To overcome the problems on performance of ordinary potentiometers by slide wire, the Vernier Potentiometer is used. This potentiometer has two ranges. The normal range of 1.6 V down to 10µV and a lower range of O. 16 down to 1μV.
This potentiometer uses Kelvin Varley arrangement as shown in figure above, in this potentiometer, there are three measuring dials. The first dial measures up to 1.5 V
(on the XI range) in steps of 0.1 volt. The middle dial has 102 studs and reads up to 0.IV in steps of 0.001 volt. The third dial also has 102 studs and reads from -0.0001 volt to 0.001 volt in steps of 0.00001 volt
(i.e.10 μ V), There is no slide wire. The resistances of the middle dial shunts two of the coils of first dial. The moving arm of middle dial carries two arms which spaced two studs apart. In actual practice the resistance of second dial is greater than that between two studs in main (first) dial, so that the voltage drops across the second dial is greater than 0.1V. The third dial is obtained from a shunt circuit which permits a true zero and a small negative setting to be obtained.
The Vernier Potentiometer reads to increment of 0.00001 V (10µV) on range XI and has readability of 1μν on X0.1 range. If a third range of X 0.01 is provided, then the readability becomes 0.1 µV.
Also Read- A.C. Potentiometers