By natural law, the farther electrical current travels, the lower its voltage becomes. Your car horn has to be connected to the car battery in order to sound. The shortest distance between two points is a wire connecting your horn to your battery. The only problem with this arrangement is that connecting the two would give you a permanent horn blast when you turned the key in your ignition.

This is an unacceptable arrangement, so a relay is included in the connection. The relay stops the horn from sounding until you activate the relay by pressing the horn. The relay then allows the horn to connect to the battery, or complete the circuit, and it sounds. As soon as you stop pressing the horn, the relay breaks the connection, or circuit, and presto-- no more horn!

Relays, with switches, are used for most of the equipment that depends on the battery for an energy source. This includes headlights, taillights, radio, etc.

A "relay" is any switching device operated by a low current circuit that controls opening and closing of another circuit of high current capacity. The purpose of the "cutout relay" is to prevent the battery from discharging through the generator when the engine is stopped or turning over slowly. A "field relay" connects the alternator field windings and voltage regulator windings directly to the battery.