The instruments that are used to measure electrical quantities (such as electric current, voltage, power, electric energy, frequency etc.) are called electrical instruments.
Classification of Electrical Instruments
The electrical instruments divided into two types
- Absolute instruments
- Secondary instruments
- Absolute Instruments: The instruments that give the value of the quantity to be measured in terms of the constants of the instrument are called absolute instruments. The examples of such instruments are Tangent Galvanometer and Rayleigh’s Current Balance. No previous calibration is necessary in such types of instruments. They are used only in standard laboratories.
- Secondary Instruments: The instruments that determine the electrical quantity to be measured directly in terms of deflection are called secondary instruments.These instruments are provided with a calibrated scale. These are the secondary instruments which are most generally used in everyday work. These instruments are most commonly used in generating stations, substations, laboratories, industries etc. Ammeters, Voltmeters, Wattmeters, Energy meters etc are the examples of secondary instruments.
Types of Secondary Instruments
The secondary instruments are further divided into following types:-
- Indicating Instruments:- The instruments which indicate the magnitude of electrical quantity being measured instantaneously are called indicating instruments. Examples of such instruments are ammeters, voltmeters and wattmeters.
- Integrating Instruments:- The instruments which measure total quantity of electrical energy in a given time are called integrating instruments. Energy meter is the example of integrating instrument.
- Recording Instruments:- The instruments which give a continuous record of the variations of the electrical quantity being measured are called recording instruments. EEG and ECG are the example of such instruments.
A recording instrument is merely an indicating instrument with a pen attached to its pointer. The motion of the drum is in a direction perpendicular to that of the deflection of the pen. The path traced out indicates the variation in the electrical quantity being measured during the given period.