**Explanation:**

**Pyrometer**: A pyrometer is an instrument used to measure high temperatures without making direct contact with the object being measured. It operates based on the principle of thermal radiation emitted by the object.

**Thermal Radiation**: All objects above absolute zero temperature emit thermal radiation in the form of electromagnetic waves. The intensity and wavelength distribution of this radiation depend on the temperature of the object.

**Working of a Pyrometer**: A pyrometer measures the temperature of a source by detecting and analyzing the thermal radiation emitted by the source.

1. **Detection**: The pyrometer consists of a sensor that detects the thermal radiation emitted by the source. The sensor is usually made of a material that is sensitive to a certain range of wavelengths.

2. **Conversion**: The detected thermal radiation is converted into an electrical signal using various techniques. One common method is using a thermopile.

3. **Thermopile**: A thermopile is a device that consists of multiple thermocouples connected in series. A thermocouple is made of two dissimilar metals joined together. When exposed to a temperature gradient, a thermocouple generates an electric voltage proportional to the temperature difference.

4. **Measurement**: The electrical signal from the thermopile is then measured and analyzed to determine the temperature of the source. The voltage generated by the thermopile is calibrated to correspond to a specific temperature range.

5. **Advantages of a Pyrometer**: Pyrometers have several advantages in temperature measurement:

- Non-contact: Pyrometers can measure the temperature of objects without physically touching them, making them suitable for measuring high-temperature sources or objects in motion.

- Fast response: Pyrometers provide instant temperature readings, allowing for real-time monitoring and control.

- Wide temperature range: Pyrometers can measure temperatures ranging from a few hundred degrees Celsius to several thousand degrees Celsius.

- Versatility: Pyrometers can be used in various industries, including metallurgy, glass manufacturing, ceramics, and food processing.

In conclusion, a pyrometer is the correct instrument used to measure the temperature of a source from its thermal radiation. It operates based on the principle of detecting and analyzing the thermal radiation emitted by the source, providing non-contact and accurate temperature measurements.

Pyrometer, device for measuring relatively high temperatures, such as are encountered in furnaces. Most pyrometers work by measuring radiation from the body whose temperature is to be measured