(Same as the Wheatstone bridge balance condition and conductivity derivation described for the earlier repeated question in this set — see the paired 5-mark Wheatstone Bridge / conductivity / temperature-coefficient question above for the full step-by-step solution: balance condition $P/Q=R/S$, conductivity $\sigma=ne^2\tau/m$, and temperature coefficient $\alpha\approx3.93\times10^{-3}\ °C^{-1}$.)