THE POTENTIOMETER
The potentiometer is a device which allows acurate values of resistance and voltage to be determined. Conventinal volt meters, let a small amount of current through to drive their meters, which of course causes a change in the p.d. they are trying to measure. A potentiometer works by balencing p.d.s so that now current actually flows. This means that also the E.M.F. of cells can be measured.
The length AB is made of uniform resistance wire, which is layed on a meter rule, the ratio of length to resistance is calibrated with a standard cell. To find an unkown resistance R, the probe is moved along the wire until the zero centered galvonmeter G shows no deflection, at this point the resistance of the length of wire will be equal to the resistance of the unkown resistor.