Speaker

A speaker turns an electrical signal into sound. A coil of wire sits in the field of a strong magnet, and when the audio signal flows through the coil, the coil is pushed back and forth. The coil is glued to a light cone, which moves the air in waves we hear as sound.

To build one, a neodymium magnet is fixed into a cup of steel with a steel plate on top. The steel channels the magnet's field into a narrow ring-shaped gap. Enameled copper wire is wound into the voice coil on a light tube that drops into that gap. The coil is glued to the narrow end of a cone of paper pulp. A flexible ring at the cone's edge (the surround) and a springy fabric disk (the spider) hold it centered, so it can move in and out but not sideways. Everything sits in a frame, and fine wires bring the signal to the coil.

In the television, two or more small speakers play the sound, often facing down or backward because a slim screen leaves no room at the front. The moving-coil design dates to 1925, from Chester Rice and Edward Kellogg at General Electric.