The Wimshurst machine is another device for generating high voltages with the advantage that it can create both positive and negative charges. It consists of two parallel insulating discs that are geared to rotate in opposite directions, neutralising rods and rods that collect the charge.
Although the discs are made of an insulating material, each disc has metal foil strips that are spaced radially on the outer surface of the discs. These plates help collect the excess charge that builds up on the insulating plates. The charge is removed by pairs of collecting combs. The Wimshurst machine works by induction, when the device is made there will be a small charge on the surface of the discs, which.
Figure 2. Schematic diagram of a Wimshurst machine operation
An insulator is very difficult to discharge completely and there will always be regions on the discs which have small imbalances in charge. Consider such a small charge at the top of the disc, the charge induces an opposite charge on the second disc and the charges rotate away from each other, the foil sectors contain the charge. When the foil sectors pass under the neutralising rods they induce the opposite charge on the opposite side of the discs. This creates more charge on the foil sector. As the induced charges rotate around the disc the induced charges create a positive feed back of charge, so that on the top half of the front disc there is a build up of positive charge, while on the bottom half of the front disc there is a negative charge. On the back disc, the induction produces a negative charge on the top of the back disc and a positive charge on the bottom half of the back disc. Positive and negative charges are removed from the by combs which collect the charge and conduct it to metal spheres, until it become strong enough to jump across an air gap.
The discs make it difficult to visualise the build up of charge, so it might become clearer by considering the discs as opposite rotating cylinders. The inner cylinder represents the front disc and the outer cylinder the outer disc. On each cylinder the foil sectors are represented by the rectangular blocks that occupy the perimeter of the cylinders. The neutralising rod for the outer disc goes around the outer half of the outer cylinder. The second neutralising rod is placed at ninety degrees to the outer neutralising rod.
If one of the foil sectors on the outer cylinder has a slight charge on its surface, positive say. It will induce a negative charge on the foil sectors that is closest to it on the inner cylinder. As the cylinders rotate in oposite directions, the charges are separated. The energy for the electricity comes from the work done in turning the discs.
At A and B on the two discs, brushes at the ends of a neutralising rod connect diametrically opposite sectors momentarily. If we consider one disc, then the sector is oppposite, say, a negatively charged sector on the other disc. A (+) charge is induced on the inner side, while a (-) charge appears on the outer side, under the brush. At the diametrically opposite sector, the polarities are reversed, so the (+) charge appears on the outer side under the other brush. The charges on the outer sides are neutralized as a brief current flows, leaving the upper sector with a greater (+) charge, and the lower with a greater (-) charge. The induction of the charges does not depend on how much charge is already there, and equal signs of charge will not neutralize. The sectors charge to higher and higher potentials each time they pass beneath a brush, and their higher potentials serve to induce even greater charges.