A copper constantan thermocouple produces an e.m.f of $40\;\mu V$ per $^{\circ}C$. The smallest temperature difference that can be measured with this thermocouple is $42.5 ^{\circ}C$, When a galvanometer capable of detecting as low as $10^{-6}\;amp$ is employed. The resistances of that galvanometer is

Question

$$\begin {array} {1 1} (1)\;50 \Omega & \quad (2)\;100 \Omega \\ (3)\;200 \Omega & \quad (4)\;400 \Omega \end {array}$$