As already you may be aware that these are divided into (1) absolute instruments and (2) secondary instruments. Absolute instruments are those which give the value of the quantity to be measured in terms of the constants of the instrument and their deflection only. No previous calibration or comparison is necessary in their case. The example of such an instrument is tangent galvanometer which gives the value of current in terms of the tangent of deflection produced by the current, the radius and number of turns of wire used and the horizontal component of earth’s field.
Secondary Instruments are those in which the value of electrical quantity to be measured can be determined from the deflection of the instruments only when they have been pre calibrated by comparison with absolute instruments. Without calibration, the deflection of such instrument is meaningless.
It is the secondary instruments which are most generally used in everyday work, the use of the absolute instruments being merely confined within lab as standardizing instruments. Read Electrical Measuring Instruments
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