What is a thermocouple?

• A. A thermocouple is a mechanical thermometer that uses a mercury column to measure temperature.
• B. A thermocouple is a single metal wire heated by a resistance heater to measure temperature.
• C. A thermocouple is an electrical thermometer containing two differing metal wires, welded at one end and open at the other. A voltage is produced with a change in temperature.
• D. A thermocouple is a device that measures temperature by changes in color of a liquid crystal.

Answer: (C)

A thermocouple works by converting a temperature difference into an electrical voltage using two dissimilar metal wires joined at one end. When the joined end (the hot junction) is at a different temperature from the other ends, electrons diffuse between the two metals and generate a small voltage proportional to that temperature difference. The free ends are connected to a measuring device, so the voltage can be read and translated into temperature. This is why the device is described as an electrical thermometer containing two differing metal wires welded at one end and open at the other, with a voltage produced as temperature changes.

The other descriptions aren’t thermocouples: a mercury-filled glass thermometer uses a liquid column, not a voltage from metal junctions; a single metal wire heated by a heater isn’t a thermoelectric pair and doesn’t produce a thermocouple voltage; and a liquid crystal thermometer measures temperature by color change, not by electrical voltage from metal junctions.