Thevenin's theorem states, that the equivalent circuit will simply be an e.m.f. in series with an internal resistance , (exactly like the power supply models introduced in the previous section).

The values of the e.m.f and internal resistance are calculated as follows.

- The value for the e.m.f., is simply the voltage across the output terminals of the original circuit, when no external components are connected.
- The value for the internal resistance of the equivalent circuit, is found by shorting out the e.m.f. in the original circuit and then calculating the resistance between the output terminals

The formal statement of Thevenin's theorem is: "The linear network behind a pair of output terminals, can be replaced with an e.m.f. in series with an internal resistance." (The term linear network ,just refers to a circuit containing components like resistors to which Ohm's law applies).

Thevenin's theorem is best illustrated with the example on the following page. This example only uses a very simple circuit to illustrate how to apply the theorem. In practice the circuits we use Thevenin's theorem with, can be much more complicated.