Understanding Meters for DC Circuits

Electrical meters are essential tools for measuring various parameters in circuits, such as current, voltage, and energy. However, not all types of meters are suitable for both alternating current (AC) and direct current (DC) circuits. The operational principle of the meter determines its compatibility with AC or DC.

Analysis of Meter Types and DC Suitability

Let's examine the meter types mentioned in the options:

• Commutator Motor Meters: These meters work based on the motor principle, where a current-carrying coil placed in a magnetic field experiences a force or torque. They can be designed to measure energy in both AC and DC circuits. For DC circuits, the torque is produced by the interaction of the current in the armature coil with the magnetic field produced by the field coils.
• Mercury Motor Meters: These are specifically designed for DC energy measurement. They operate on the principle of a copper disc immersed in mercury, placed in a magnetic field. DC current flows through the disc and mercury, and the interaction with the magnetic field causes the disc to rotate, driving the meter's counter.
• Induction Meters: Induction type meters, such as the common energy meter used in households (for AC supply), operate on the principle of electromagnetic induction. They utilize the interaction between alternating magnetic fluxes and the eddy currents induced by these fluxes in a metallic disc (usually aluminum). A changing magnetic flux is required to induce eddy currents.

Why Induction Meters are Not Used on DC Circuits

The core principle of operation for induction meters relies on inducing eddy currents in a disc. This induction requires a continually changing magnetic field, which is naturally provided by alternating current. A steady direct current produces a constant magnetic field, which does not induce the necessary eddy currents in the disc for the meter to operate correctly or at all. Therefore, induction meters are inherently unsuitable for measuring energy or other parameters in DC circuits.

Comparing the options:

• Commutator motor meters can work on DC.
• Mercury motor meters are primarily designed for DC.
• Induction meters require AC to operate due to their reliance on changing magnetic fields and induced eddy currents.

Thus, the meter type that is not used on a D.C. circuit among the given options is the induction meter.

Conclusion

Based on their operating principles, commutator motor meters and mercury motor meters can be used on DC circuits, while induction meters cannot. The question asks which meter is not used on a D.C. circuit.

Revision Table: Meter Types and Applications

Here's a quick summary table to reinforce the concepts:

Additional Information: Principles of Operation

Understanding the fundamental principles helps clarify why certain meters are suitable for AC, DC, or both.

• DC Meters (like PMMC): Often rely on the permanent magnet moving coil principle. A coil carrying DC current is placed in a static magnetic field. The interaction produces a steady torque, causing deflection proportional to the DC current.
• AC Meters (like basic Induction): Rely on principles that require changing quantities. Induction meters specifically use the interaction of changing magnetic fields produced by AC with eddy currents induced by these same changing fields in a conductive disc.
• Universal Meters (like Dynamometer, Moving Iron, some Motor Meters): Operate on principles where the deflecting torque is proportional to the square of the current or voltage, or the product of voltage and current (for power). Squaring makes the torque unidirectional regardless of AC or DC supply polarity changes, or the interaction happens between magnetic fields produced by the circuit current/voltage itself, which aligns regardless of polarity changes.

The induction meter's dependency on a constantly changing magnetic flux makes it specific to AC circuits and unusable for measuring parameters in steady-state DC circuits.