What device is used to measure temperature on a piston engine?

The correct choice is a thermocouple, which is specifically designed to measure temperature in various applications, including piston engines. A thermocouple consists of two different metal wires that are joined at one end. When the junction experiences a change in temperature, it generates a voltage that correlates to the temperature change, allowing for precise readings. This capability to withstand extreme temperatures and provide rapid response times makes thermocouples particularly suitable for measuring the high temperatures often found in piston engines.

Other devices listed serve different purposes. A thermometer, while it can measure temperature, is not the most effective or reliable device for the extreme environments encountered in engine operations. A barometer measures atmospheric pressure and is irrelevant for temperature measurement. A thermostat is used to regulate temperature by controlling the flow of coolant in an engine rather than measuring the temperature itself. Understanding the specific application of each device clarifies why a thermocouple is the most appropriate choice for measuring temperature on a piston engine.