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 (V0) and unknown output resistance (R0). Attach a variable load resistor (RL) and determine V0 and R0 by measuring the voltage across the load (V) and the current through the load (I) for at least two different values of RL.
Now measure the V and I for several values of RL, plot the power delivered to the load, PL = IV (= I2RL = V2/RL) as a function of RL and verify that the power is a maximum when RL = R0. (This condition is sometimes referred to as ‘impedance matching’.)