First power supply is connected across the
two bulbs in series, set to 12 volts or so, so that the bulbs light.
The second power supply is connected across a single bulb, and set to 6
V. Show that closing the switch to the second power supply has no
effect on the circuit. This illustrates one of the Mazur questions.
NOTE
Connect second power supply only momentarily,
in case the voltages are not identical.
