Switches

Switches are the most common circuit control device. They usually have two or more sets of contacts. Opening the contacts is called "opening" or "breaking the circuit," while closing the contacts is called "closing" or "making" the circuit. "Poles" refer to the number of input circuit terminals. "Throws" refer to the number of output circuits. Such switches are referred to as SPST (singlepole, single-throw), SPDT (single-pole, double-throw), and MPMT (multiple-pole, multiple-throw).

The various types of switches include:

• Hinged pawl - a simple SPST switch to make or break a circuit.

• Momentary contact - another SPST switch, normally open or closed, which makes or breaks the circuit when pressed ... typically used for the horn switch.

• SPDT - one wire in, two wires out ... commonly used in high-beam / low-beam headlamp circuits.

• MPMT - movable contacts are linked to sets of output terminals ... may be used for the transmission neutral start switch.

• Mercury switch - liquid mercury flows between contacts to make circuit ... commonly used to turn engine compartment and trunk lamps on and off.

• Temperature-sensitive switch - a bimetal element bends when heated to make contact completing a circuit or to break contact opening a circuit. The same principle is also used in time-delay switches and flashers.

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