The Wimshurst influence machine is an electrostatic generator, a machine for generating high voltages developed between 1880 and 1883 by British inventor James Wimshurst (1832–1903).
It has a distinctive appearance with two large contra-rotating discs mounted in a vertical plane, two cross bars with metallic brushes, and a spark gap formed by two metal spheres.
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These machines belong to a class of generators called influence machines, which separate electric charges through electrostatic induction, or influence. Earlier machines in this class were developed by Wilhelm Holtz (1865 and 1867), August Toepler (1865), and J. Robert Voss (1880). The older machines were less efficient and exhibited an unpredictable tendency to switch their polarity. The Wimshurst did not have this defect.
In a Wimshurst machine, the two insulated discs and their metal sectors rotate in opposite directions passing the crossed metal neutralizer bars and their brushes. An imbalance of charges is induced, amplified, and collected by two pairs of metal combs with points placed near the surfaces of each disk. These collectors are mounted on insulating supports and connected to the output terminals. The positive feedback increases the accumulating charges exponentially until the dielectric breakdown voltage of the air is reached and an electric spark jumps across the gap.
The machine is theoretically not self-starting, meaning that if none of the sectors on the discs has any electrical charge there is nothing to induce charges on other sectors. In practice, even a small residual charge on any sector is enough to start the process going once the discs start to rotate. The machine will only work satisfactorily in a dry atmosphere. It does require mechanical power to turn the disks against the electric field, and it is this energy that the machine converts into electric power. The output of the Wimshurst machine is essentially a constant current that is proportional to the area covered by the metal sectors and to the rotation speed. The insulation and the size of the machine determine the maximum output voltage that can be reached. The accumulated spark energy can be increased by adding a pair of Leyden jars, an early type of capacitor suitable for high voltages, with the jars’ inner plates independently connected to each of the output terminals and the jars’ outer plates interconnected. A typical Wimshurst machine can produce sparks that are about a third of the disc's diameter in length and several tens of microamperes.
The two contra-rotating insulating discs (usually made of glass) have a number of metal sectors stuck onto them. The machine is provided with 4 small earthed brushes (2 on each side of the machine on conducting shafts at 90 degrees to each other), plus a pair of charge-collection combs. The conducting shafts that hold the brushes on a typical Wimshurst machine would form an 'X' on an x-ray photograph; the charge-collection combs are typically mounted along the horizontal and equally contact the outer edges of both front and back discs. The collection combs on each side are usually connected to respective Leyden jars.
Any small charge on either of the two discs suffices to begin the charging process. Suppose, therefore, that the back disc has a small, net electrostatic charge. For concreteness, assume this charge is positive and that A rotates counter-clockwise. As the charged sector rotates to the position of the brush next to disc B, it induces a polarization of charge on the conducting shaft holding the brush, attracting negative charge to the near side, so that positive charge accumulates on the far side (across the disc, 180 degrees away). The shaft's polarized charges attach to the nearest sectors on disc B, resulting in negative charge on B closest the original positive charge on A, and positive charge on the opposite side of B. As A continues to rotate, its charge is accumulated by the first charge-collection comb it encounters (after an additional 45-degree rotation).
As B rotates 90 degrees clockwise, the charges that have been induced on it line up with the brushes next to disc A. Naturally the charges on B induce the opposite polarization of the A-brushes' shaft (viz., negative next to positive and positive next to negative), and the shaft's polarization is transferred to its disc. Disc B keeps rotating and its charges are accumulated by the nearest charge-collection combs.
Disc A rotates 90 degrees so that its charges line up with the brush of disc B, where an opposite charge-polarization is induced on the B conducting shaft and the nearest sectors of B, similar to the description two paragraphs above.
The process repeats, with each charge polarization on A inducing polarization on B, inducing polarization on A, etc. All of these induced positive and negative charges are collected by combs to charge the Leyden jars, electrical charge-storage devices similar to capacitors. It is the mechanical energy required to separate the opposing charges on the adjacent sectors that provides the energy source for the electrical output.