The looking back resistance of a network is the equivalent electrical resistance of the network when someone looks back into the network from the terminals where said branch is connected. During calculating this equivalent resistance, all sources are removed leaving their internal resistances in the network. Actually in Norton theorem, the branch of the network through which we have to find out the current, is removed from the network. After removing the branch, we short circuit the terminals where the said branch was connected. Then we calculate the short circuit current that flows between the terminals. This current is nothing but Norton equivalent current I_{N} of the source. The equivalent resistance between the said terminals with all sources removed leaving their internal resistances in the circuit is calculated and said it is R_{N}. Now we will form a current source that's current is I_{N} A and internal shunt resistance is R_{N} Ω.
For getting clearer concept of this theorem, we have explained it by the following example,
In the example two resistors R_{1} and R_{2} are connected in series and this series combination is connected across one voltage source of emf E with internal resistance R_{i} as shown. Series combination of one resistive branch of R_{L} and another resistance R_{3} is connected across the resistance R_{2} as shown. Now we have to find out the current through R_{L} by applying Norton theorem.
First, we have to remove the resistor R_{L} from terminals A and B and make the terminals A and B short circuited by zero resistance.
Second, we have to calculate the short circuit current or Norton equivalent current I_{N} through the points A and B.
The equivalent resistance of the network,
To determine internal resistance or Norton equivalent resistance R_{N} of the network under consideration, remove the branch between A and B and also replace the voltage source by its internal resistance. Now the equivalent resistance as viewed from open terminals A and B is R_{N},
As per **Norton theorem**, when resistance R_{L} is reconnected across terminals A and B, the network behaves as a source of constant current I_{N} with shunt connected internal resistance R_{N} and this is Norton equivalent circuit.

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Created at 2016-10-18 04:51:47

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