Classification of instruments can be carried out in two categories:

• Absolute Instrument These instruments give the magnitude of the quantity under measurements in terms of physical constants of the instrument. The quantity to be measured will be given in terms of deflection and the constant of the instrument. The examples of this class of instruments are Tangent Galvanometer and Rayleigh’s current balance.

Absolute instruments are very rarely used except in some laboratories and for other scientific works. No calibration is needed in this type of instruments.

• Secondary Instrument  :    These instruments are so manufactured that the qty can be measured only by monitoring the instrument indicated output. These instruments are calibrated by comparing it directly with an absolute instrument or with other secondary instrument which has been calibrated against an absolute instrument.Working with absolute instruments for routine work is time consuming since every time a measurement is made, it takes a lot of time to compute the magnitude of the quantity under measurement. Therefore, secondary instruments are most commonly used.

The Secondary instruments primarily work in two modes:

1. Analog mode : Signals that are continuous in nature and take on infinity of values in any given range are called Analog signals. The devices, which generate these signals, are called analog devices.An analog signal uses the property of the medium to distribute/pass the information contained in it. Lets understand it with an example : an aneroid barometer uses the rotary condition as the signal to pass the pressure applied. If we consider Electrical things, the most commonly used property is voltage, frequency, current, and charge.
2. Digital mode : Signals which vary in discrete steps and take up only finite different values in a given range, are called Digital signals. The devices that generate such signals are called digital devices.

I hope you have understood about the classification of instruments. Feel free to ask if there is any doubt. To know about the methods of measurement, kindly visit http://electricalfundablog.com/blog/methods-of-measurement/663/.