrier is within A let the charge on A and C be x, and that on the carrier z, then, since the potential of the carrier is zero, being in contact with earth, its charge is z = −QU.

The carrier enters B with this charge and communicates it to B. If the capacity of B and C is B, their potential will be

178 Q U. B If the other carrier has at the same time carried a charge −QV from C to Q′ D, it will change the potential of A and O from U to U − V, if Q′ is A the coefficient of induction between the carrier and C, and A the capacity of A and D. If, therefore, Un and Vn be the potentials of the two inductors after n half revolutions, and Un+1 and Vn+1 be the potentials after n + 1 half revolutions, Q′ Un+1 = Un − V , A n Q Vn+1 = Vn − Un . B ′ Q Q If we write p2 = and q 2 = , we find B A changed from V to V − pUn+1 + qVn+1 = (pUn + qVn ) (1 − pq) = (pU0 + qV0 ) (1 − pq)n+1 , pUn+1 − qVn+1 = (pUn − qVn ) (1 + pq) = (pU0 − qV0 ) (1 + pq)n+1 . Hence q Un = U0 {(1 − pq)n + (1 + pq)n } + V0 {(1 − pq)n − (1 + pq)n } , p p Vn = U0 {(1 − pq)n − (1 + pq)n } + V0 {(1 − pq)n + (1 + pq)n } . q

It appears from these equations that the quantity pU + qV continually di- minishes, so that whatever be the initial state of electrification the receivers are ultimately oppositely electrified, so that the potentials of A and B are in the ratio of q to −p.

On the other hand, the quantity pU − qV continually increases, so that, however little pU may exceed or fall short of qV at first, the difference will be increased in a geometrical ratio in each revolution till the electromotive forces become so great that the insulation of the apparatus is overcome. Instruments of this kind may be used for various purposes.WATER DROPPING ACCUMULATOR.

For producing a copious supply of electricity at a high potential, as is done by means of Mr. Varley’s large machine.

For adjusting the charge of a condenser, as in the case of Thomson’s elec- trometer, the charge of which can be increased or diminished by a few turns of a very small machine of this kind, which is then called a Replenisher. For multiplying small differences of potential. The inductors may be charged at first to an exceedingly small potential, as, for instance, that due to a thermo-electric pair, then, by turning the machine, the difference of potentials may be continually multiplied till it becomes capable of measure- ment by an ordinary electrometer. By determining by experiment the ratio of increase of this difference due to each turn of the machine, the original electromotive force with which the inductors were charged may be deduced from the number of turns and the final electrification.

In most of these instruments the carriers are made to revolve about an axis and to come into the proper positions with respect to the inductors by turning an axle. The connexions are made by means of springs so placed that the carriers come in contact with them at the proper instants. 199*.] Sir W. Thomson∗ , however, has constructed a machine for multi- plying electrical charges in which the carriers are drops of water falling out of the inside of an inductor into an insulated receiver. The receiver is thus continually supplied with electricity of opposite sign to that of the inductor. If the inductor is electrified positively, the receiver will receive a continually increasing charge of negative electricity.

The water is made to escape from the receiver by means of a funnel, the nozzle of which is almost surrounded by the metal of the receiver. The drops falling from this nozzle are therefore nearly free from electrification. Another inductor and receiver of the same construction are arranged so that the induc- tor of the one system is in connexion with the receiver of the other. The rate of increase of charge of the receivers is thus no longer constant, but increases in a geometrical progression with the time, the charges of the two receivers being of opposite signs. This increase goes on till the falling drops are so diverted from their course by the electrical action that they fall outside of the ∗ Proc. R. S., June 20, 1867.HOLTZ’S MACHINE. 180 receiver or even strike the inductor. In this instrument the energy of the electrification is drawn from that of the falling drops.

200.] In Holtz’s ‘Influence-Machine’ a plate of varnished glass is made to rotate in front of a fixed plate of varnished glass. The inductors consist of two pointed pieces of card sometimes covered with tin foil and placed on the further side of the fixed plate so that their points are at opposite extrem- ities of a diameter. Holes are cut in the fixed plate opposite the points of the inductors.

The electrodes are first put in connexion with each other and the machine is set in rotation. One of the inductors is then electrified, either by an ordinary machine or by an excited piece of ebonite. Let us suppose it electrified positively. The comb in front of the charged inductor immedi- ately begins to glow and discharges negative electricity against the rotating disk. This negative electrification is carried round by the disk to the other side where it is free from the influence of the positive inductor.

The other inductor now discharges positive electricity from its point and becomes itself negatively charged, and the comb of the negative electrode discharges posi- tive electricity, which is carried round the disk on the other side back to the positive electrode. In this way there is kept up an electric current from the positive to the negative electrode. A rushing noise is heard and in the dark a glow is seen extending itself from the positive comb over the surface of the rotating disk in the direction opposite to its motion. If the electrodes are now gradually separated a succession of sparks will pass between them.