Wheatstone's Bridge - 2
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, 102 and 103
, 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 K2 and K2 are also provided in the post office box. The key K1 is internally connected to the point A and the K2 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 K1 and the galvanometer between D and key K2. 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 