Write principle of Wheatstone bridge.

In a meter bridge the point $D$ is neutral point as shown in the figure.

In the experimental setup of meter bridge shown in the figure, the null point is obtained at a distance of $40\,cm$ from $A$. If a $10\,\Omega $ resistor is connected in series with $R_1$, the null point shifts by $10\,cm$. The resistance that should be connected in parallel with $\left( {{R_1} + 10} \right)\,\Omega $ such that the null point shifts back to its initial position is ………….. $\Omega$

In a meter bridge, the wire of length $1\, m$ has a nonuniform cross-section such that, the variation $\frac{{dR}}{{d\ell }}$ of its resistance $R$ with length $\ell $ is $\frac{{dR}}{{d\ell }} \propto \frac{1}{{\sqrt \ell  }}$ Two equal resistances are connected as shown in the figure. The galvanometer has zero deflection when the jockey is at point $P$. What is the length $AP$ ? ……………. $m$

The resistances in $WSB$ circuit shown in the diagram all are different and the current through the galvanometer is zero. If all thermal effects are negligible, the current through the galvanometer will not be zero, when