Refer to figures.
EASA learning objective 022.01.02.01.04 states: Identify temperature measurements that are applicable to an aircraft: gas temperature measurement (ambient air, bleed-air systems, air-conditioning systems, air inlet, exhaust gas, gas turbine outlets); liquid-temperature measurement (fuel, oil, hydraulic); component-temperature measurement (generator, transformer rectifier unit (TRU), pumps (fuel, hydraulic), power transfer unit (PTU).

Aircraft use a variety of methods for measuring the temperature of liquids, gases and aircraft components, the most common are as follows:

Bi - Metallic Strip - Used in direct reading thermometers for measuring OAT in light aircraft. This is an expansion type system. Two strips of metal with different expansion properties are fused together, when heated one metal expands more than the other causing the strip to bend. This movement can be used to drive a pointer over a temperature scale up to a maximum of 400°C

Temperature - dependent resistor - The electrical resistance of some materials changes with temperature, this characteristic can be used to measure temperature. Resistance temperature devices are solid state devices which commonly use platinum as the sensing element, they require an electrical current to operate and are able to accurately measure temperatures up to 500° These devices are commonly used to measure aircraft component temperatures but aren't suitable for measuring very high temperatures such as exhaust gas.

Thermocouples - Thermocouples are used for measuring the very high temperatures found in exhaust gases and cylinder heads and can be used up to approximately 2000°C. A thermocouple uses a junction of two different metals which produce a voltage related to the difference in temperature between hot and cold junctions. Multiple thermocouples are used in jet engines to measure EGT, they require no electrical power as they produce their own voltage.