Class 4 (4th Engineer) MET 📅 Jul 2023

Exam Question

A moving-coil instrument has a resistance of 10 Ohms and requires a current of 15mA to give full-scale deflection. Calculate the resistance value of the resistor necessary to enable it to be used to measure (16)

(a) Currents upto 25A

(b) Voltages upto 500V

Reference Answer

### Principle of Moving-Coil Instruments
A Permanent Magnet Moving Coil (PMMC) instrument operates on the motor principle. A current flowing through a coil placed in a magnetic field experiences a force, causing it to rotate. The deflection of the pointer is directly proportional to the current flowing through the coil. These instruments are highly sensitive but are limited by:
1. **Low Current Capacity**: The fine wire of the coil can only carry a very small current (in this case, 15 mA).
2. **Low Voltage Drop**: Due to the low internal resistance, the voltage drop across the coil is minimal.

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