A Wheatstone bridge is used to determine the value of unknown resistance $$\mathrm{X}$$ by adjusting the variable resistance $$\mathrm{Y}$$ as shown in the figure. For the most precise measurement of $$\mathrm{X}$$, the resistances $$\mathrm{P}$$ and $$\mathrm{Q}$$:

 1 do not play any significant role. 2 should be approximately equal to $$2\mathrm{X}$$. 3 should be approximately equal and are small. 4 should be very large and unequal.
Subtopic: Â Wheatstone Bridge |
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In the Wheatstone Bridge arrangement shown, the resistance $$P$$ is $$2~ \Omega.$$ The bridge is initially balanced. If P and Q are interchanged: an amount equal to $$S$$ has to be added to $$R$$ to restore the balance. The resistance $$Q$$ equals (nearly):

1. $$2 ~\Omega$$
2. $$3.2 ~\Omega$$
3. $$4 ~\Omega$$
4. $$6.4 ~\Omega$$
Subtopic: Â Wheatstone Bridge |
From NCERT
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In a Wheatstone Bridge arrangement, as shown in the figure, the bridge is balanced. However, when the resistances in the arms P, Q are switched, the bridge is balanced only when $$R$$ is replaced by $$4R$$ in the other two arms. If the value of R is $$100$$ $$\Omega$$, that of S is:
1. $$100$$ $$\Omega$$
2. $$50$$ $$\Omega$$
3. $$200$$ $$\Omega$$
4. $$400$$ $$\Omega$$