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Applied Electricity Paper 2 , Nov/Dec. 2008  
Questions:   1 2 3 4 5 6   Main
General Comments
Weakness/Remedies
Strength










































Question 4

Explain how the following works in a moving-coil meter:
  1. deflecting device;
  2. control device.
  3. A moving-coil instrument gives a full-scale deflection with a current of 30mA and a voltage of 90mV. Calculate the value of a resistor be connected in series with the instrument so that it can be used to read 0-100V.
_____________________________________________________________________________________________________
Observation

The expected answers were:

     (i) Deflecting device in a moving-coil works by the  interaction of the field set up by the permanent             magnet with the field set up by the current flowing through the coil.
     (ii) Control device ensures that the deflecting device returns  to its zero position after measurement with             the aid of the restoring force in the hair spring.

  1. I(at f.s.d.) = 0.030A

V(at f.s.d.)= 0.090V
VT(voltage across terminals) = 100V
Rm =resistance of multiplier
Voltage across resistor = (100 – 0.090)V = 99.91V
Rm = 99.91  =3330.33
         0.030A
The question was on Measuring Instruments. It was quite popular.
The performance was relatively satisfactory.

Many candidates did not do well in part(a) but most performed well in the calculation aspect of part(b).
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