10. Back EMF

The voltage in the circuit above is adjusted so that the light bulb glows dimly. When the switch is opened, the bulb will flash much brighter momentarily as the magnetic field collapses around the inductor.

A neon bulb can be substituted and the voltage adjusted far below threshhold. The neon bulb will flash when the switch is opened, showing that the back EMF is much larger than the steady voltage.

The circuit can be arranged to produce a large spark when the switch is opened.