Which statement reflects how a thermocouple responds to temperature changes?

• A. Electrical resistance changes, and the resulting current is interpreted as temperature
• B. Thermal expansion causes a length change
• C. The device emits light proportional to temperature
• D. The pressure across a diaphragm changes

Answer: (A)

A thermocouple responds to temperature changes by producing a small electrical voltage, not by changing its resistance. This voltage arises from the Seebeck effect: when two dissimilar metals are joined, a temperature difference between the junctions creates an electromotive force that a meter reads as temperature. The signal is typically a millivolt-level voltage that the instrumentation converts into a temperature reading; the amount of voltage depends on the temperature gradient and the metal pair used. This is different from resistance-based sensors, where temperature is inferred from changes in electrical resistance. The other effects listed—thermal expansion, light emission, or pressure changes—belong to different types of sensors and do not describe how a thermocouple generates its signal. The key point is that a thermocouple’s output is an electrical voltage proportional to temperature difference, interpreted by the measurement system to determine temperature.