Editing Output impedance (section)

== Measurement ==
The source resistance of a purely resistive device can be experimentally determined by increasingly loading the device until the voltage across the load (AC or DC) is one half of the open circuit voltage.  At this point, the load resistance and [[internal resistance]] are equal. 

It can more accurately be described by keeping track of the voltage vs current curves for various loads, and calculating the resistance from [[Ohm's law]].  (The internal resistance may not be the same for different types of loading or at different frequencies, especially in devices like chemical batteries.)

The generalized source impedance for a reactive (inductive or capacitive) source device is more complicated to determine, and is usually measured with specialized instruments, rather than taking many measurements by hand.