A transistor is an electrical component that functions, most basically, as a switch — in principle not so different from a light switch. Instead of a physical movement, however, a transistor is controlled by a flow of electricity. And unlike your basic light switch, a transistor can be on, off, or somewhere in between.

Most transistors have three connections: one for current in, one for current out, and one that controls the “switch.” The current flowing through a transistor can be larger than the current controlling it, so it can become an amplifier: Connect the input to a power source (like a battery), and the control lead to a weak signal (like a guitar pickup), and the output will sound like the control signal, only louder. Just how much louder depends on a lot of things, but a factor of 200 is routine. This number is called gain.

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If you use the output from one transistor to control another, the gains multiply. With two transistors, the ideal gain becomes 200 × 200 = 40,000, and with three transistors (as in this circuit), 200 × 200 × 200 = 8,000,000! That huge gain lets you use it to detect the tiniest movements of electricity — even those created at a distance by induction or static charge!

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