@gunrunnerjohn posted:I'm having a hard time visualizing how you get RPM from this instrument.
Humboldt Vibration Indicator, Tachometer Type
System resonance. I had one of these forty years ago, perhaps I still do, they work well for model airplane engines. You slide the needle in and out to peak the vibration. Or perhaps to null ? Not sure if the amplitude of electric motors vibration would be enough. perhaps a little modeling clay on the flywheel would make it work till the clay flew across the room. A bit related to why barrels climb a smooth rather steep incline in Lionel's barrel loader. The cheap laser tachs work better and are more accurate ! j
