The Tesla Coil we bring with us is small, and only generates a couple hundred thousand volts of electricity and will light up a fluorescent bulb held in your hand.
A fluorescent light bulb held near a Tesla coil will light up and spark, even without being plugged in!
Quick Physics: The Tesla coil creates an electric field that pushes electrons through the light bulb. This is the same way the lights in your house work, except in your house, the electricty comes through a wire instead of through the air.
A Tesla coil is a device for making very high voltages. Voltage is a way to measure how much energy an electric charge has. Tesla coil can make voltages of more than a million volts. The small one used in the demonstration makes about 60,000 volts. Normally, such high voltages are very dangerous, but the Tesla coil makes very high frequency electricity. This means the coil turns on and off very quickly so they electricity flows on the outside of your skin instead of through your body.
The Tesla coil is very different from the Van de Graaff generator. A Van de Graaff generator makes static electricity; the charges don’t move on their own. A Tesla coil makes current electricity; the charges are flowing. One end of the Tesla coil is connected to the ground. Because the coil makes very high voltages, the electricity can leave the Tesla coil and go through the air to get back to the ground. If a fluorescent light bulb is held near the coil, the electricity will then go through the light bulb to get to the ground, which makes it light up. If you get the light bulb close enough to the Tesla coil, you can see the electricity jumping into the light bulb.
Also see the page on Plasma Balls.