Physics Lab Observation Sheet
CAREY FOSTER’S BRIDGE
2011-2012
PREPARED BY:
Mr. CHARIS ISRAEL ANCHA M.Sc., (Ph.D.)
Asst. Prof. of PHYSICS
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CAREY FOSTER’S BRIDGE
Physics Lab Observation Sheet
Exp. No:
2011-2012
CAREY FOSTER’S BRIDGE
Date:
Aim: To determine the specific resistance of the material of a wire by Carey Fosters Bridge.
Apparatus: Carey Foster’s Bridge Board, Battery eliminator, Galvanometer, High resistance (5000 Ω), Low
resistance box (0.1 – 10 Ω), two standard resistances (2Ω each), Copper Strip, Copper wire, meter scale,
Screw gauge, connecting wires.
Formulae:
R
Ω / cm.
l1 − l2
where l1 & l2 are the balancing lengths when the low resistance box is in the left and right
gaps of the bridge and R is the resistance in the low resistance box.
2) Resistance of the given copper wire, RCopper wire = ρ (l3 − l4 ) + R Ω.
where l3 & l4 are the balancing lengths when the low resistance box is in the left and right
gaps of the bridge and R is the resistance in the low resistance box.
RCopper Wireπ r 2
Ω − cm. where r & l are the radius
3) The Specific Resistance of the given of the wire, S =
l
and length of the given copper wire.
1) Resistivity of the bridge wire, ρ =
Figure:
K
Battery
2Ω
L.R.
2Ω
C
P'
P
Copper Strip
R
Q
A
R'
S
B
J
H .R.
Procedure:
G
(A) To determine the Resistivity ‘ρ’ of the bridge wire:
1)
2)
3)
4)
5)
6)
7)
8)
9)
Make the circuit such as shown in the figure with tight connections.
Connect the low resistance box in the left gap ‘P’ and the given copper strip in the right gap ‘S’.
The two gaps ‘Q’ & ‘R’ are to be connected by the standard resistance of 2Ω each.
From the centre ‘C’ of the circuit board connect the galvanometer ‘G’ with a high resistance of 5KΩ
and a jockey ‘J’ in series.
Make sure that the connections made are tight.
A battery of around 2-4V is to be connected between P’ and R’ through a key ‘K’.
By changing the resistance in the L.R. box, identify the balancing lengths ‘l1’.
Now interchange the L.R. and the copper strip and again identify the balancing lengths ‘l2’.
Tabulate the readings of R, l1 & l2 and calculate the resistivity ‘ρ’ of the bridge wire.
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2011-2012
CAREY FOSTER’S BRIDGE
Physics Lab Observation Sheet
(B) To determine the unknown resistance of the copper wire:
1) Now replace the copper strip with the given copper wire.
2) Repeat the experiment to find the balancing lengths l3 & l4.
3) Tabulate the readings of R, l3 & l4 and calculate the resistance of the given copper wire.
Observations:
Least Count of the screw Gauge:
Pitch of the Screw =
distance moved on the main scale
mm = _________ mm.
no. of rotations of the screw
Least Count, L.C. =
pitch of the screw
mm = ________ mm.
no. of divisions on the circular scale
Length of the given copper wire, ‘l’ = _____cm.
Tabular -I:
To determine the radius of the given copper wire
Readings of Screw Gauge
S. No.
Trial Position
1.
First end
2.
Middle
3.
Last end
Tabular-II:
M.S.R
‘a’
C.S.R
‘b’
Total
a+(b x L.C.)
mm
Mean
Diameter of
the wire
‘d’ mm
Radius of the
wire
‘r’ mm
To determine the Resistivity ‘ρ’ of the bridge wire
S.
No
Resistance
RΩ
1.
0.1
2.
0.2
3.
0.3
4.
0.4
Distance of the balancing point when the L.R. is in
Left Gap
Right Gap
Trial Trial Mean Trial Trial
Mean
I
II
l2 cm
I
II
l1 cm
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l1 – l2
cm
ρ=
R
Ω / cm
l1 − l2
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Physics Lab Observation Sheet
Tabular-III:
S.
No
Resistance
RΩ
1.
1.1
2.
1.2
3.
1.3
4.
1.4
2011-2012
CAREY FOSTER’S BRIDGE
To determine the Unknown Resistance of the copper wire
Distance of the balancing point when the L.R. is in
Left Gap
Right Gap
Trial Trial Mean Trial Trial
Mean
I
II
l1 cm
I
II
l2 cm
l3 – l4
cm
RCopper wire =
ρ (l3 − l4 ) + R
Ω
Mean
Resistance
Ω
Calculations:
The Specific Resistance of the given Copper wire, S =
RCopper Wireπ r 2
l
=_________Ω − cm.
Precautions:
1)
2)
3)
4)
All the connections are to be made tightly.
Allow the current to flow through the circuit on when the readings are to be taken.
Jockey should not be dragged on the wire, it should be lifted and made in contact with the wire.
Confirm the connections by checking the deflection in the galvanometer.
Result:
1) The Resistivity of the bridge wire, ρ = ________ Ω/cm.
2) The resistance of the given copper wire, Rcopper wire = __________ Ω.
3) The Specific Resistance of the given copper wire, S = ________ Ω - cm.
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