PURPOSE: To show self inductance and back EMF.
DESCRIPTION: Two circuits are compared. One circuit is a 500 Watt lamp with the resistance of the lamp being the lone element of the circuit. The lamp is simply turned on by a closing switch. The lamp is seen to glow quickly. There is a small thermal delay due to the time needed to heat the filament, about half second of time.
Another circuit has the same 500 Watt lamp. But the lamp has a very large inductor connected in series. By closing a knife switch the current to the series R L circuit is turned on. The lamp will take a long time to turn on, about six seconds to achieve its full brightness from the current. After coming to full brightnes opening the knife switch to cut the current will produce a large spark from back EMF. This is seen as a giant arc of flame across the poles of the knife switch.
The large coil is almost three miles of wire. It is 26 gauge wire with a linear resistance of 11.4 Ohm per 100 ft. It has 165.1 Ohm total resistance. It is is 12 in. tall and 12 in. diameter. The core filled with soft iron and is 5 inches wide.
EQUIPMENT: Coil, lamp, knife switch, power supply as shown in the photos.
SETUP TIME: Click on the photos below that show how to close the knife switch to turn on the lamp:
Below is a photo of how to open the knife switch. Note that the spark from the back EMF is not seen in the photo because the photographer missed it.
Below is a different coil of wire being used as a parallel R L circuit with an AC neon lamp to show the polarity and the energy of the magnetic field:
Udated by JZ in 3/13/2000
Click here to watch movie of this demo!
