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A relay is an electromagnetic device in which contacts are made and
subsequently broken. An example of this would be your car's horn.
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.
In 1922, a Model T was the first car equipped with a radio. In 1927,
the first commercially produced car radio came on the scene.
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.
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