sentences:  is found by replacing the extra element with a second test signal source (either current source or voltage source as appropriate).Then, is defined as the ratio of voltage across the terminals of this second test source to the current leaving its positive terminal when the output of the system's transfer function is nulled for any value of the primary input to the system's transfer function.In practice, can be found from working backwards from the facts that the output of the transfer function is made zero and that the primary input to the transfer function is unknown.