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Question:

Inductors ( Confused with results)?

I have built a basic telegraph to test the inductor on my scope to verify what I have been taught. I wrapped my coil around a drill bit from bottom to top in a clockwise direction with the negative lead on the bottom, positive on top. I connect the battery positive directly to the top of the coil ( positive) and the negative battery to the key switch, from the switch to the lower end of my coil. I expected as the switch was closed and current was increasing that the coil will attempt to keep it the same ( zero) by reducing the voltage by an equal amount while it is building its magnetic field. However, I am seeing a positive spike rather than a negative and I do not know why. I have connected my scope negative to the bottom of the coil ( negative) and positive to the top of the coil ( positive). Like i said, I expected a negative spike in voltage, but i am seeing a positive spike which reduces to the positive battery voltage. Please Explain ?

Answer:

It is a bit difficult to visualize the experimental setup, but what I would expect that you see is this: when you close the key, the voltage across the coil should rise rapidly to the battery voltage, as the current increases, limited only by the coil and battery resistance. The scope will measure the sum of the voltage due to the coil inductance and the coil resistance, the voltage due to inductance will quickly deline to zero, and the voltage will approach the battery voltage asymptotically. Life gets much more interesting when you open the key: the current flowing through the inductor attempts to flow through the opening key contacts and will produce a spark, and you will see a large voltage spike on your oscilloscope. Figure out what the polarity should be, and see if that is what you get.
Dec 7, 2017

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