When current flows through a power source, some energy is lost and not all energy transferred to the charge is available for the circuit. The terminal p.d. is the measured at the terminals of the power source is therefore less than the actual
e.m.f., and this difference is called lost volts. Increasing the current increases the lost volts, lowering the terminal p.d.
Due to Kirchhoff 2:
Determining the internal resistance and e.m.f. of a chemical cell: use a variable resistor to change the resistance of a circuit, drawing different currents from the power source. Plot a graph of terminal p.d. (V) against current (I). Since = − + , the y intercept gives the e.m.f. and the gradient gives the internal resistance (where the current is 0 there are no lost volts so the terminal p.d. equals the e.m.f.).