The Wimshurst machine is a simple, portable electrostatic generator that makes for a great demo for a variety of audiences.
Tribocharging
The triboelectric effect describes the development of net charge that develops when two dissimilar materials are brought into contact. Laser printing, which is based on Xerogrpahy uses this principle to charge toner particles that stick selectively to a drum that has been patterned with light. But the most familiar manifestation of tribocharging is the static electricity that gives you a little shock when you touch a metal object like a door knob. That electricity results from the net charge that develops from your feet repeatedly contacting the floor.
Electrostatic Generators
Van de Graaf generator like the Westinghouse Atom Smasher use belts to convey charge from the triboelectric effect. The Wimshurst machine, which predates the Van de Graaf generator, works on the same principle. Two counter-rotating discs constantly rub against metal brushes. The discs are insulating, but have metallic pads spaced at regular intervals, that shuttle the charge from each revolution to Leyden jars that store the accumulated charge capacitively.
You can download a high quality PDF of this diagram here
One of way of thinking about how charge accumulates is by considering that all surfaces are hydrated by the water vapor that is present in air. At certain ranges of relative humidity nanoscopic drops of water form. When two surfaces come into contact they are bridged by these drops of water. Different materials exhibit different electrostatic properties such as work functions, which induces dissolved ions in the water bridges to distribute between them. When the two surfaces are separated, the water bridge breaks faster than the ions can move, trapping more positive ions on one side and more negative ions on the other, creating a difference in electric potential.
Why this is a great demo
The Wimshurst machine is very simple, allowing observers to see all of the moving parts.
Wimshurst machines can be built or purchased as kits or fully assembled.
Wimshurst machines are compact, portable and do not require electricity, chemicals or any other consumables.
Wimshurst machines can generate tens of thousands of volts of electric potential, but are safe to operate because the maximum currents are minuscule
The best part of the Wimshurst machine is how many scientific concepts it is capable of demonstrating, making it fun and informative for just about any audience. Static electricity is familiar to everyone, making it easy to lead natural curiosity from an every day phenomenon to tribocharging, Xerography, particle accelerators and the nature of electricity. It can also be used to discuss more advanced concepts such as electric double layers, the Gibbs-Donnan effect and other principles associated with surfaces, interfaces, ions and electric charge.
What can you do with it?
You can perform simple experiments like changing the gap-size and timing the “lighting bolts” or making qualitative observations about their size or how loud they get.
You can put objects between the electrodes to see how they interact with huge electric potentials.
You can dare people to touch it and get a shock.
You can have two people each touch an electrode and then another person, who touches another person and so-on until the last two people close the circuit and get a shock; you will be surprised by how many people can be “energized” by this simple machine.