Question 66

The Correct Answer is A. **Explanation for A (Thermocouple):** A thermocouple operates based on the Seebeck effect (thermoelectric effect). It consists of two dissimilar electrical conductors joined together at one end (the hot junction or sensing junction). When this junction is heated, and the other ends (the cold junction or reference junction) are kept at a different, often cooler, temperature, a voltage (electromotive force or EMF) is generated. The magnitude of this voltage is directly proportional to the magnitude of the temperature difference between the hot and cold junctions. **Why the other options are incorrect:** * **B) Resistance temperature detector (RTD):** An RTD operates by changing its electrical resistance as the temperature changes. It requires an external current source (excitation current) to measure the voltage drop across it, which is then converted to temperature. It does not generate its own voltage based on temperature difference. * **C) Bimetallic sensor:** A bimetallic sensor uses two strips of different metals welded together. When heated, the difference in thermal expansion rates causes the strip to bend or coil. It is a mechanical device used for indication or switching and does not generate an electrical voltage. * **D) Thermistor:** Similar to an RTD, a thermistor changes its electrical resistance significantly with changes in temperature (often having a negative temperature coefficient, NTC). It is a passive sensor and requires an external excitation voltage/current to measure the change in resistance; it does not generate a voltage itself.