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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 3

(a)        List two advantages of moving-coil instruments over the moving-iron
type.
(b) A moving-coil instruments has a resistance of 20Ω and a full-scale deflection of 10mA.  Determine, using the circuit diagram, the values of the resistance that would be connected to the instrument to enable it read:

    1. O – 2A 
    2. O – 100V.                                                                            
_____________________________________________________________________________________________________
observation

The expected answers were:
(a) Any two correct advantages  of the moving- coil instrument

 

    • high sensitivity
    • uniform scale
    • not affected by  external magnetic field
    • can be used as a multi-range instrument.

(b)(i)   

 

 

 

Same voltage across AB i.e.
20   x   0.010   =  1.990X
       X  =  20 x 0.010
               ee 
                    1.990

       X  =  0.1Ω

 

 

 

 

 

 

From the diagram,
100  =  Vx + V20Ω
100 = 0.010X + 20 x 0.010
          0.010X  =  100 – 0.2
          X  =  99.8
                 0.010
          X  = 9980Ω
                                                                                                                        240
This question was on measuring instruments.  Part (a) was attempted by many candidates and scored high marks in it.
Candidates’ performance in part (b) was not as good as in part (a).

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