How a Wimshurst Machine Works


The Wimshurst machine has three major parts: rotating parallel plates, neutralizing rods, and collecting combs.

A Wimshurst machine develps a maximum electrostatic potential based on the number of plates used and their diameter, and the spacing between the plates. Increasing the rotating speed of the plates does not increase the maximum discharge voltage it only decreases the time the machine takes to gain its full electrostatic potential (charge).

Metal foil strips called "sectors" are evenly spaced along the outer surface of each plate. These sectors help conduct away the plates excess charge. The charges that accumulate on the sectors are removed by pairs of collecting combs made of tinsel threads.

The Wimshurst machine works with induction and doesn't depend on friction to make an electrostatic charge unlike the Van de Graff generator. Quadrants of negative and positive charge are created across the plates by induction between the front and rear plates. As the plates rotate these positive and negative charges are conducted away through the metal sectors by the collecting combs. Induction requires that an object be initially charged before you can use it to induce a charge in something else. The plates on the Wimshurst machine have an initial imbalance of charge just from being handled when they were put together, when the machine is spun it actually amplifies this initial imabalance of charge.

Looking at the figure to the left let's say that quadrant CD of the rear plate had a slight negative charge before the crank was turned. That would induce a small positive charge on the front plate in the same sector. As you turn the crank, electrons on the front plate sector at point C are repelled by the electrons built up on the rear plate. That pushes them up the neutralizing rod to A, which is a more desirable place to be because the rear plate has a positive charge there. That leaves electron-starved quadrant CD with a net positive charge, and electron-rich quadrant AB becomes more negatively charged.

Note that the rear plate that is rotating in the opposite direction works in a reciprocal fashion. Electrons move through the rear neutralizing rod from B to D. So on the rear plate, quadrant AB becomes more positive and quadrant CD becomes more negative. That permits the front neutralizing rod to scoop up more repelled electrons and so on.

In the region of the collecting combs, the front and rear plates have the same charge. The charges on the plates repel each other in those areas. That permits the collecting combs to scoop up the excess charges and send them to the discharge balls. The charges will continue to accumulate until surface leakage or a spark between the discharge balls dissipates them.