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Solution:- The average of l1 and l1' is calculated as L1; and the average of l2 and l2' is calculated as L2. The given observation table is completed as follows:
S. No. |
Resistance taken from R.B. R( ) |
Balancing length |
Mean Value |
X = 
() |
Unknown
resistance X
in right gap |
Unknown
resistance
in the right gat |
l1 (cm) |
l2 (cm) |
l1 (cm) |
l2 (cm) |
L1 (cm) |
L2 (cm) |
1 |
8 |
55.4 |
44.6 |
55.6 |
44.4 |
55.5 |
44.5 |
9.97 |
2 |
9 |
52.6 |
47.4 |
52.6 |
47.4 |
52.6 |
47.4 |
9.98 |
3 |
10 |
50.0 |
50.0 |
50.2 |
49.8 |
51.1 |
49.9 |
10.24 |
4 |
11 |
47.8 |
52.2 |
47.4 |
52.6 |
47.1 |
52.4 |
9.99 |
5 |
12 |
45.5 |
54.5 |
45.5 |
54.5 |
45.5 |
54.5 |
10.02 |
6 |
13 |
43.4 |
56.6 |
43.0 |
57.0 |
43.2 |
56.8 |
9.98 |
The final value of the unknown resistance is the mean of the values of the last column, i.e.
= 10.01
Dumb Question
:- which method is better for measuring resistance, metre bridge or Ohm's law ?
Ans:- Ohm's law setup is highly inaccurate because it measures resistaance when the current is present in wire and thus the meausured value includes the resistance of source too, hence its highly inaccurate, this inaccuracy is curtailed in metre bridge apparatus hence it is better.
Post Office Box
:-
It is a compact form of the Wheatstone bridge. It consists of compact resistance so arranged that different disired values of resistances may be selected in the three arms of Wheatstone bridge, as shown in fig. 14.5.
Each of the arms AB and BC contains three resistances of 10, 10
2
and 10
3
, respectively. These are called the ratio arms. Using these resistances the ratio
can be made to have any of the following values: 100:1, 10:1, 1:1, 1:10 or 1:100.
The arm AD is a complete resistance box containing resistances from 1 to 5000
. The tap keys K
2
and K
2
are also provided in the post office box. The key K
1
is internally connected to the point A and the K
2
to the point B(as shown by dotted line in the fig.14.5). The unknown resistance X is connected between C and D, the battery between C and key K
1
and the galvanometer between D and key K
2
. The circuit shown in fig.14.5(A) is exactly the same as that of the Wheatstone bridge shown in fig. 14.5(B). Hence, the value of the unknown resistanceee is given by
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