or anything that spins??Its got a rubber tip that you physically put on the end of a shaft . Maybe its for checking the rpm of an electric motor ?
True but its still a handy and novel tool to have around . Besides it fits well with the 40s-70s themeOn small engines you can get RPM's from exhaust pulses and plug wire induction.
That is what I use mine for and dad taught me using his. The machinery has a speed setting in the book for shaft speeds and this is what is used on the old stuff. The newer stuff went with a separate tach in the cab and now is full digital from the drivers seat.The tachometer works great on combines and farm equipment. Measures shaft speed.