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Monday, June 17, 2013

The Wimshurst machine.

Wimshurst machine with two Leyden jars.

The Wimshurst influence machine is an electrostatic device 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.


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 a 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.


Suppose that the conditions are as in the figure that is the segment A1 is positive and the segment B1 negative. Now, as A1 moves to the left and B1 to the right, their potentials will rise on account of the work done in separating them against attraction. When A1 comes opposite the segment B2 of the B plate, which is now in contact with the brush Y, it will be at a high positive potential, and will therefore cause a displacement of electricity along the conductor between Y and Y1 bringing a large negative charge on B1 and sending a positive charge to the segment touching. As A1 moves on, it passes near the brush Z and is partially discharged into the external circuit. It then passes on until, on touching the brush X it is put in connection with X, and has a new charge, this time negative, driven into it by induction from B2. Positive electricity, then, being carried by the conducting patches from right to left on the upper half of the A plate, and negative from left to right on its lower half.

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 shafts at 90 degrees to each other), plus a pair of charge collection combs.

Assume that a single sector on one glass plate has a positive electrostatic charge. As the sector passes a sector on the other plate, the positively charged sector attracts the electrons in the neighbouring sector to the surface next to the positive sector. This process repeats as the discs revolve until one of the sectors on the other disc passes one of the earthed brushes. This brush discharges the positive charge on the opposite side of the sector to where the electrons have migrated. As the sectors part, this second sector now retains a net negative charge. This negative charge will now repel electrons on sectors on the other disc as they pass. Once again earthed brushes dissipate the negative charge on the outside of those sectors leaving a net positive charge. The repeated induction of charges is cumulative and if it were not for the current drawn would continue until the insulation broke down. The positive and negative charges are collected by combs to charge the Leyden jars. It is the mechanical energy required to separate the opposing charges on the adjacent sectors that provides the energy source for the electrical output.


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