The measurement of a given quantity is the result of comparison between the quantity to be measured and a definite standard. The instruments which are used for such measurements are called measuring instruments. The three basic quantities in the electrical measurement are current, voltage and power. The measurement of these quantities is important as it is used for obtaining measurement of some other quantity or used to test the performance of some electronic circuit or components etc.
The necessary requirements for any measuring instruments arer:
1- With the introduction of the instrument in the circuit, the circuit conditions should not be altered. Thus the quantity to be the instrument used.
2- The power consumed by the instruments for their operation should be as small as possible.
The instrument which measure the current flowing in the circuit is called ammeter while the instrument which measures the voltage across any two points of a circuit is called voltmeter. But there is no fundamental difference in the operating principle of analog voltmeter and ammeter. The action of almost all the analog ammeters and voltmeters depends on the deflecting torque produced by an electric current. In ammeters such a torque is proportional to the current to be measured. In voltmeters this torque is decided by a current which is proportional to the voltage to be measured. Thus all the analog ammeters and voltmeters are basically current measuring devices. The instruments which are used to measure the power are called power meters or wattmeters.