Classification of Instruments

Classification of Instruments Points : Classification of Instruments There are many ways in which instruments can be classified. Broadly, instruments are classified into two categories: -
(1) Absolute Instruments.
(2) Secondary Instruments. Absolute Instruments: 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 precious calibration or comparison is necessary in their case. The example of such an instrument is the target 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 ire used and the horizontal component of earth’s field. The RaIeigh’s current balance and absolute electrometer are the other examples of absolute instruments. Such instruments are seldom used except in standard laboratories and in similar institutions standardizing instruments. Secondary Instruments: The Secondary Instruments are those, in which the value of electrical quantity to he 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. These are the secondary instruments, which are most generally used in e every day. These instruments are so constructed that the quantity being measured can only he measured by observing the output indicated by the instrument. A voltmeter, a glass thermometer and a pressure gauge are typical examples of secondary instruments.