The instruments used for all electrical measurements are called measuring instruments. They include ammeters, voltmeters, wattmeters, energy meters etc. The various electrical instruments may broadly be divided into two groups:
- Absolute instruments. Absolute instruments are those instruments which indicate the quantity to be measured in terms of the constants of the instrument (dimensions, turns etc.) and in, order to find out the quantity in the practical units it is necessary to multiply such deflections with an instrument constant. No previous calibration or comparison is necessary in this case. The most, common absolute instrument is tangent galvanometer which gives the measured current in terms of tangent of the deflected angle, the radius and the number of turns of the galvanometer. Such instruments are rarely used (the use being merely confined within laboratories as standardizing-instruments).
- Secondary instruments. Secondary instruments are those in which the value of electrical, quantity to be measured can be determined from the deflection of instrument only when they have been pre-calibrated by comparison with an absolute instrument. The deflection of the instrument gives directly the quantity to be measured. These instruments are most generally used in everyday work.
Secondary instruments may also be classified as follows:
- Indicating instruments. Indicating instruments are those which indicate the instantaneous value of the electrical quantity being measured at the time at which it is being measured. Their indications are given by pointers moving over calibrated chief.
Example. Ammeters, voltmeters and wattmeters.
2. Recording instruments. Recording instruments are those which give a continuous record of the variations of an electrical quantity over a selected period of time. The pointer in these types instruments is an infed pen which leaves a trace on a paper put over a moving drum.
3. Integrating instruments. Integrating instruments are those which measure the total quantity of electricity delivered in a particular time.
Example. Ampere-hour.and watt-hour meters.
Electrical measuring instruments may also be classified as follows:
1. According to the quantity being measured:
Ammeters …… for measuring the magnitude of current.
Voltmeters ….. for measuring voltages.
Ohmmeters and resistance bridges for measuring resistances.
Wattmeters …. for power measurements.
Watt-hour meters …… for energy measurements.
Frequency meters …. for frequency measurements.
Power factor meters …. for power-factor measurements.
2. According to the kind of current:
Instruments are classified into D.C., A.C. and A.C./ D.C. instruments.
3.According to accuracy limits:
4.According to the principle of operation :
Instruments are grouped into:
- Moving coil
- Moving iron
- Rectifier types.
5. According to the type of indication:
Instruments may be :
- Indicating type
- Recording type.
6. According to application:
- Switch board