Measuring Instruments
The instruments used for all electrical measurements are called measuring instruments. The include ammeters, voltmeters, wattmeters, energy meters etc.
The various electrical instruments divided into two groups
1. Absolute instruments:
This instruments are which indicate the quantity to be measured in terms of the constants of the instruments and in order to find the quantity in the practical units it is necessary to multiply such deflections with an instruments constant. No previous calibration or comparison in 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.
2. Secondary Instruments:
This instruments are those in which the value of electrical quantity to be measured can be determined from the deflection of instruments only when they have been pre-calibrated by comparison with an absolute instruments. The deflection of the instruments gives directly the quantity to be measured. These instruments are most generally used in everyday work.