Apparatus
- Slide Wire Bridge
- resistance box
- unknown resistance
- Galvanometer
- rheostat, cell
- tapping key
- Connecting wires and sand paper.
? Fig.1 (a)
Theory
Determination of an unknown resistance by slide wire bridge or meter bridge is based on the principle of Wheatstone bridge .A Wheatstone bridge consist of four resistance P,Q,R and X Connected to form a network ABCD .A galvanometer G is connected between points B and D and a cell between points And C(fig .1.a)
If these resistances are so adjusted that the point B and D are at the same potential, no current flows through the galvanometer .in this condition the bridge is said to be balanced and the following relation holds good.
If three of the four resistance (i.e,.P,Q and R)are known ,the fourth unknown resistance X can be easily determined .
Slide bridge or meter bridge :this is a practical application of Wheatstone bridge .it consist of meter or half a meter long uniform wire of constantan or manganin, stretched on a wooden board along a meter rod between two terminals A and C (fig :1.a).the terminals Mand L respectively .A straight copper strip ,provided with three terminals .N,O and T is fixed on the board between the terminals Mand L ,thus forming two gaps MT.A cell is connected between the terminals Dand a jockey or a sliding constant which can be made to touch the wire at any desired position B.
A suitable resistance is inserted in the gap NL by taking out some plugs from the resistance box R.B and the position of the jockey is adjusted so that the galvanometer gives no deflection .such a position B of the jockey is called the balance or null point which is at a distance (say)from A and
from C,As the arrangement of resistance in the slide wire bridge is the same as that in the Wheatstone bridge (resistance P and Q are represented by the resistance of two segment of the wire A Cin which the balance point B divides it ), therefore
As the resistance of a uniform cross sectional of a wire is proportional to its length so
=
orX=R(
)
R,being known resistance X can be determined .
Procedure
- Draw a net circuit diagram as show in fig .1.(a).
- Clean the ends of connecting with sand paper and arrange the apparatus as shown in fig .1(a).
- Where G is galvanometer R.B is resistance box, X is unknown resistance and K is tapping key.
- In order to test the circuit close key after suitably adjust the restate with all the plugs in the resistance box intact and tight, place the jockey somewhere in the middle of wires .see the direction of deflection in the galvanometer .now take out the infinity plug from the resistance box and place jockey somewhere in the middle of middle of wire .if the deflection is in the opposite direction, than the connection is correct, otherwise check the connection.
- Till the balance point is somewhere in the middle of wires (between 40-60).measure the length
(fig .1a).
Note length of a balance point from end A asalso length of a balance point from end Cas
=100-
X=R(
)
Take three more observation with different values of R and find the mean value of X record your observation as following.
Observation & calculation
| No of obs. | Resistance applied by resistance box R (ohms) | Distance of the balance point B from the end point A | Distance of the balance point B from the end point C. | Unknown resistance X=Rx |
| 1 2 3 | .. .. .. | .. .. .. | .. .. .. | .. .. .. |
Mean resistance of wire =X=——————- (ohms)
Precautions
- All the connection should be clean, neat and tight.
- Shunt the galvanometer so as to save it from being damaged in the approximate determination of balance point.
- Stop the current for some time after each observation to ensure that the temperature of bridge wire remains constant.
- Close the battery key first and then complete the galvanometer circuit.
- The balance point should be as near the center of wire as possible because the bridge is more sensitive when the balance point is near the middle.
- The jockey is not pressed too much while sliding it over the bridge wires.
- All the plug in the resistance box should be well pressed .after removing a plug in the resistance box press the two plugs laying on either side of plug removed.
Viva voice
Q.1: what is the condition for balancing a Wheatstone bridge?
Ans .the resistance should be adjusted so that the potential difference across the galvanometer is zero.
Q.2: Does the null point indicate the absence of current or two equal and opposites current in the galvanometer?
Ans .it indicates the absence of current.
Q.3: what is the advantage of this method over the others for resistance measurement?
Ans .the superiority of this method lies in the fact that null method and not a deflection method it is easier to see that there is no deflection than to measure a deflection accurately.
Q.4: what is a shunt?
Ans. It is a wire of low resistance which is connected across o sensitive galvanometer
It provides an alternative path and so only a fraction of the total current pass through galvanometer .its fraction is to save the galvanometer from being damaged.
Q.5: Why should the connections be clean?
Ans. The dust and other greasy matters in a dirty condition provide a resistance which is not accounted for in the relation of the experiment.
Q.6: Define electric current?
Ans .the time rate of flow of charge is called current.
Q.7: what is the unit of current? Define it?
Ans. The unit of current is ampere. The current will be one ampere if a charge of one coulomb passes through a certain cross section in one second.
Q.8What is resistance? Define its unit.
Ans. It is the opposition of flow of current through a conductor .its unit is ohm.
Q.9: Define ohm?
Ans. The resistance of a conductor will be one ohm, if current of one ampere flows through it when a potential difference of one volt is applied across its ends.
Q.10: What is the effect of temperature onthe resistance?
Ans. The resistance of the conductor most rise with the increase of temperature due to increase in collision of free electrons with the atom of conductor.
Q.11: State ohm ‘law?
Ans .According to ohm law the current is directly proportional to the potential difference at the ends of conductor, provided the physical state of conductor does not change.
Q.12: Upon which factors the resistances of a conductor depend?
Ans. The resistance of a conductor depends upon nature, dimensions and Temperature of the material
Q.13: What is the conductance? Give its unit.
Ans. Reciprocal of resistance is called conductance .its unit is (0hm )1
Or mho or Siemon.
Q.14: What id equivalent resistance in case of (a)in parallel,(b) in series combination?
Ans .(a) In parallel arrangement, the equivalent
Resistance is given by =
=
=
+———
(B) in series arrangement, the equivalent resistance is given by
Q.15: Define specifie
Ans. The resistance of meter cube of a wire is called specific resistance or resistivity. Its SI unit is ohm meter (m).
Q.16: what is conductivity? Give its unit.
Ans. The reciprocal of resistivity is called conductivity. Its SI unit is (Ohm-meter)1
Q.17: what is eureka wire?
Ans. It is the alloy of 40% nickel and 60% copper.
Q.18: why is the slide wire is so called?
Ans. it is because we use a jockey which acts as sliding content to balance it.
Q.19: what is the principle of wheat stone bridge?
Ans. The principle of side Wire Bridge is the principle of wheat stone bridge.
Q.20: what is the principle of wheat stone bridge?
Ans. When four resistances and
are joined in from of the bridge and galvanometer shows no deflection, then
=
Q.21: what does the null point or zero deflection in the galvanometer indicate?
Ans. Null point indicates the absence of current through the galvanometer in either direction. The resistance of the conductor most rise with the increase of temperature due to increase in collision of free electrons with the atom of conductor.
Q.22: why should the balance point necessarily lie between 40cm and 60cm of a wire of the slide wire bridge?
Ans It is a pre-condition for balancing the bridge to give more accurate result. In that case the ratio arms are nearly equal in resistance and the bridge is most sensitive and accurate.
Q.23: Why should the connection should be clean neat and tight?
Ans. Because loose and dirty connections add to the resistance of the circuit and consequently decreases the accuracy if the result.
Q.24: what is the effect of thickness of wire in the specific resistance?
Ans. The specific resistance depends upon the nature and temperature of the material but not on its dimension thus specific resistance remains unchanged.
