Wimshurst machine

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The Wimshurst machine is a historical electrostatic machine for generating high voltages, and was developed between 1880 and 1883 by British inventor James Wimshurst (1832 – 1903). It is an electrical generator with a distinctive appearance, having two large contra-rotating discs mounted in a vertical plane, two crossed bars across them, and a spark gap formed by two metal spheres.

[edit] Description

The machine belongs to a class of generators called influence machines. These machines work by separating electric charges by 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). They were more efficient than the earlier machines that worked by friction. The older influence machines exhibited a tendency to suddenly and without warning switch their polarity. The Wimshurst machine did not suffer from this defect.

The working principle of the machine is that as the two counter-rotating disks made of insulating material porting metal sectors pass one another in front of the crossed metal bars with brushes, or neutralizers, they induce a charge imbalance on one another, with a positive feedback effect that increases the charges exponentially until ionization of the air and sparking limit the charge concentration. The generated charges are then partially drained off at the collecting electrodes, that consist of combs of sharp metal points mounted on horseshoe-shaped conductors, placed at a small distance from the surfaces of the disks, without touching them, at the sides of the machine. The collectors are mounted on insulating supports, and connected to the terminals of the machine.

The machine is self-starting, meaning that it requires no external electrical power supply to create the initial charge. The machine quickly amplifies any minute charge imbalance in the disks, always present. It does, however, require mechanical power to turn the discs against the electric field, and is this power that the machine converts into electric power. The output of the machine is essentially a constant current, proportional to the area covered by the sectors and to the rotation speed. The spark energy can be increased by adding a pair of Leyden jars, which are an early type of capacitor suitable for high voltages, with the inner plates connected to the terminals and the outer plates interconnected. A typical Wimshurst machine can produce sparks with about a third of the disk's diameter in length, and several tens of microamperes of current.

[edit] External links and references

• "History of Electrostatic Generators". Hans-Peter Mathematick Technick Algorithmick Linguistick Omnium Gatherum.
• de Queiroz, Antonio Carlos M., "Electrostatic Machines".
  • de Queiroz, Antonio Carlos M., "The Wimshurst Electrostatic Machine"
• Weisstein, Eric W., "Wimshurst Machine".
• Bossert, François, "Wimshurst machine". Lycée Louis Couffignal, Strasbourg. (English version)
• Charrier Jacques "La machine de Wimshurst". Faculté des Sciences de Nantes.