Potentiometer (measuring instrument)
A potentiometer is an instrument for variable potential (voltage) in a circuit. Before the introduction of the moving coil and digital volt meters, potentiometers were used in measuring voltage. The method was described by Johann Christian Poggendorff around 1841 and became a standard laboratory measuring technique.
In this arrangement, a fraction of a known voltage from a resistive slide wire is compared with an unknown voltage by means of a galvanometer. The sliding contact or wiper of the potentiometer is adjusted and the galvanometer briefly connected between the sliding contact and the unknown voltage. The deflection of the galvanometer is observed and the sliding tap adjusted until the galvanometer no longer deflects from zero. At that point the galvanometer draws no current from the unknown source, and the magnitude of voltage can be calculated from the position of the sliding contact.
This null balance measuring method is still important in electrical metrology and standards work and is also used in other areas of electronics.
Measurement potentiometers are divided into four main classes listed below.
Potentiometer is a simple device used to measure the EMF, TPD, internal resistance of a cell. It consists of a board where a tungsten or manganese wire is fitted on it. It works on the principle that the potential dropped between two points in a wire of uniform cross section is directly proportional to the distance between the points.
Driving cell (E) of some EMF which is always greater than the EMF (E’) to be measured is used to send current through the circuit. It drops uniform potential along the potentiometer wire AB.
Between A and X (some point in between A and B), some potential (say V) is dropped. Consider the alternative path wire AGX for the flow of current in between A and X except the potentiometer wire. The potential due to driving cell is same for both AX segment of wire and the long wire AGX since they are in parallel. Thus, electric field exists along AGX. When a cell E’ is introduced with positive connected to A in the path AGX, it creates opposite field to that created by the driving cell, and thus when V = E’, no current flows through the portion AGX which is confirmed by the galvanometer (G). Thus, we get E’ ∝d (from principle and the figure).
In this circuit, the ends of a uniform resistance wire R1 are connected to a regulated DC supply VS for use as a voltage divider. The potentiometer is first calibrated by positioning the wiper (arrow) at the spot on the R1 wire that corresponds to the voltage of a standard cell so that
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