Norton's theorem

Norton's theorem is Thévenin's twin: any linear circuit, seen from two terminals, is equivalent to a single current source in parallel with a single resistor. Where Thévenin pictures the source as a fixed 'push' (voltage) behind a series resistor, Norton pictures it as a fixed 'flow' (current) feeding a parallel resistor — two different sketches of the very same behavior.

The Norton current I_n is the short-circuit current that flows when you connect the two terminals with a wire, and the Norton resistance R_n equals the Thévenin resistance R_th. The two equivalents convert into each other by Ohm's law: V_th = I_n · R_n. Engineers pick whichever view simplifies the problem — Norton meshes neatly with nodal analysis and parallel combinations, while Thévenin suits series loops. Both compress a daunting subnetwork into two elements you can reason about instantly.