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.).