Question 1
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(a) List three electronic measuring instruments.
(b) State the instrument used for measuring the magnitude and period of waveform.
(c) With the aid of a circuit diagram, explain how the range of an ammeter can be extended to read high value currents.
The expected responses were:
(a)Electronic measuring instruments
- Galvanometer
- Moving coil instrument
- Moving iron instrument
- Ohmmeter
- Voltmeter
- Ammeter
- Wattmeter
- Multimeter
- Frequency Counter
- Cathode Ray Oscilloscope
(b) Cathode Ray Oscilloscope (CRO)
(c)In order to extend the range of an ammeter to read higher current value, a shunt of low value resistor is connected in parallel to the meter resistance to bypass the excess current whilst the meter maintains its full scale deflection current.
The requirement in the first part of the question is to list three measuring instruments (ammeter, voltmeter, oscilloscope, etc), and most candidates, according to the Chief Examiner, could list at least two. Only a few candidates recalled that the amplitude (not magnitude, as in the question) of a signal can be measured with the use of an oscilloscope. Question 1(c), a rather popular question, concerns the use of a shunt, to extend the range of indicating instruments. The report further stated that not many candidates understood the requirements of this question, which is simply that of connecting a low resistance resistor across (in parallel) with the ammeter.