A simple potentiometer circuit as shown in the fig. is used to measure the emf of a iron constantan thermocouple. A fixed voltage of $1\mathrm{.}215$ V is applied across points A and B $.$ A current of $3$ mA flows through the resistors. The range of temperature variations is $150C$ to $650C.$ Find the value of $R_1,R_2$ and $R_G$ for an ambient temperature of $20C.$ Data from thermocouple tables for iron$-$constantan with $0C$ reference junction temperature is: emf for $20C=1\mathrm{.}022mV$ emf for $150C=14\mathrm{.}682$mVemf for $650C=53\mathrm{.}512mV$