Which of the following measuring instruments operates based on the heating effect of electric current?

Analysis

The question asks for a measuring instrument that functions specifically by utilizing the heating effect of electric current.

Concept Explanation

The heating effect of electric current (also known as Joule heating) occurs when an electric current passes through a conductor, causing it to heat up. The heat produced is proportional to the square of the current ($H = I^2Rt$).

• Moving iron ammeter: Operates based on the magnetic effect of current (the repulsion between two pieces of iron placed in a magnetic field).
• Hot wire ammeter: Contains a fine wire that expands when heated by the current passing through it. This expansion is used to move a pointer on a scale. Because the heat produced is proportional to $I^2$, it can measure both direct current (DC) and alternating current (AC).
• Galvanometer: Operates based on the magnetic effect of current (a coil rotating in a magnetic field).
• Electric heater: While it operates on the heating effect of current, it is an appliance, not a measuring instrument.

Step-by-Step Solution

1. Evaluate the options: We are looking for a device that measures current and uses heat.
2. Eliminate non-measuring instruments: Option D (electric heater) is a device that converts electrical energy to heat, but it does not measure current.
3. Analyze the measuring instruments: • Moving iron ammeters and galvanometers rely on magnetic fields. • The hot wire ammeter is specifically designed so that the current passes through a wire, causing it to get hot and expand. The degree of expansion is calibrated to show the value of the current.
4. Conclusion: The hot wire ammeter is the only measuring instrument listed that relies on the thermal (heating) effect of electricity.

Exam Tip

When you see questions about ammeters, remember the "Effect" rule:

• Moving Coil/Moving Iron: Magnetic Effect.
• Hot Wire: Heating Effect.
• Digital: Electronic/Semiconductor properties.