The Wimshurst electrostatic generator was invented in the 1880s. The modern version consists of two plastic discs which are are rotated in opposite directions by a hand-crank and drive belt mechanism.
Wimshurst Electrostatic Generator
The way it works is that cemented onto each disc are a number of metal foil sectors which both generate and carry small charges of electricity. These charge will be stored in a Leyden Jar capacitor until discharged.
As the discs turn, each metal foil sector accumulates the charges through contact with brushes on bars near the front and back of the discs. Two additional pairs of brushes collect the accumulated charges and transmit them to the storage capacitor.
The brushes also are connected to spark gap electrodes. As the discs revolve, a high voltage spark can jump between the electrodes if they are gradually brought together.
The Wimshurst electrostatic generator can generate up to 75,000 volts. Since the current is very low, there is little danger from the high voltage, but yet it is effective for creating sparks and performing interesting static electricity experiments.
The Van de Graaf generator can develop potential energy of as high as 400,000 volts, and it will develop sparks up to 15 inches (38 centimeters) in length.
It is relatively safe to use though, since the current is only 10 microamps. (10 / 1,000,000 amps). This is because electrical power equals the voltage times the amperage. 400,000 volts times 10 microamps equal only 4 watts—the energy to light a flashlight bulb.
Van de Graaf Generator
The Van de Graaff generator is powered by a high speed electric motor. It has a belt running on two pulleys. The lower pulley is made of an insulating material and the upper pulley is made of metal. There is a grounded comb close to the belt in front of the lower pulley, and another inside the terminal, close to the belt in front of the upper pulley, connected to the terminal.
Adhesion between the belt and the lower pulley charges the pulley, and it attracts opposite charges to the outer surface of the belt from the grounded comb. The belt then transports these charges to the terminal, where they are collected by the upper comb and transferred to the terminal. The shielding provided by the terminal allows the continuation of the process even if the terminal is already strongly charged. The high speed of the belt, along with the large discharge electrode, allows the continuous buildup of charges and great voltages to be achieved.
When you take a fluorescent bulb and bring it near a Van de Graaf generator, the charges excite the atoms inside the bulb, cause it to glow--even if the bulb is not plugged in.
Other Electrostatic Generators