WIMSHURST MACHINE: HISTORY, HOW IT WORKS AND APPLICATIONS - PHYSICAL - 2025

The Wimshurst machine is a high voltage, low amperage electrostatic generator, capable of producing static electricity by separation of charges, thanks to the turn of a crank. On the other hand, the currently used generators such as batteries, alternators and dynamos are rather sources of electromotive force, causing movements of charges in a closed circuit.

The Wimshurst machine was developed by the British engineer and inventor James Wimshurst (1832-1903) between 1880 and 1883, improving versions of electrostatic generators proposed by other inventors.

Wimshurst machine. Source: Andy Dingley (scanner)

It stands out over previous electrostatic machines for its reliable, reproducible operation and simple construction, being able to generate a staggering potential difference of between 90,000 and 100,000 volts.

Wimshurst machine parts

The base of the machine are the two characteristic insulating material discs, with thin metal sheets attached and arranged in the form of radial sectors.

Each metal sector has another diametrically opposite and symmetrical. The discs are usually between 30 and 40 cm in diameter, but they can also be much larger.

Both discs are mounted in a vertical plane and are separated by a distance of between 1 to 5 mm. It is important that the discs never touch during spinning. The discs are rotated in opposite directions by means of a pulley mechanism.

The Wimshurst machine has two metal bars parallel to the plane of rotation of each disk: one towards the outside of the first disk and the other towards the outside of the second disk. These bars intersect at an angle with respect to each other.

The ends of each bar have metal brushes that make contact with opposing metal sectors on each disc. They are known as neutralizer bars, for good reason that will be discussed shortly.

The brushes keep in electrical (metallic) contact the sector of the disc that touches one end of the bar, with the sector diametrically opposite. The same thing happens on the other album.

The triboelectric effect

The brushes and sectors of the disc are made of different metals, almost always copper or bronze, while the blades of the discs are made of aluminum.

The fleeting contact between them while the discs rotate and the subsequent separation, creates the possibility of exchanging charges through adhesion. This is the triboelectric effect, which can also occur between a piece of amber and a woolen cloth, for example.

A pair of U-shaped metal collectors (combs) are added to the machine with metal tips or spikes, located in opposite positions.

The sectors of both discs pass through the internal part of the collector U without touching it. The collectors are mounted on an insulating base and are in turn connected to two other metal bars ending in spheres, close but not touching either.

When mechanical energy is supplied to the machine by means of the crank, the friction of the brushes produces the triboelectric effect that separates the charges, after which the electrons already separated are captured by the collectors and stored in two devices called bottles of Leyden.

The Leyden bottle or jug ​​is a condenser with cylindrical metal frames. Each bottle is connected to the other by the central plate, forming two capacitors in series.

Turning the handle produces such a high difference in electrical potential between the spheres that the air between them ionizes and a spark jumps. The complete device can be seen in the image above.

In the Wimshurst machine, electricity comes out of matter, which is made up of atoms. And these in turn are made up of electric charges: negative electrons and positive protons.

In the atom the positively charged protons are packed in the center or nucleus and the negatively charged electrons around its nucleus.

When a material loses some of its outermost electrons, it becomes positively charged. Conversely, if you capture some electrons, you get a net negative charge. When the number of protons and electrons is equal, the material is neutral.

In insulating materials, electrons stay around their nuclei without the ability to stray too far. But in metals the nuclei are so close to each other that the outermost electrons (or valence) can jump from one atom to another, moving throughout the conductive material.

If a negatively charged object approaches one of the faces of a metal plate, then the electrons of the metal move away by electrostatic repulsion, in this case to the opposite face. The plate is then said to have become polarized.

Now, if this polarized plate is connected by a conductor (neutralizing bars) on its negative side to another plate, the electrons would move to this second plate. If the connection is suddenly cut, the second plate is negatively charged.

Load and storage cycle

For the Wimshurst machine to boot, one of the metal sectors on the disk needs to have a load imbalance. This happens naturally and frequently, especially when there is little humidity.

When the discs begin to spin, there will be a time when a neutral sector of the opposite disc opposes the loaded sector. This induces on it a charge of equal magnitude and opposite direction thanks to the brushes, since the electrons move away or closer, according to the sign of the sector facing.

Schematic of the Wimshurst machine. Source: RobertKuhlmann

The U-shaped collectors are responsible for collecting the charge when the disks repel each other because they are charged with charges of the same sign, as shown in the figure, and store said charge in the Leyden bottles connected to them.

To achieve this, the internal part of the U protrudes comb-like peaks directed towards the external faces of each disc, but without touching them. The idea is that positive charge concentrates on the tips, so that the electrons expelled from the sectors are attracted and accumulate in the central plate of the bottles.

In this way the sector facing the collector loses all its electrons and remains neutral, while the central plate of the Leyden is negatively charged.

In the opposite collector, the opposite happens, the collector delivers electrons to the positive plate that faces it until it is neutralized and the process is repeated continuously.

Applications and experiments

The main application of the Wimshurst machine is to obtain electricity from each sign. But it has the disadvantage that it supplies a rather irregular voltage, since it depends on the mechanical actuation.

The angle of the neutralizer bars can be varied to set high output current or high output voltage. If the neutralizers are far from the collectors, the machine delivers a high voltage (up to more than 100 kV).

On the other hand, if they are close to the collectors, the output voltage decreases and the output current increases, and can reach up to 10 microamperes at normal rotational speeds.

When the accumulated charge reaches a high enough value then a high electric field is produced in the spheres connected to the central plates of the Leyden.

This field ionizes the air and produces the spark, discharging the bottles and giving rise to a new charge cycle.

Experiment 1

The effects of the electrostatic field can be appreciated by placing a sheet of cardboard between the spheres and observing that the sparks make holes in it.

Experiment 2

For this experiment you will need: a pendulum made of a ping pong ball covered with aluminum foil and two L-shaped metal sheets.

The ball is hung in the middle of the two sheets by means of an insulating wire. Each sheet is connected to the electrodes of the Wimshurst machine by cables with clamps.

As the crank is turned, the initially neutral ball will oscillate between the blades. One of them will have an excess of negative charge that will yield to the ball, which will be attracted by the positive sheet.

The ball will deposit its excess electrons on this sheet, it will be briefly neutralized and the cycle will repeat again as long as the crank continues to turn.

References

1. De Queiroz, A. Electrostatic Machines. Recovered from: coe.ufrj.br
2. Gacanovic, Mico. 2010. Electrostatic Application Principles. Recovered from: orbus.be