APPARATUS REQUIRED
i) Carey foster bridge
ii) Thick copper strips
iii) Fractional resistance box
iv) Connecting wires
v) Resistance
vi) A Jockey
vii) A sensitive galvanometer
viii) A cell
xi) A key.
THEORY
Carey foster Bridge is an improved form of meter bridge. To increase the effective length of the meter bridge two more gaps are provided. Rest of the construction is similar to the meter bridge as shown in figure below. Here, P&Q are two ratio arms and X and Y are resistors in which Y can be known from a fractional resistance box. The unknown low resistance x can be determined as follow .If p is resistance per unit length that is, ohm 1 cm of the wire α1 and α2 respectively are the end resistance at a and b and a balance point is obtained at B where aD=l1
Then,
P/Q = X+l1p+α1 / Y(100-l1)p+α2 ………..(i)
If x and Y are interchanged and the balanced point is obtained at distance l2 from a Then we get,
P/Q = Y+l2p+α1 / X+(100-l2)p+α2 ………..(ii)
Comparing equation (1) and (2) we get,
X+l1p+α1 / Y+(100-L1)p+α2 = X+l2p+α1 / Y+(100-l2)p+α2
Adding 1 on both sides
X+l1p +α1 / Y+(100-l1) P+α2 = Y +l2p+α1+X+(100-l2)p+α2 / X+(100-l2)+α2
Y+(100-l)p+α2 = X+(100-l2)p+α2
Y-X = (l3-l2)p
X= Y-(l3-l2)p
PROCEDURE
i) Complete the circuit as shown in the given diagram-Make the gaps 1, 2, 3 and 4 on the bridge. If necessary, clean the ends of connecting wire and the thick copper strip with a sand paper.
ii) Connect two equal resistance P and Q in inner gaps 2 and 3. If P and Q are two resistance boxes, take out 5 ohms flags from the either and make all other plugs tight.
iii) Without taking out any play from the frictional resistance box adjust the position of the Jockey to obtain a balance point and note the reading length aD = x
iv)Take out the plug from the jockey and k and again interchange x and Y. Now take out 0.1 ohm plug from tractional resistance Y and after again interchanging x and y.
v) Repeat the observation with the values of x equal 0.2, 0.3, 0.4 and 0.5 ohm etc.
vi) Remove the copper stripe and connect by a short thick Copper wire or metal bar keeping resistance in the fractional and resistance box zero.
vii) Repeat the observation by taking 0.1, 0.2, 0.3,0.4 and 0.5 ohm resistance from the fraction resistance box.
OBSERVATION
Determination of ρ the resistance per cm of the bridge wire, balanced point with Y zero in the left gap(x)=45cm
Balanced point with Y zero and in the right gap (Y) = 46.8cm
Therefore, Correction = x-y = 46.8-45 =1.8cm
| S.N | X | Y | Position of balance point with copper strip left gap(l1) | Position of balance point with copper strip right gap(l2) | shift (l1-l2) | Resistance ρ=1/α per cm | Correct shift d= (l1-l2)-ρl | |
| 1 | 0 | 0 | 45 | 46.8 | 1.8 | 0 | 0 | |
| 2 | 0 | 0.1 | 48.1 | 44.6 | 3.5 | 0.05 | 1.7 | Mean R= 0 ohm |
| 3 | 0 | 0.2 | 48.6 | 39 | 9.6 | 0.01 | 7.8 | |
| 4 | 0 | 0.3 | 49.8 | 41.5 | 7.7 | 0.05 | 5.9 | |
| 5 | 0 | 0.4 | 46 | 39.5 | 6.6 | 0.08 | 48 | |
| 6 | 0 | 0.5 | 48.5 | 37.5 | 11 | 0.05 | 9.2 |
Determination of unknown new resistance.
| S.N. | Y ohm | Position of balance point with unknown resistances left gap | Position of balance point with unknown resistances right gap | Shift (l1-l2) cm | Correct shift d= (l1-l2)ρ | Unknown resistance Y= ρd |
| 1 | 0 | 46.3 | 51.5 | 5.2 | 4.4 | 0 |
| 2 | 0.1 | 46.7 | 50.7 | 4 | 2.2 | 0.167 |
| 3 | 0.2 | 47 | 48.9 | 1.5 | 0.7 | 0.198 |
| 4 | 0.3 | 50.7 | 47.4 | 3.3 | 1.4 | 0.213 |
| 5 | 0.4 | 51.1 | 45.4 | 5.7 | 3.6 | 0.112 |
| 6 | 0.5 | 53.8 | 44.5 | 9.4 | 7.6 | 0.22 |
Mean Y = 0.1370 Ohms
RESULT
The value of ρ the resistance per cm of the bridge wire is 0.04 ohm cm and unknown resistance 0.1370 Ohms.
PERCENTAGE ERROR
Actual value of the given resistance (ρ) = 2 ohms
Obtain value of the low resistamer (O) = 0.5 ohms
Therefore: % error = ρ-0/ρ x100%
= 2-0.5/2 x100%
= 7.5 %
PRECAUTIONS AND SOURCE OF ERROR
i) The ends of the connecting wires should be clean and should be firmly secured in the binding terminal.
ii) A plug bey should be used in the call circuit and the Current should be allowed to flow only for the time reading are taken.
iii) The jockey should be pressed gently and instantly.
iv) The use of a rheostat to obtain the ratio arms P and Q whose values should nearly be equal.
V) To Avoid any induction effects the cell circuit should be completed first and then the galvanometer circle.