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Temperature Coefficient of Resistance Aim of the experiment is to determine the
resistance of a given resistor in different temperature and hence to find the temperatrure
co-efficient of resistance using the relation XT= X0(1+aT)
XT= Resistance of wire at T 0C X0= Resistance of wire at 00C
a= temperature co-efficient of resistance T=Temperature
Apparatus The Carey Foster bridge is an electrical circuit
that can be used to measure very small resistances. It works on the same principle as Wheatstone’s
bridge, which consists of four resistances, P, Q, R and S.
Thick copper strip: Fractional resistance box:
Lead accumulator: Galvanometer:
Unknown low resistance: One way key:
Connecting wires: Jockey
There are four gaps in this arrangement. The standard low resistances, P and Q, of 2 O
each are connected in the inner gaps 2 and 3. The known resistance, i.e., the fractional
resistance box X and the unknown resistance Y whose resistance is to be determined are
connected in the outer gaps 1 and 4, respectively. A one meter long resistance wire EF of uniform
area of cross section is soldered to the ends of two copper strips. Since the wire has uniform
cross-sectional area, the resistance per unit length is the same along the wire. A galvanometer
G is connected between terminal B and the jockey D, which is a knife edge contact that
can be moved along the meter wire EF and pressed to make electrical contact with the wire.
A lead accumulator with a key K in series is connected between terminals A and C.
Procedure Determination of resistance per unit length,
?, of the Carey Foster bridge wire Make the circuit connections. Set the resistance
of the fractional resistance box to a minimum. Plug in the battery key so that a current
flows through the bridge. Press down the jockey so that the knife edge
makes contact with the wire, and observe the galvanometer deflection.
Move the jockey to different positions along the bridge wire from left to right.
Notice the reading in galvanometer. The needle of galvanometer will start depleting to the
right side and when the jockey reaches at a particular point of bridge wire, the galvanometer
shows a zero reading. This is the balancing point. If the jockey has been moved right
to this point the galvanometer will start depleting to the right side. Even after the
jockey reaches the right end of bridge wire, if the balance point not found, then increase
the resistance of the fractional box and repeat the previous step. Continue this until we
get a balance point. Take the length from the left end to the balancing
point of bridge wire. This is L1. Interchange the position of the copper strip
and resistance box (i.e., copper strip to the left and resistance box to the right)
and find out the balancing length L2 using the previous step.
Apply the L1 and L2 in the equation and find the ?
? = r / (L2 - L1), where r is the resistance of the resistance box.
Using ?, find out the resistance of a given wire of unknown resistance in room temperature
and applied temperature. Remove the copper strip and insert the unknown
low resistance (i.e., a beaker with water, thermometer, heater and a test tube with unknown
resistance inside it) in one of the outer gaps of the bridge.
Set the resistance of fractional resistance box little high (ranging from 1 to 10).
Repeat the same procedure of finding the balancing lengths,L1 and L2.
Apply ?, L1 and L2 (of the current circuit) in the equation and find out the resistance
of the unknown. It is the resistance in the room temperature.
X= ? (L2-L1) Change the temperature and repeat the steps
to find out the resistance in the changed temperature.
A graph is plotted with temperature along X axis and resistance along Y axis. Then the
graph is extrapolated to cut the X axis at absolute zero, ie -2730C . From the graph
we can find out the temperature co-efficient of resistance a using the relation
XT= X0(1+aT)