The Wimshurst machine was developed by British engineer and inventor James Wimshurst (1832-1903) between 1880 and 1883, improving the versions of electrostatic generators proposed by other inventors.
It stands out from previous electrostatic machines for its reliable and reproducible operation and its simple construction, capable of generating an incredible potential difference between 90,000 and 100,000 volts.
Wimshurst Machine Parts
The base of the machine is the two characteristic discs of insulating material, with thin metal plates attached and arranged in the form of radial sectors.
Each metal sector has another sector that is diametrically opposite and symmetrical. 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 separate a distance between 1 and 5 mm. It is important that during rotation the discs never touch. The discs are rotated in opposite directions by a pulley mechanism.
The Wimshurst machine has two metal bars parallel to the plane of rotation of each disc: one facing the outside of the first disc and the other facing the outside of the second disc. These bars cross at an angle to each other.
The ends of each bar have metal brushes that make contact with opposite sectors of metal on each disk. They are known as neutralizing bars, for a good reason that will be seen shortly.
The brushes keep the disc sector touching one end of the bar in electrical (metallic) contact with the diametrically opposite sector. The same happens on the other disc.
The triboelectric effect
The brushes and disc sectors are made of different metals, almost always copper or bronze, while the disc plates are made of aluminium.
The fleeting contact between them as 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) is added to the machine and terminated in metal spikes or spikes located in opposite positions.
The sectors of both disks pass through the inside of the collector U without touching it. The collectors are mounted on an insulating base and, in turn, are connected to two other metal bars that end in spheres, closed but not touching.
When mechanical energy is supplied to the machine through the crank, brush friction produces the triboelectric effect that separates the charges, after which the already separated electrons are picked up by the collectors and stored in two devices called Leyden bottles
The Leyden bottle or pitcher is a condenser with cylindrical metal reinforcements. Each bottle is connected to the other by the central plate, forming two capacitors in series.
Turning the crank produces such a high electrical potential difference between the spheres that the air between them ionizes and a spark jumps out. The complete device can be seen in the image above.
Physical principles involved
In the Wimshurst machine, electricity leaves matter, which is made up of atoms. And these, in turn, are composed of electrical charges: negative electrons and positive protons.
In an atom, positively charged protons are packed in the center or nucleus and negatively charged electrons around its nucleus.
When a material loses some of its outermost electrons, it is positively charged. On the contrary, 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 remain around their nuclei, without the possibility of moving very far. But in metals, the nuclei are so close together that the outermost (or valence) electrons can jump from one atom to another, moving through all the conducting material.
If a negatively charged object is approached from one side of a metal plate, electrons in the metal are driven away by electrostatic repulsion, in this case to the opposite side. The plate is said to have polarized.
Now, if this polarized plate is connected by a conductor (neutralizing bars) on its negative side to another plate, electrons will move to this second plate. If the connection is suddenly interrupted, the second card will be negatively charged.
Loading and storage cycle
In order for the Wimshurst machine to start, one of the metal sectors on the disk needs to have a load imbalance. This occurs naturally and frequently, especially when there is low humidity.
When the disks start to spin, there will be a moment when a neutral sector of the opposite disk opposes the loaded sector. This induces a charge of equal magnitude and opposite direction, thanks to the brushes, as the electrons move away or come closer, according to the sign of the sector ahead.
U-shaped collectors are responsible for collecting the charge when the discs are repelled, being charged with charges of the same sign as shown in the figure, and store this charge in Leyden bottles connected to them.
To achieve this, the peaks protrude into the U like combs directed towards the outer faces of each disk, but without touching them. The idea is that the positive charge is concentrated at the ends, 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 Leyden plate is negatively charged.
The opposite happens in the opposite collector, the collector delivers electrons to the positive plate facing it until it is neutralized and the process is repeated over and over.
Applications and experiments
The main application of the Wimshurst machine is to get electricity from each signal. But it has the disadvantage of providing a very irregular voltage, as it depends on the mechanical drive.
The angle of the neutralizer bars can be varied to set high output current or high output voltage. If the neutralizers are far away from the collectors, the machine will supply 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 microamps at normal speeds of rotation.
When the accumulated charge reaches a high enough value, a high electric field is produced in the spheres connected to the central Leyden plates.
This field ionizes the air and produces the spark, discharging the bottles and giving rise to a new charging cycle.
The effects of the electrostatic field can be seen by placing a sheet of cardboard between the spheres and noticing that sparks bore holes in it.
For this experiment, you need: a pendulum made from 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 insulating wire. Each sheet is connected to the electrodes of the Wimshurst machine by means of wires with staples.
When the crank is turned, the initially neutral ball will swing between the sheets. 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, will be neutralized shortly, and the cycle will be repeated again as long as the crank continues to turn.