Demos: 6D-10 Self-Induction

A neon lamp with circular electrodes is connected to a 110 V DC supply through a rheostat and a switch K₁. A low-resistance, high self-inductance coil is connected parallel with the lamp through switch K₂. With K₂ open and K₁ closed, the cathode of the neon lamp lights up when the voltage across the bulb is 80 V or greater, and stays lit. If K₂ is then closed, the light goes out because L has a low resistance. But on opening or closing K₁, the cathode or anode will glow momentarily, because of the emf induced in L following Lenz’s Law.

Directions: The circuit should be set up ahead of time by the lec-demo staff. Close the key K₁ to cause the neon bulb to glow. Then close K₂. Noting the bulb has now gone out, open K₁. After noting that the bulb momentarily lit, close K₁ to note that it lights again.

Suggestions for Presentation: Briefly describe the operation of the neon bulb, particularly in regard to its threshold voltage. Then remind the students that a coil has a fairly low DC resistance, so that when it is placed in parallel with the bulb, there is almost a direct short of the bulb. After opening and closing K₁, have the students speculate as to how the voltage across the bulb could suddenly rise to the threshold and why different halves of the bulb light, respectively.

Applications:

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