Section 3.4

Generation of Electricity

Production of electricity by light, heat, friction, pressure, chemical action and magnetism.

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There are six recognised methods of producing electricity: friction, chemical action, heat, light, pressure, and magnetism.
Chemical action produces electricity in primary and secondary cells — the aircraft battery.
Heat applied to the junction of two dissimilar metals produces an EMF: the thermocouple, used for exhaust gas and cylinder head temperature.
Light falling on a photocell produces an EMF; pressure applied to a crystal produces an EMF (piezoelectric effect), used in some vibration and pressure sensors.
Magnetism — electromagnetic induction — is the primary method of generating electrical power on an aircraft.
⚠ Exam trap: a thermocouple converts heat to EMF; a piezoelectric crystal converts pressure to EMF. Do not confuse the two.

Solved examples

An engine's exhaust gas temperature indicating system uses which method of electricity generation?
Heat — a thermocouple made of two dissimilar metals (commonly chromel/alumel) produces an EMF proportional to the temperature difference between the hot and cold junctions.

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