In a null type of instrument, a zero or null indication leads to determination of the magnitude of measured quantity. The null condition is dependent upon some other known conditions.
In contrast to deflection type of instruments, a null type instrument attempts to maintain the deflection at zero by suitable application of an effect opposing that generated by the measured quantity. Therefore, for the operation of a null type of instrument, the following are required:
- The effect produced by the measured quantity;
- The opposing effect, whose value is accurately known. This is necessary in order to determine the numerical value of the measured quantity accurately;
- A detector, which detects the null conditions, a device which indicates zero deflection (balance conditions) when the effect produced by the measured quantity is equal to the effect produced by the opposing quantity. The detector should be capable of displaying unbalance, a condition when the effect produced by the measured quantity is not equal to the opposing effect. Also, the detector should have means (automatic or manual) for restoring balance.
Fig shows the elementary form of a D.C potentiometer. It is a null type of instrument wherein an unknown emf, Ex is measured. The slide wire of the potentiometer has been calibrated in terms of emf with the help of a standard emf source. The null detector is a current galvanometer whose deflection is proportional to the unbalance emf, the difference between the emf Eab across portion a b of slide wire and the unknown emf Ex. As soon as the two are equal, there is no current through the galvanometer and therefore it shows zero deflection thereby indicating null conditions Therefore, the unknown emf Ex is equal to Eab, which is directly indicated by the calibrated scale placed alongside the slide wire.