Applied Electricity Paper 2 , May/June 2011
 Questions: 1 2 3 4 5 6 Main
Weakness/Remedies
Strength

Question 4
.
1. List two types of control device in measuring instruments.
2.  A milliammeter gives full-scale deflection with 3 mA and has a resistance of   10 Ω. Calculate the resistance required to enable the instrument to read up  to:
(i)      10 A;
(ii)      100 V.

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Observation

• The two types of controlling device in measuring instruments are:
• Gravity control device
• Spring control device
•   (i) The resistance Rsh to read up to 10 A is

Ish = I – I­m = 10 – 0.003 = 9.997 A
Vsh = RshIsh = Vm = ImRm
Rsh = ImRm/ Ish
Rsh =
Rsh = 3 x 10-3 = 3 mΩ.
(ii) The resistance Rseries  to  read up to 100 V is obtained by using:
VT = Vm + Vs = ImRm + IsRs = 100 V
IsRs = 100 - ImRm
Rs =
= Rseries
Rseries = 33 kΩ
The question was on Measuring Instruments. It was very popular with the candidates. Many candidates attempted this question but more than half of those who attempted could not list control devices.
It was reported that the calculation aspect was well-done by almost all the candidates. However, few candidates did not subtract the current through the meter and voltage drop across the meter from the determination of shunt and multiplier values respectively.