Basic Electricity Paper 2 , May/June 2014  
Questions:   1 2 3 4 5 6 7   Main
General Comments

Question 4
  1. Explain damping as applied to electrical measuring instruments.

                 (b)   State two advantages of a digital meter over an analogue meter.
                 (c) A moving-coil instrument has a resistance of 10 Ω with a full-scale   
                   deflection (f.s.d.) of 8 mA. With the aid of a sketch, calculate the value of

                   a multiplier that will enable it read up to 100 V

The expected answers were:
(a)  In an electrical measuring instrument, the combination of the inertia of the moving system and the controlling torque of the spiral springs or of gravity gives the moving system a natural frequency of oscillation called damping.      
 (b) (i) Digital meter has high impedance hence less error
            (ii)Digital meter can be read by an inexperienced person from a far








(c)V = Va + Vm = Ira + IRm                                                                  
                          100 = 0.008 x 10 + 0.008Rm                                                                        
                          100 – 0.08 = 0.008Rm                                                                                              
                          Rm=  = 12490 = 12.49 kΩ
The question was on Measuring Instruments. It was very popular with the candidates. Those who attempted this question did better in part (c) than they did in parts (a) and (b).

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