O.I8.

In, actual circuitiif the

altmeter are
the working?
positions of
interchanged, howammeter
will it and Q.19. A student wrongly connects the polarity of
affecr ammeter in the circuit, what difficulty he/she
Ans. Voltmeter will now be
Cieruit. Resistance of connected in the
series in the
will face?
trhe order of kilo voltmeter being Ans. The ammeter will not show any reading and the
esiit will be very ohms), the currentextremely
flowing in
hiph
the experiment cannot be performed.
small and the
be performed. experiment cannot 20. State two conditions under which Ohm's law
in valid.,
Ans. I Valid only for
conductors
2. Constant temperature.

Experiment A2
AIM lo tind the resistance of a
given wire/standard resistor using metre bridge.
APPARATUS REQUIRED
. Metre bridge
Given wire of about 1m length
Connecting wire
Sandpaper
" Plug key
Cell or battery eliminator
" Resistance box
. Galvanometer
Jockey
" Rheostat

DESCRIPTION OF THE APPARATUS

It consists of one metre long constantan or manganin wire
(AC) of uniform crosS-sectional area mounted on
a wooden board parallel to a metre scale. The two ends of the
wire are
L-shaped metal strips are used to provide two gaps between ab and cd toattached
to terminals A and C. Two thick
bridge. The connect resistors forming a Wheatstone
terminal D between the gaps is used for connecting galvanometer and the other
galvanometer is connected to a jockey J. terminal of the

THEORYIPRINCIPLE/FORMULA
The metre bridge works on the principle of Wheatstone bridge. It consists of four resistances P, Q, R and
S connected in the circuit as shown in Fig. (b). It can be used to determine any one of the
four resistances in
terms of the remaining three.
For a balanced bridge with the keys K and K, closed, no current flows through the galvanometer. In this
condition we have;
LAB MANUAL OF PHYSICSCLASS XI|
 of Wheatstone bridge. If R and Sare
practical form lono .."8
called slide wire bridge, is a balance point on a one metre
Metre bridge, also is the
and cd (Fig. a) of the bridge and B
resistances in the gaps ab
cross-section, we have
of uniform area of
Resistance of wire AB
Resistance of wire BC resistances of AB and
of cross-section and r
uniform area
manganin wire of
AC= l00 cm is a constantan eor
lengths
are pruportional to their
R
100

DIAGRAM
Unknown Resistance

R.B.
Resistance
,-l,
Box
C K,
(G) Galvanometer
-I,
60 7O 80 0 100

B
R 1-,+h
(100-) K
F

Lechlanche Rh
cel
Fig. (b)
Fig. (a) Metre Bridge

PROCEDURE
the connections as shou
Clean the ends of the connecting wires by means of a sandpaper. Make
1.
connections are tight.
the circuit diagram. Ensure that all the R by mese
are correct, introduce a suitable resistance
2. In order to check whether the connections If the deflecthion i .
at end A and then at C.
resistance box R. Insert the key K. Press the jockey
correct. If required, adjust the valne
galvanometer is in opposite directions, the connections are
connections.
resistance R in the resistance box or check the
the balance point is obtained near :
3. Adjust the value of the resistance in the resistance box R so that
R and position of
mid-point of the wire AC i.e., between 40 cm and 60 cm marks. Note the value of
jockey B.
4. Change the value of resistance R in the resistance box and repeat step 3. The balancing length sho:
always be between 40 cm and 60 cm. Record five observations.

LAB MANUAL OF PHYSICS-CLASS XII

 | O8SERVATIONS
unknown Resistance
L Name of the material of wite
Sandard value of specitic resistance from table 2 m

TABLE

Resistance
Rin left gap R in right gap
from resistance Position of Mean S
Balancing Length Position of Balancing Length
R(100-1) R(100-)
box X(2) balancing length, I 100 Balancing of length, 100 -r 2
(Q1 ()
point B (em) (cm) point, B (cm) (em)
(em) (em)

Mean value of unknown resistance, S =

5

PRECAUTIONS
1. Draw the circuit diagram before making connections.
2. Clean the ends of the connecting wires before making connections.
3. The connections should be neat and tight.
4. The plugs of the resistance box should be tight.
5. The value of R should be varied in small steps only (preferably 0.1 ohm each time).
6. The current should be passed for minimum possible time.
7. The balance point should lie between 40 cm and 60 cm.
8. The resistance wire should not be allowed to form a loop.

SOURCES OF ERROR
1. There may be error due to contact resistances.
estimated.
2. Length of the wire used up in connections may not be correctly
continuously for some
3. There may be a change in resistance due to heating of wires when current flows
time.
and connecting
4. The measurement of resistance is affected by the end resistances due to copper strips
SCrewS.

Viva-Voce
Q.2. You can also determine resistance of a wire
Q.I. What for are you using metre bridge in this
experiment? using Ohm's law. Which of these two methods
Ans. It is used to determine the resistance of the given is better? Why?
Ans. Metre bridge is better because, it uses null method.
wire.

LAB MANUAL OF PHYSICS-CLASS XII 9