Energy conversion principle — thermal to electrical: Identify the device that directly converts a temperature difference into an electrical signal or voltage.

Introduction / Context:
Transducers convert one form of energy to another. Recognizing which device maps a temperature gradient to an electrical output is crucial in instrumentation and control, where accurate measurements feed feedback loops and safety systems.

Given Data / Assumptions:

• The question asks for direct thermal-to-electrical conversion.
• No external excitation beyond temperature difference is implied.
• Focus on the primary operating principle, not incidental effects.

Concept / Approach:
A thermocouple uses the Seebeck effect: when two dissimilar metals are joined and their junctions are at different temperatures, a voltage proportional to the temperature difference is produced. This is a direct thermal-to-electrical conversion and forms the basis for many industrial temperature sensors.

Step-by-Step Solution:
List candidates and their energy domains.Thermocouple → thermal gradient to voltage (Seebeck effect).Solar cell → light (photons) to electrical (photovoltaic effect).Chemical cell → chemical potential to electrical (electrochemistry).Piezoelectric → mechanical strain to electrical charge.Select the device that maps thermal to electrical directly: thermocouple.

Verification / Alternative check:
Thermocouples are standard in process industries and household appliances (e.g., gas furnaces) where measuring temperature via millivolt outputs is common.

Why Other Options Are Wrong:
(b) Photovoltaic devices are light-powered, not heat-powered. (c) Batteries convert chemical energy. (d) Piezo devices respond to pressure or vibration, not directly to temperature difference.

Common Pitfalls:
Confusing thermopiles (series thermocouples) with thermistors. Thermistors change resistance with temperature and require a measurement circuit; they do not directly generate a voltage from thermal gradients.

Final Answer:
thermocouple.