Figure 3.33 shows a potentiometer with a cell of 2.0 V and
internal resistance 0.40 ÃŽÂ© maintaining
a potential drop across the resistor wire AB. A standard cell
which maintains a constant emf of 1.02 V (for very moderate
currents up to a few mA) gives a balance point at 67.3 cm length
of the wire. To ensure very low currents drawn from the standard
cell, a very high resistance of 600 kÃŽÂ©
is put in series with it, which is shorted close to the balance
point. The standard cell is then replaced by a cell of unknown
emf ÃŽÂµ and the balance point
found similarly, turns out to be at 82.3 cm length of the wire.
(a) What is the value ÃŽÂµ
(b) What purpose does the high resistance of 600
(c) Is the balance point affected by this high resistance?
(d) Is the balance point affected by the internal
resistance of the driver cell?
(e) Would the method work in the above situation if the
driver cell of the potentiometer had an emf of 1.0 V instead of
(f ) Would the circuit work well for determining an
extremely small emf, say of the order of a few mV (such as the
typical emf of a thermo-couple)? If not, how will you modify the
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