First power supply is connected
across the two bulbs in series, set to 8 volts or so, so that the bulbs
light. The second power supply is connected across a single bulb,
and set to 4
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.