Wimshurst machine
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The Wimshurst influence machine is an
electrostatic generator An electrostatic generator, or electrostatic machine, is an electrical generator that produces ''static electricity'', or electricity at high voltage and low continuous current. The knowledge of static electricity dates back to the earliest ci ...
, a machine for generating
high voltage High voltage electricity refers to electrical potential large enough to cause injury or damage. In certain industries, ''high voltage'' refers to voltage above a certain threshold. Equipment and conductors that carry high voltage warrant spec ...
s developed between 1880 and 1883 by British inventor
James Wimshurst James Wimshurst (13 April 1832 – 3 January 1903) was an English inventor, engineer and shipwright. Though Wimshurst did not patent his machines and the various improvements that he made to them, his refinements to the electrostatic generator led ...
(1832–1903). It has a distinctive appearance with two large contra-rotating discs mounted in a vertical plane, two crossed bars with metallic brushes, and a
spark gap A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the potential difference between the conduct ...
formed by two metal spheres.


Description

These machines belong to a class of electrostatic generators called
influence machine An electrostatic generator, or electrostatic machine, is an electrical generator that produces ''static electricity'', or electricity at high voltage and low continuous current. The knowledge of static electricity dates back to the earliest civil ...
s, which separate
electric charge Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respe ...
s through
electrostatic induction Electrostatic induction, also known as "electrostatic influence" or simply "influence" in Europe and Latin America, is a redistribution of electric charge in an object that is caused by the influence of nearby charges. In the presence of a charg ...
, or ''influence'', not depending on friction for their operation. Earlier machines in this class were developed by
Wilhelm Holtz Wilhelm Holtz (15 October 1836 – 27 September 1913) was a German physicist who was a native of Saatel bei Barth, Mecklenburg. Between 1857 and 1862, he studied physics and natural sciences in Berlin, Dijon and Edinburgh. Afterwards, he perfor ...
(1865 and 1867),
August Toepler August Joseph Ignaz Toepler (7 September 1836 – 6 March 1912) was a German chemist and physicist known for his experiments in electrostatics. Biography August Toepler was born on 7 September 1836. He studied chemistry at the Gewerbe-Insti ...
(1865), J. Robert Voss (1880), and others. The older machines are less efficient and exhibit an unpredictable tendency to switch their polarity, while the Wimshurst machine has neither 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 disc. 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 An electric spark is an abrupt electrical discharge that occurs when a sufficiently high electric field creates an ionized, electrically conductive channel through a normally-insulating medium, often air or other gases or gas mixtures. Michael F ...
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 requires mechanical
power Power most often refers to: * Power (physics), meaning "rate of doing work" ** Engine power, the power put out by an engine ** Electric power * Power (social and political), the ability to influence people or events ** Abusive power Power may a ...
to turn the disks against the electric field, and it is this energy that the machine converts into the electric power of the spark. The steady-state output of the Wimshurst machine is a direct (non-alternating)
current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...
that is proportional to the area covered by the metal sector, the rotation speed, and a complicated function of the initial charge distribution. The insulation and the size of the machine determine the maximal output voltage that can be reached. The accumulated spark energy can be increased by adding a pair of
Leyden jar A Leyden jar (or Leiden jar, or archaically, sometimes Kleistian jar) is an electrical component that stores a high-voltage electric charge (from an external source) between electrical conductors on the inside and outside of a glass jar. It typi ...
s, an early type of
capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of ...
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 available voltage gain can be understood by noting that the charge density on oppositely charged sectors, between the neutralizer bars, is nearly uniform across the sectors, and thus at low voltage, while the charge density on same charged sectors, approaching the collector combs, peaks near the sector edges, at a consequently high voltage relative to the opposite collector combs. Wimshurst machines were used during the 19th century in physics research. They were also occasionally used to generate high voltage to power the first-generation Crookes
X-ray tube An X-ray tube is a vacuum tube that converts electrical input power into X-rays. The availability of this controllable source of X-rays created the field of radiography, the imaging of partly opaque objects with penetrating radiation. In contrast ...
s during the first two decades of the 20th century, although Holtz machines and
induction coil An induction coil or "spark coil" (archaically known as an inductorium or Ruhmkorff coil after Heinrich Rühmkorff) is a type of electrical transformer used to produce high-voltage pulses from a low-voltage direct current (DC) supply. p.98 To ...
s were more commonly used. Today they are only used in science museums and education to demonstrate the principles of electrostatics.


