Potentiometer is a
device used to compare the Emf’s of two cells and also used to find the
internal resistance of a given cell.It works on a
principle that the potential drop across a wire is directly proportional to the
length of the wire through which the current is passing.
The device consists
of two parts called primary and secondary circuit. The primary circuit consists
of a strong battery to supply the potential requirement of the circuit. The
connection from the cell is given to a long wire. This long wire is divided
into pieces of wires each of one meter length and they are connected with small
copper pieces of negligible length. They behaves like multiple wires connected
in series and the total length is available between the copper strips.A
galvanometer is connected from the wire with a connecting wire.
Comparison of EMF's of two cells
The secondary
circuit consists of cells whose electromotive forces has to be compared. There
is key that acts like a on and off switch and it can be used to control the
flow of current through a particular cell.
In two different
cases the two cells are connected in the secondary circuit separately and
corresponding balancing lengths are measured with it. When the emf of secondary
circuit is balanced with that of primary, the galvanometer shows zero
deflection. This happens at a particular length of the wire and that length is
called balancing length. We will find the balancing length with the second cell
whose EMF has to be compared.
As per the
principle, the ratio of EMF’s is nothing but the ratio of the balancing lengths
in the two cases. Thus we are able to compare the EMF’s of two cells using the
Potentiometer.
Determination of
internal resistance of a cell
We need to connect
cell alone first whose internal resistance has to be measured. We can find the
balancing length in the circuit. In the next case, the same cell is connected
to a external resistance in parallel and again balancing length is measured
using the potentiometer. There we will get one more balancing length. The ratio
of emf of that cell and its potential drop across a resistor is nothing but the
ratio of balancing lengths.
Basing on the
definition of potential drop, we can get one more relation between emf and
voltage and by comparing both of them, we can measure the internal resistance
of a cell as shown below.
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