Adjust the load resistor (at the right) until the power to the load resistor is maximized, as read by a joule meter (the box with all the connections in the center of the picture). The resistance of the load will also be about 70 ohms.
This demonstration has been used as an introduction to impedance matching between the power amplifier and loudspeakers of an audio system.
Note: a video camera can be made available upon request to aid in displaying this to large classes.