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A measuring instruments always provides us the data on [physical value of some variables measured. Selection, function, classifications are the main part of a measuring instruments. They are of the type analog and digital.An indicating instruments always have some essential features called controlling devices, damping devices, deflecting devices.Instruments are mainly classified into 7 categories they are:-

1. Analog and digital type instruments. 
2. Null output and deflection type instruments.
3. Automatic and manual type instruments.
4. Secondary and absolute type instruments.
5. Electrical, electronic and mechanical type instruments.
6. power and self operated instruments.
7. remote indicating and self-contained instruments.

Digital instruments always have high accuracy, high speed and will not have the human operational errors.Electrical methods are more used widely than mechanical methods.but fore more significant and much more faster operations cant be met by mechanical and electrical instruments thus we uses the electronic instruments and the very weak results obtained from the electronic instruments can be detected by using amplifiers or pre-amplifiers.The basic functions of an instrument are recording function, indicating functions, controlling functions.Electrical instruments operates on many physical effects the effects are Thermal effect, chemical effects,magnetic effects, electrostatic effect, electro-magnetic effect.The types of instrument which effects the thermal effect is voltmeter,maximum demand meter, wattmeter etc.The types of instrument which effects the magnetic effect is voltmeter, power factor meter, ammeter, wattmeter etc.The types of instrument which effects the electrostatic effect is multipurpose voltmeter which also used to measure current and power etc.The types of instrument which effects the chemical effect is integrating meter(DC ampere hour meter).The types of instrument which effects the electro-magnetic induction effect is voltmeter, power factor meter, ammeter, wattmeter, AC using integrating meters etc.

There are different types of instruments having different types of control, suitability, type of damping etc.

Moving iron type can used for both AC and DC, control type is spring or gravity control, damping is air friction,(Cheaper construction,accurate on AC or DC, common for indicating instrument). Moving coil is further classified into two types they are permanent magnet type and dynamo meter type can be used for both AC and DC measurement used for spring control damping is eddy current for permanent magnet type and air friction for dynamo-meter type(usage as wattmeter, more accurate for DC measurement, used for calibration). 

Galvanometer is a device which is used to measure small deflections in current and voltage in a closed circuitory and to measure the magnitude too. Galvanometers are of different types they are D'Arsonval Galvanometer, Ballistic Galvanometer, Grassot flux meter, Vibration Galvanometer, Duddel's Galvanometer, selection of galvanometer is based on (selection criterion) sensitivity which is very important in selection of galvanometer, resistance is is the next criteria is resistance, next is damping, damping will always save the time and it should be equal or less than that of the critical, period is other criteria which is nothing but should be of less reaction time, next is promptness of response which gives a critical damping and short period response, freedom from mechanical disturbances, freedom from stray field effect which is unavoidable notice of local field should not be influence the instrument, definiteness of zero reading which is zero reading should be definite, next criteria is law of deflection which is nothing but deflection range should be proportional to current, coil damping...