Secondary instruments are devices used to measure electrical quantities such as current or voltage, and they can only determine the value of the quantity being measured if the instrument has been calibrated beforehand.

Calibration is typically done by comparing the instrument with either an absolute instrument, which is highly accurate and precise or a calibrated instrument. The calibration is necessary to ensure the accuracy and reliability of the measurement taken by the secondary instrument.

There are three types of secondary instruments: indicating instruments, recording instruments, and integrating instruments.

Indicating instruments provide a reading of the magnitude of the quantity being measured through the use of a dial and pointer.

Recording instruments, on the other hand, provide a continuous record of the quantity being measured over a specified period. They use a pen attached to the moving system of the instrument to record variations in the quantity being measured on a sheet of paper that moves perpendicular to the pen's movement.

Integrating instruments are used to record totalized events over a specified period of time. They calculate the product of time and an electrical quantity, which provides the summation of the events being measured. Examples of integrating instruments include ampere-hour and watt-hour meters, which are used to measure the amount of electrical energy consumed over a period of time.

In general, secondary instruments are widely used in most applications, whereas absolute instruments are typically only used in standard laboratories or similar institutions.