How does an electronic delay timer (solid state) work?

You know, those 2" by 2" by 3/4" (WAGNER hockey puck) adjustable solid state timers? How do they work? Are they a capacitive resister? Sorry, I'm not much on electronic theory. Are they susceptable to voltage/current surge? Magnetic flux? Will that screw them up so they might instantly... show more You know, those 2" by 2" by 3/4" (WAGNER hockey puck) adjustable solid state timers? How do they work? Are they a capacitive resister? Sorry, I'm not much on electronic theory. Are they susceptable to voltage/current surge? Magnetic flux? Will that screw them up so they might instantly reset or infinite time out? I've got one in my furnace circuit, and I think that when the high voltage transformer for spark is energized it messes with the timer and I end up having to get them replaced every so often as they intermittantly fail.
Thanx!
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