The Kollsman Electric Tachometer Indicator

May be of interest to those reading this:

I asked a few weeks ago about a good oscilliscope. I purchased a $30 oscilliscope off of amazon that doesn't even have a package, it's basically on a breadboard case.

and it works good enough to test things like this. I used it yesterday to test two separate signals (speedometer signal, an AC wave), and the resulting VSS output (converted to a 12v DC signal), and also for fun hooked into my tach. So... highly recommend

Magnetic coupling is clever. I always figured they used centrifugal force instead. Perhaps some do.