Question

Which instrument has the manual Null balance operation?

• A. Optical pyrometer
• B. Resistance Thermometer
• C. Glass Thermometer
• D. Thermistor

Hint:

Only the \textbf{optical pyrometer} requires \textbf{manual adjustment} to match the brightness of a filament with the target — a classic example of \textbf{null balance operation}.

Answer 1

The Correct Option is A

An optical pyrometer is a temperature-measuring device used especially for high temperatures where physical contact with the object is not possible.
It is commonly used to measure the temperature of incandescent bodies, such as molten metal or heated furnaces.
The key working principle involves the comparison of brightness between a calibrated filament inside the pyrometer and the hot object.
- The observer manually adjusts the current flowing through the filament.
- The goal is to make the filament "disappear" against the background when it visually matches the object's brightness.
- This matching process is referred to as the manual Null balance operation.
Why others are incorrect:
- Resistance Thermometers (RTDs) operate based on change in resistance and do not require manual balancing.
- Glass Thermometers work on the expansion of liquid and do not use any null balance mechanism.
- Thermistors are automatic sensors based on temperature-dependent resistance.
Therefore, only the optical pyrometer involves a manual nulling process by visual observation, making it the correct answer.