The Improvised Tachometer

Above 300 RPM, everything is a blur.

Every machine you can build has a speed it wants, and a shaft turning too fast or too slow looks exactly like one turning right. A tachometer settles it in a second. This book is for when you have no tachometer.

Six ways to measure shaft speed with what is in the shed: a chalk mark and a clock, a pendulum built from string and a rock, a strobe, a tuning fork and your own ears, and the alternator's own electrical hum. Each one checked against the others so a confident wrong answer cannot get past you.

Then the arithmetic of pulleys, belts, and slip, so the number you measured becomes the number you need.