Which instrument is used to measure the difference between two temperatures?

The correct choice is the thermocouple. A thermocouple is specifically designed to measure the temperature difference between two junctions. It operates based on the principle of thermoelectric effect, where two dissimilar metals are joined at one end, creating a voltage that is proportional to the temperature difference between the junctions. This allows it to effectively measure variations in temperature across different points in a system.

In contrast, a thermometer primarily measures absolute temperature rather than the difference between two temperatures. It provides a direct reading of the temperature at a specific location, which is useful but not for detecting a difference. A thermistor, a type of resistor that changes resistance with temperature, is also typically used for measuring temperatures rather than differences, although it can be utilized in differential temperature applications with additional circuitry. A barometer measures atmospheric pressure and is unrelated to temperature measurements. Hence, the thermocouple is the appropriate instrument for measuring the difference between two temperatures.