Question

QUESTION TWO (a) Electrical measuring instruments may be classified according to their functions. List three (03) such classes of electrical instruments. [4 marks] (b) Describe the three (03) main features of an indicating instrument. [6 marks] (c) A voltmeter is connected across a circuit consisting of a milli-ammeter in series with an unknown resistor, R. if the readings on the instruments are 0.5V and 1mA respectively, and if the resistance of the milli-ammter is 99, determine the true resistance of R. [10 marks]

Answer 1

a)Three classes of instruments are :

1)Indicating instrument , example Voltmeter , Ammeter

2)Integrating Instrument , example Energy Meter

3) Recording Instrument , Potentiometric Recorder

b)Three main features are:

1)Deflecting torque - produced due to magnetic or electrical effect produced by quantity under measurement and so used to deflect the pointer over scale.

2)Controlling torque - Used to keep pointer at steady state so that readings can be measured . In absence of this torque pointer will show full scale reading which is highly inaccurate . It is provided by spring and gravity control mechanisms .

3)Damping torque- To damp out oscillation of pointer about equilibrium so that we get steady and stable reading more quickly and without ongoing oscillations . Provided by Air friction , Eddy current .

C)

Votmal A Jnod BA 9J2 AMMETER 0.CV Raado D-5V ImA D KVL , ansund (1MA x 3 0.5 (RX ImA)= 0 ImA x (9+R) 9tR) 0.5 0 S 0-SX 103 IMA SOD 3tR Ra (SD-) R4912 49)

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