5J20.20 Light Bulb in Parallel with Large Inductor

PURPOSE: To demonstrate self-inductance.

DESCRIPTION: A small light bulb in parallel with a large inductor, which is in series with a DC source and a resistor; when the switch is closed the bulb flares up brightly for about 1 second, then dims down.

SETUP TIME: Medium

COMPONENT LIST:

    


  1.  Small Light Bulb			Light Bulb Drawers


  2.  Large Inductor			


  3.  DC source/resistor			lab bench/resistor drawer

SUGGESTIONS: See Vincent's 2122 Demonstration Booklet for further instructions.

REFERENCES: 2122 4.79; PIRA 5J20.20

OTHER NOTES: Explanation: While the inductor is "charging" through the resistor a counter EMF develops which makes the source see a high impedance (infinite, in fact, at first); the early current flows more through the light bulb. But after the current through the inductor reaches a steady state, the inductor's effective impedance declines to a low value, most of the voltage drop appears across the resistor, and the bulb is starved. Use typically Howey coil 05-40-2 or equivalent; this coil is also known as HP-311711153. The large Ealing electromagnet 4.55 can also be used as the inductor for a really long time constant. A No. TS305 bulb, No. 308, or 1/2 of a 1666, is good with 28 VDC and roughly 14 ohms. A sneon bulb will show reversal of polarity if the circuit is fed with 115 VDC.