33 Deriving the Formula
With no current in the galvanometer circuit, the voltages at either end of it must be the same.
This means that the voltages across Rl & R2 must be the same and similarly those across R3 & R4.
With no current in the galvanometer, the current in R1 must the same as that in R3 and the current in R2 must equal that in R4