Which of the following meter type is suitable only for direct current measurement?

Explanation:

Moving Coil Type Meter

Definition: A moving coil type meter, also known as a permanent magnet moving coil (PMMC) instrument, is a type of electrical measuring instrument used for measuring direct current (DC). It operates on the principle that a current-carrying conductor, placed in a magnetic field, experiences a mechanical force.

Working Principle: The moving coil meter consists of a rectangular coil of wire, which is wound on a non-magnetic frame and is placed between the poles of a permanent magnet. When a current passes through the coil, it experiences a force due to the magnetic field. This force produces a torque, causing the coil to rotate. The rotation of the coil is opposed by a control spring, and the resulting deflection is proportional to the current passing through the coil. This deflection moves a pointer over a calibrated scale to indicate the current value.

Advantages:

• High accuracy and sensitivity, making it suitable for precise measurements.
• Linear scale, which simplifies the reading of measurements.
• Low power consumption due to the efficient design.

Disadvantages:

• Limited to DC measurements; cannot measure alternating current (AC) directly.
• Can be damaged by mechanical shocks or strong external magnetic fields.

Applications: Moving coil meters are commonly used in laboratories, testing equipment, and other applications where accurate DC measurement is required.

Correct Option Analysis:

The correct option is:

Option 2: Moving coil type

This option is correct because moving coil meters are designed specifically for measuring direct current (DC). The operation of these meters relies on the interaction between a current-carrying coil and a permanent magnetic field, which produces a force proportional to the DC flowing through the coil.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: Moving iron type

Moving iron meters can measure both AC and DC. They operate on the principle of the attraction or repulsion of iron pieces in the presence of a magnetic field generated by the current. While they are versatile, they are not limited to DC measurement, making this option incorrect for the given question.

Option 3: Electrodynamometer type

Electrodynamometer meters can measure both AC and DC. They use two coils, a fixed coil and a moving coil, and the torque is produced by the interaction between the magnetic fields generated by these coils. Since they are not restricted to DC measurement, this option is also incorrect for the question.

Option 4: Hot wire instrument

Hot wire instruments can measure both AC and DC by heating a wire through which the current passes. The expansion of the wire due to heating causes a deflection that is proportional to the current. As they can measure both types of current, this option is not suitable for the question.

Conclusion:

In conclusion, the moving coil type meter is specifically designed for accurate and sensitive DC measurements. Its operation is based on the interaction between a current-carrying coil and a magnetic field, making it suitable only for direct current measurement. Other types of meters, such as moving iron, electrodynamometer, and hot wire instruments, can measure both AC and DC, making them unsuitable for the given question which asks for a meter type suitable only for DC measurement.