The melting point of wax is measured by using a thermometer which is not exact, and it indicates a temperature of ${50}^{0}C$ as shown in figure-1

A wheatstone bridge is used to measure this melting point exactly. The deviation from ${50}^{0}C$ is being measured by observing the variation of the resistance of a coil embedded in wax.
The Wheatstone bridge is setup so that the resistance of each arm is $50\Omega$ exactly at ${50}^{0}C$, and there is no current in the galvanometer as shown in figure-II. The resistance of the galvanometer is  $50\Omega$

One arm of the bridge containing the coil of resistance $50\Omega$ is now immersed in a wax-box. The wax is just melted as shown in the figure-III. It is now observed that the current through the galvanometer is 0.1 $\mu A$.
Given, temperature coefficient of resistance is ${10}^{-3}$ per ${ }^{0}C$. Resistances other than inside wax-box are assumed to be constant and are exactly $50\Omega$ while the resistance of wax box is very close to $50\Omega$