Measurement of Small Resistance:
In figure necessary circuit is shown.
Primary circuit is as usual. In secondary circuit, unknown small resistance r, known resistance R, rheostat Rh, key K2 and emf ε’ all are connected in series. Higher potential end of R, connects at point A which has higher potential, while lower potential end of R and r connects with the ends 1 and 3 of two way key. Terminal 2 of two way key connects with galvanometer end. Galvanometer also connects with jockey.
Working: Key K1 is closed, key K2 is also closed, now plug is inserted between terminal 1 and 2
of two way key. Now find out potential difference across resistance R by potentiometer. If I current flowing through secondary circuit, potential difference V across the ends of resistance R and the balancing length is l1 for V.
By the principle of potentiometer
V = kl1 ………….. (1)
but V = IR (by Ohm’s law)
IR = kl1 ………… (2)
Eliminate the plug between 1 and 2 and insert it between 2 and 3. Now R and r connect in series. Current I remains same in this circuit also now the potential difference across the ends of the (R + r) is V1 and balancing length for it is l2.
Putting the values l1, l2 and R, then internal resistance of primary cell (r) can be determined.
