A potentiometer circuit is used to investigate the electromotive force (e.m.f.) of a cell X.
The e.m.f. of cell X is known to be approximately 
. The driver cell has negligible internal resistance and an e.m.f. of 
. The sliding contact is moved along the uniform resistance wire between ends Q and R to a point P where the reading on the galvanometer is zero. What is an expression for the approximate length QP?
