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Re: Interesting induction phenomenon



I had left out my oscilloscope attached to an Air Core Solenoid that I use
to demonstrate EM induction by pushing a magnet in and out of the coil. I
had done this in a different location in the room than normal and
uncovered, therefore, a new event for me. This morning some students were
repeating the demo, playing, and I noticed that instead of a dot being
deflected it was a vertical line. Why the line? The answer came quickly as
the coil was moved away from a computer that was operating today, but not
yesterday. A horizontal sweep gave us a frequency of 50kHz. Then I changed
the screen resolution and the amplitude of the induction dropped and
depending on which resolution I used I found a different wave form with
different frequencies. I do not know much about computer monitors but am
hoping to learn.

Any ideas?

You are observing the EMI from what is called a multisync monitor.
It supports a variety of horizontal swep frequencies. If you try
the same thing with, say, an iMac, you won't see a change in the
frequency because all resolutions have the same horizontal sweep
frequency (I think - I figured this out myself when I saw the odd
refresh frequencies and did some numerology on it). It is cheaper
and easier to make a display which runs at constant sweep frequency.

Leigh