Operation

The two contra-rotating insulating discs (usually made of glass) have a number of metal sectors stuck onto them. The machine is provided with four small brushes (two on each side of the machine on conducting shafts at 90° to each other), plus a pair of charge-collection combs. The conducting shafts, that hold the brushes on a typical Wimshurst machine, would form the shape of an "X", if one could see through the insulating discs, as they are perpendicular to each other. 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 jar A Leyden jar (or Leiden jar, or archaically, sometimes Kleistian jar) is an electrical component that stores a high-voltage electric charge (from an external source) between electrical conductors on the inside and outside of a glass jar. It typi ...
s. 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 (red) and that the back disc ( lower chain) rotates counter-clockwise (right to left). As the charged sector (moving red square) rotates to the position of the brush ( down arrow tip) next to front disc ( upper chain near center), it induces a polarization of charge on the conducting shaft ( -Y1upper horizontal black line) holding the brush, attracting negative (green) charge to the near side ( upper square becoming green), so that positive (red) charge accumulates on the far side (across the disc, 180 degrees away) ( 1upper square becoming red). The shaft's polarized charges attach to the nearest sectors on disc B, resulting in negative charge on B closer to the original positive charge on A, and positive charge on the opposite side of B 1 After an additional 45° rotation ( near lower chain middle), the positive (red) charge on A (lower chain) is repelled by a positive (red) charge on B ( upper chain) approaching. The first collection comb ( arrow-tipped lines to triangles) encountered allows both positive (red) charges to leave the sectors neutral (squares becoming black), and accumulate in the Leyden jar anode (red triangle) attracted to the Leyden jar cathode (green triangle). The charge completes the cycle across the discs when a spark (yellow zigzag) discharges the Leyden jar (red and green triangles). As B rotates 90° clockwise (left to right), the charges that have been induced on it line up with the brushes next to disc A , X1 The charges on B induce the opposite polarization of the A-brushes' shaft, 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° so that its charges line up with the brush of disc B , Y1 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. The "influence" of neighboring attractive sectors induces exponentially larger charges, until balanced by the conducting shaft's finite capacitance. All of these induced positive and negative charges are collected by combs to charge the Leyden jars, electrical charge-storage devices similar to capacitors. The mechanical energy required to separate the opposing charges on the adjacent sectors provides the energy source for the electrical output.


See also

*
Kelvin water dropper The Kelvin water dropper, invented by Scottish scientist William Thomson (Lord Kelvin) in 1867, is a type of electrostatic generator. Kelvin referred to the device as his water-dropping condenser. The apparatus is variously called the Kelvin hydro ...
*
Pelletron A Pelletron is a type of electrostatic generator, structurally similar to a Van de Graaff generator. Pelletrons have been built in many sizes, from small units producing voltages up to 500 kilovolts (kV) and beam energies up to 1 megaelectronvolt ...
*
Van de Graaff generator A Van de Graaff generator is an electrostatic generator which uses a moving belt to accumulate electric charge on a hollow metal globe on the top of an insulated column, creating very high electric potentials. It produces very high voltage direct ...
*
Tesla coil A Tesla coil is an electrical resonant transformer circuit designed by inventor Nikola Tesla in 1891. It is used to produce high-voltage, low-current, high-frequency alternating-current electricity. Tesla experimented with a number of different ...


References

*"
History of Electrostatic Generators
'". Hans-Peter Mathematick Technick Algorithmick Linguistick Omnium Gatherum. *de Queiroz, Antonio Carlos M., "

'" *Weisstein, Eric W., "

'". *Bossert, François, "

'". Lycée Louis Couffignal, Strasbourg.

version) *Charrier Jacques "

'". Faculté des Sciences de Nantes.


External links

{{commons category, Wimshurst machines
The Wimshurst Machine Website: Photos and Video Clips of a Wimshurst MachineMIT video demonstration and explanation of a Wimshurst machine
(MIT TechTV physics demo) Electrostatic generators Electrical instruments Historical scientific instruments Articles containing video clips