| Fig. 7.—Wimshurst’s Machine. |
| Fig. 8.—Action of the Wimshurst Machine. |
The operation of the machine is as follows: Let us suppose that one of the studs on the back plate is positively electrified and one at the opposite end of a diameter is negatively electrified, and that at that moment two corresponding studs on the front plate passing opposite to these back studs are momentarily connected together by the neutralizing wire belonging to the front plate. The positive stud on the back plate will act inductively on the front stud and charge it negatively, and similarly for the other stud, and as the rotation continues these charged studs will pass round and give up most of their charge through the combs to the Leyden jars. The moment, however, a pair of studs on the front plate are charged, they act as field plates to studs on the back plate which are passing at the moment, provided these last are connected by the back neutralizing wire. After a few revolutions of the disks half the studs on the front plate at any moment are charged negatively and half positively and the same on the back plate, the neutralizing wires forming the boundary between the positively and negatively charged studs. The diagram in fig. 8, taken by permission from S.P. Thompson’s paper (loc. cit.), represents a view of the distribution of these charges on the front and back plates respectively. It will be seen that each stud is in turn both a field plate and a carrier having a charge induced on it, and then passing on in turn induces further charges on other studs. Wimshurst constructed numerous very powerful machines of this type, some of them with multiple plates, which operate in almost any climate, and rarely fail to charge themselves and deliver a torrent of sparks between the discharge balls whenever the winch is turned. He also devised an alternating current electrical machine in which the discharge balls were alternately positive and negative. Large Wimshurst multiple plate influence machines are often used instead of induction coils for exciting Röntgen ray tubes in medical work. They give very steady illumination on fluorescent screens.
In 1900 it was found by F. Tudsbury that if an influence machine is enclosed in a metallic chamber containing compressed air, or better, carbon dioxide, the insulating properties of compressed gases enable a greatly improved effect to be obtained owing to the diminution of the leakage across the plates and from the supports. Hence sparks can be obtained of more than double the length at ordinary atmospheric pressure. In one case a machine with plates 8 in. in diameter which could give sparks 2.5 in. at ordinary pressure gave sparks of 5, 7, and 8 in. as the pressure was raised to 15, 30 and 45 ℔ above the normal atmosphere.
The action of Lord Kelvin’s replenisher (fig. 9) used by him in connexion with his electrometers for maintaining their charge, closely resembles that of Belli’s doubler and will be understood from fig. 9. Lord Kelvin also devised an influence machine, commonly called a “mouse mill,” for electrifying the ink in connexion with his siphon recorder. It was an electrostatic and electromagnetic machine combined, driven by an electric current and producing in turn electrostatic charges of electricity. In connexion with this subject mention must also be made of the water dropping influence machine of the same inventor.[1]
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| Fig. 9.—Lord Kelvin’s Replenisher. | |
C, C, Metal carriers fixed to ebonite cross-arm. F, F, Brass field-plates or conductors. | a, a, Receiving springs. n, n, Connecting springs or neutralizing brushes. |
The action and efficiency of influence machines have been investigated by F. Rossetti, A. Righi and F.W.G. Kohlrausch. The electromotive force is practically constant no matter what the velocity of the disks, but according to some observers the internal resistance decreases as the velocity increases. Kohlrausch, using a Holtz machine with a plate 16 in. in diameter, found that the current given by it could only electrolyse acidulated water in 40 hours sufficient to liberate one cubic centimetre of mixed gases. E.E.N. Mascart, A. Roiti, and E. Bouchotte have also examined the efficiency and current producing power of influence machines.
Bibliography.—In addition to S.P. Thompson’s valuable paper on influence machines (to which this article is much indebted) and other references given, see J. Clerk Maxwell, Treatise on Electricity and Magnetism (2nd ed., Oxford, 1881), vol. i. p. 294; J.D. Everett, Electricity (expansion of part iii. of Deschanel’s Natural Philosophy) (London, 1901), ch. iv. p. 20; A. Winkelmann, Handbuch der Physik (Breslau, 1905), vol. iv. pp. 50-58 (contains a large number of references to original papers); J. Gray, Electrical Influence Machines, their Development and Modern Forms (London, 1903).
(J. A. F.)
[1] See Lord Kelvin, Reprint of Papers on Electrostatics and Magnetism (1872); “Electrophoric Apparatus and Illustrations of Voltaic Theory,” p. 319; “On Electric Machines Founded on Induction and Convection,” p. 330; “The Reciprocal Electrophorus,” p. 337.