2.7 Potentiometer

Wheatstone's Bridge

The resistance of a resistor can be determined by measuring potential difference across it and current flowing through it. But when we connect a voltmeter or ammeter to a circuit , due to the finite resistance of voltmeter or ammeter , circuit current and potential difference changes.Hence the measured values will not be accurate. The measurement can be made accurate only when the current through the measuring device is zero.

        It was C.F. Wheatstone's who first suggested a circuit consisting of four resistors and a galvanometer  to zero . The circuit proposed by him  is know as Wheatstone's Bridge Circuit.

Superconductivity

Is it possible to reduce the resistance of any material to zero? the answer is 'yes' .Such materials whose resistance is negligibly small are called superconductors.This phenomenon was discovered by H.Kamerlingh Onnes in 1911. He found that when mercury is cooled to 4.2 K its resistivity suddenly falls to zero.This phenomenon is called superconductivity. The phenomenon by which the resistance certain substances reduce to zero when cooled to extremely low temperature is called superconductivity and the temperature below which superconductivity occurs is called critical temperature.

                              Mercury is not only the substance which exhibit superconductivity . Now researches are going o to obtain superconducting materials with critical temperature of the order of room temperature . Most of the good conductors do not exhibit superconductivity. The element with highest critical temperature is niobium . In 1986 superconducting ceramic was discovered with critical temperature 30 K. As of 2006 the highest temperature superconductor is mercury thallium barium calcium copper oxide at 138 K

Measurement of  E.m.f- Potentiometer

Any practical voltmeter has a finite resistance and draws current from circuit to which it is connected. Hence if we use a voltmeter to measure e.m.f of a cell , due to the finite current through the cell, the potential difference measured will be only terminal voltage and not the e.m.f. E.m.f of a cell can be measured only if no current is drawn from cell while measuring the emf ,This made possible in a potentiometer.

             Potentiometer is a device used to measure potential difference between two points in a circuit. The Working principle of a potentiometer is that the potential  drop per unit length of a uniform resistance wire is constant , if a constant current is maintained through it. Generally ,potentiometer consists of 10 m long uniform resistance wire fixed on a wooden platform. Let r0 be the resistance per unit length .Then potential difference between two points separates by l length of the wire is

V=IR=Ir0l