Absolute Instruments
are those which give the value of the quantity to be measured, in terms of the constants of the instrument and their deflection only. No previous calibration or comparison is necessary in their case. An example of such an instrument is a tangent galvanometer, which gives the value of current, in terms of the tangent of deflection produced by the current, the radius and number of turns of wire used, and the horizontal component of the earth’s field.
Secondary Instruments
are those, in which the value of electrical quantity to be measured can be determined from the deflection of the instruments, only when they have been pre-calibrated by comparison with an absolute instrument. Without calibration, the deflection of such instruments is meaningless. It is the secondary instruments, that are most generally used in everyday work; the use of the absolute instruments being merely confined within laboratories, as standardizing instruments.
Difference Between Absolute And Secondary Instruments
| Absolute instruments | Secondary instruments |
| The values of the quantity to be calculated in terms of physical constants and their deflection are given by Absolute Instruments. | Secondary instruments are tools whose production is calculated to give the quantity’s value. |
| It is not necessary to calibrate them and there is no need to compare them with the other standard instruments. | It is important to calibrate these tools against an absolute instrument. Secondary instruments are utilized in day-to-day life. |
| Absolute instruments are being used as standardization methods in laboratories. | Examples – Absolute electrometer, tangent Galvanometer, etc |
| Examples – Voltmeter, ammeter, etc | Examples – Voltmeter, ammeter etc |
