6. Principle of Operation
• The basic principle of operation of a Slide Dc potentiometer is illustrated as
shown in the Fig.
• A basic potentiometer circuit consists of a slide wire AB of uniform cross
section and Unit length. Generally slide wire is made up of manganin. Let r
be the resistance per unit Length of slide wire.
• The battery supplies a current through the slide wire which is limited with
the help of regulating resistance i.e. rheostat.
• The battery B1 Whose e.m.f. is to be measured is connected in series with a
galvanometer G and switch K.
• When the switch K is opened, the current through slide wire is i. If the
sliding contact is at position C, let the length AC be Units, then the voltage
drop across AC is given by ir1.
7. • Consider that switch K is closed which puts the battery B1 in the circuit.
• The battery B1 whose e.m.f. is to be measured is connected such that
the voltage drop along the slide wire and e.m.f. of B1 oppose each
• The deflection in the galvanometer G depends on of magnitude of
voltage drop across the slide wire portion AC and e.m.f. of B1.
• If the voltage drop across length l of the slide wire is greater than e.m.f.
of battery B1, then the current will flow in the direction A to C through
the galvanometer. Similarly if the e.m.f. of battery B1is greater than the
voltage drop across the length l of the slide wire, then the current will
flow in the direction C to A through the galvanometer.
• The most important condition exhibiting the basic principle of the
potentiometer is that no current flows through the galvanometer when
the two e.m.f’s are equal.
8. • Generally a scale is provided along with the slide wire which enables to
measure the length of portion AC.
• Hence to measure e.m.f. of a battery, first adjust a current through slide
wire with switch K open.
• Then insert battery whose e.m.f. is to be measured.
• By closing switch K, adjust sliding contact such that the galvanometer
shows zero deflection. Measure the length of the portion of the slide wire
with the help of scale provided. Then the unknown e.m.f. E of battery is
• E = i(r.l)
• where r is the resistance per unit length, i is the working current adjusted
using rheostat R.
• If e.m.f.s of the two batteries B1 andB2 are to be compared then insert the
first battery B1 in series with the galvanometer and then adjust the sliding
contact such that no current flows through the galvanometer.
9. • Measure the length of the slide wire portion say Repeat the same procedure
with battery B2 in the circuit. Measures the length of the slide wire portion
be l1,Repeat the same procedure for the battery B2 circuit, the length is
l2.Let e.m.f. of batteries B1 and B2 be E1 and E2, Respectively then we
10. • From above equation it is clear that the ratio of two lengths gives the ratio
of the two e.m.f’s. If one of the batteries used is a standard cell, say battery
B2 of known voltage, the e.m.f. of battery B1 is given by