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The Electrician Problem

An electrician has two two-way switches (single-pole, double-throw), a light bulb, and a power source. How should he connect the terminals so that either switch can be used to turn the light on or off?

Answer:

The terms pole and throw are used to describe switch contact variations. A pole is a set of contacts or terminals that are connected to a single circuit. A throw is one of two or more positions that the switch can adopt. A single-throw switch has one position that closes contacts, a double-throw switch has two positions.

"Single-pole, double-throw" switches, such as the ones illustrated in this puzzle, are called "two-way" switches in British English, whereas they are generally called "three-way" switches in American terminology. A "two-way" switch in American English refers to a "single pole, single throw" switch.

Terminal D must connect to terminal I, and terminal E must connect to terminal J.   D to J and E to I will also work. The light bulb and the power source must be connected in series, A to G, for example. Terminal F then connects to C and terminal B connects to H.   F to H and B to C will also work. Either switch can then turn the bulb on or off.

Two-way switches

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