Norton's theorem states, that the equivalent circuit is a constant current source in parallel with an internal resistance.

The values of the constant current source and internal resistance are calculated as follows.

- The value for the constant current source current, is the current that would flow from the output terminals, if they were short circuited.
- The value for the internal resistance for 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 Norton's theorem is: "The linear network behind a pair of output terminals, can be replaced with a constant current source in parallel with an internal resistance." (The term linear network just refers to a circuit containing components like resistors, to which Ohm's law applies)

Norton'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 to which we apply Norton's theorem can be much more complicated.