An exposed iron core is wound with a primary coil on one side and a secondary coil on the other. The primary is supplied with a DC current through a switch operated by the visitor. The secondary is connected to a light bulb. The visitor notices that the light only flashes at the moment the current is turned on or off. When the switch is held on, sending current through the primary and making the iron core into an electromagnet, a spring loaded steel hinge is attracted to the core. The visitor can easily see that even though the steel is attracted to the core, the light won’t flash unless the current (and hence the magnetic field) is changing.