Which one of the following instruments is best suited for accurately measuring very high temperatures inside metallurgical furnaces and kilns?

Introduction / Context:
Industrial processes such as metal smelting, glass making, and ceramic firing require accurate measurement of very high temperatures, often far above the boiling point of water. Ordinary clinical or laboratory thermometers cannot withstand or measure such extreme conditions. This question asks you to identify which instrument among the given options is specially designed for measuring high temperatures inside metallurgical furnaces and similar environments.

Given Data / Assumptions:
- The environment is a metallurgical furnace or kiln, with very high temperatures.- Contact with the hot material may be difficult or undesirable.- We need an instrument suitable for high temperature measurement, not for moderate or low temperatures.

Concept / Approach:
A pyrometer is an instrument specifically designed to measure high temperatures, often by detecting thermal radiation emitted by the hot body. Optical or radiation pyrometers can measure temperature from a distance without direct contact with the material. Thermocouples can also measure high temperatures, but in many textbooks and general knowledge questions, the most appropriate term connected with furnace temperature measurement is pyrometer. Mercury thermometers and common thermistors are limited to much lower temperature ranges and cannot survive or function reliably in a metallurgical furnace.

Step-by-Step Solution:
1. Recognise that metallurgical furnaces can reach temperatures of more than 1000 degree Celsius.2. Note that mercury thermometers typically operate only up to about 350 degree Celsius and would fail at such temperatures.3. Understand that thermistors are semiconductor devices mainly used for moderate temperature ranges in electronics and household devices.4. Recall that thermocouples can measure high temperatures but generally require contact and are sometimes less emphasised in general knowledge questions.5. Recall that pyrometers are widely mentioned as the standard instruments for measuring high furnace temperatures, often by sensing emitted radiation.6. Therefore, identify pyrometer as the best suited instrument for measuring temperature inside metallurgical furnaces.

Verification / Alternative check:
Industrial practice supports this reasoning. In steel plants and glass factories, optical pyrometers are regularly used to monitor furnace and molten metal temperatures. These instruments can be pointed at the hot object without physical contact, which is practical and safe. Manuals and textbooks on industrial instrumentation specifically list pyrometers under high temperature measurement devices, confirming that this is the correct choice for the scenario described.

Why Other Options Are Wrong:
- Thermocouple: While capable of measuring high temperatures, it is not usually described as the specialised furnace instrument in simple general knowledge questions.- Mercury thermometer: Unsuitable at furnace temperatures because mercury and the glass bulb would not withstand such heat.- Thermistor: Typically used for lower temperature ranges in electronic circuits and home appliances, not in metallurgical furnaces.

Common Pitfalls:
Students sometimes choose thermocouple because they remember it is used in some high temperature applications. However, exam questions often expect the more specific term pyrometer when the context is large industrial furnaces. To avoid confusion, associate pyrometers with red hot metals and kilns, and thermocouples with more general industrial temperature sensing including but not limited to engines and pipelines.

Final Answer:
The most suitable device for measuring very high furnace temperatures is the pyrometer.