Absolute vs. Secondary Instruments

Measuring instruments may be classified as either absolute instruments or secondary instruments.

Absolute Instruments

This type of instruments gives the value of the measurand in terms of instrument constant and its deflection; this instrument type doesn’t require comparison with any other standard. For instance, the tangent galvanometer gives the value of the current to be measured in terms of the tangent of the angle of deflection produced by the current, the radius and the number of turns of the galvanometer coil, and the horizontal component of the earth’s magnetic field. No calibration of the instrument is thus necessary.

Absolute instruments are mainly used in standard laboratories for standardizing.

Secondary Instruments

Secondary instruments are built so that the deflection of such instruments gives the magnitude of the electrical quantity (current, voltage, or other quantity to be measured) to be measured directly. Calibration of these instruments is necessary and is done by comparing with either an absolute instrument or with another secondary instrument that has been calibrated before. Secondary instruments can be further classified as indicating, recording and integrating instruments. Secondary instruments are what you definitely find in general use for measurement purposes.

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