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CAREY FOSTER BRIDGE

EXPT NO. Total Marks: 8

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Rh

C

Fig 1: Schematic circuit diagram ()AB : Bridge Wire , 1m length

R : Fractional resistance box

Rx : Unknown resistance

P,Q : Standard resistance

G : Table galvanometer

E : Lechlanches cellC : Plug commutator

Rh : Rheostat

WORKING FORMULA:

When the Carey Fosters bridge is balanced by putting resistance R on the left end and unknownresistance RX on the right end of the bridge, the null point is obtained at l 1 cm from the left end .

Then plotting R vs. l1, we get a straight line with slope M1.Again when the Carey Fosters bridge is balanced by putting unknown resistance RXon the

left end and R on theright end of the bridge, the null point is obtained at l2cm from the left end. Then plotting R vs. l 2

on the same graph,we get another straight line with slope M2.

These two straight lines do not pass through the origin.

The resistance per unit length of the bridge wire is given by:

E

B

A

G

RxQP

R

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12MM

1

From the intersection of these two straight lines, we obtain the unknown resistance RX.

APPARATUS USED:

Carey Fosters Bridge of wire 1 m long.

Fractional resistance box. Unknown resistance

P,Q: Standard 1 ohm resistance coils

Plug commutator

Power supply

Table galvanometer

EXPERIMENTAL RESULTS:

Table 1: Readings for null points (2)

A : ( to measure L1 cm from the left end with R in left gap and RXin the right gap )

B: (L2from the left end with RXin the left gap and R in the right gap) (2)

Obs. Resistance Null point in cm

Left gapR (ohm)

Right gap(ohm) Direct Reverse Mean( L1)

RX

Obs. Resistance Null point in cm

Right gap

R (ohm)

Left gap

(ohm) Direct Reverse Mean( L2)

RX

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Table 2 :Consolidated Results from graph (SHOW THE CALCULATIONS OF THE TWO SLOPES)

(1)

GRAPH ( 11/2

)

PERCENTAGE ERROR: (Show The Calculation) (1)

1212

12

ll

l2

ll

ll

=

L1= Max. value of null point from Table1A

and L2= Min. value of null point from Table1B

P.E. =

100%

PRECAUTIONS:

1. By Carey-Fosters bridge the end corrections can be eliminated from the calculation of unknownresistance which is not possible using a meter bridge.

2. As the bridge wire is not perfectly uniform, R should be chosen such that the length ( l 2- l1) betweentwo null point is as large as possible.

3. To prevent the flow of harmfully large currents in the battery circuit the rheostat (Rh) has beenemployed.

4. The ratio of P and Q should be equal to 1 or nearly equal to one, otherwise null points may not be

obtained within the bridge wire. For greater sensitiveness of the bridge, the values of P and Q shouldbe each equal to one ohm.5. For first observation, the value in R should be so chosen that the two balance points should be very

near to the two ends of the bridge wire. By this (l2- l1) will be more accurate so that the calculatedvalue of is nearer to accuracy. For the

successive observations the value of R should be so chosen so as to shift the null point gradually

towards the middle of the wire by steps of about 5cm.

Slope M1From L1vs.

R graph

Slope M2from L2vs. R

graph

= 1 / (M2 M1)

ohm/cm

Unknownresistance RXOhm

l = minimum division of the meter scale

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PROCEDURE :

1. Disconnect the resistance box R and the unknown resistance Rx and the gap should be connected

by a metal strip.2. Find the null point (This should be around 50). This is called electric midpoint

3 a) Connect the resistance box R and unknown resistance Rx in the left gap and right gap

respectively.

b) Now, keeping the jockey still at the electric midpoint, choose R such that the null point is again

obtained at the electric

midpoint. This value of R gives you a rough estimation of the value of the unknown resistance.

4.

Choose a resistance (R1) from the resistance box so that the null point should be found between 5-10cmand similarly

choose another resistance (R2) so that the null point should be found between 90-95 cm.

5. Obtain the null points for different resistances between R1and R2. The values should be so chosen that

they are at regular

intervals.

6. The same steps should be repeated for the previous values of R after interchanging the positions ofR and Rx.

7. A graph should be plotted by taking resistance along x-axis and null point along y-axis.

NATURE OF THE GRAPH :

lincm

Rx

R in ohm