Any dc voltage source can be represented in terms of an equivalent circuit consisting of an ideal voltage source (emf) and a series resistor (the ‘output resistance’). This is referred to as ‘Thevenin’s Theorem’. This concept is useful when determining the maximum voltage output of the source and the maximum power that the source can deliver to an external load resistor.

You are given a voltage source that has unknown emf (V_{0}) and unknown output resistance (R_{0}). Attach a variable load resistor (R_{L})
and determine V_{0} and R_{0} by measuring the voltage across
the load (V) and the current through the load (I) for at least two different
values of R_{L}.

Now measure the V and I for several values of R_{L},
plot the power delivered to the load, P_{L} = IV (= I^{2}R_{L}
= V^{2}/R_{L}) as a function of R_{L} and verify that
the power is a maximum when R_{L} = R_{0}. (This condition is sometimes referred to as
‘impedance matching’.)