The expected answers were:
(a) Advantage
- High sensitivity
- Uniform scale
- Well shielded from stray magnetic field
- Low power consumption
Disadvantage
- More expensive than moving-iron instrument
- Only suitable for d.c. currents and voltages
- Easily damaged/Not rugged
(b)(i) The voltage drop across the instrument, Vm = Im rm
Vm = 0.015 x 3 = 0.045V = 45mV
(ii) Voltage drop across shunt Rs = Voltage drop across the instrument
The current through Rs, Is = (0.2 – 0.015)A ; Is = 0.185ASince Vm = IsRs = 45 x 10-3V
Rs = Vm / Is = 45 x 10-3 / Is
= 45 x 10-3) / 0.185
Rs = 0.2432Ω = 243.2mΩ
This question was on measuring instruments. The question was popular. Almost all the candidates attempted and got good marks in it.