Which of the following devices is made of two different metals and produces a voltage when the metals are exposed to heat?

A. An accelerometer

B. A photovoltaic cell

C. A crystal

D. A thermocouple

This is the mechanism behind a thermocouple (choice D). The two metals generate a magnetic field between them when the temperature is varied, and this causes a voltage (potential difference) to form between the metals. An electronic meter is calibrated to translate this voltage difference into a temperature value for a user to read.