The potential difference is obtthe difference of all voltage across a terminal. From the circuit above I think the resistors are in series and parallel to each other. The two resistors in line CB are in series and resistance of a resistor in series is obtain by the formulae ; R=R+R. and for the parallel resistors we have I/R= 1/R1+1/R2....1/RX. And we also know the formula for calculating potential difference of a cercuit that involve current and resistance. Thus; E= I(R+r). But in this case the value of the resistor is not given so we must only get the value through practical evaluation. Through the use of multimeter. Meanwhile. We must not forget that must I meter or voltemeter measures voltage accurately when connected in parallel. So you should just place the meter in Parnell to that place with positive and negative. But incase you need to measure the total resistance of the circuilt.just place the multimeter in series with the circuit and you will get the resistance. But ise you