How does an analog speed indicator / tachometer work?

Factual Questions
1

I’ve been wondering about this for an embarrassingly long time. How does an analog tachometer work ?

I have no problem imagining how to do it digitally if the device has some memory and a reliable time reference (quartz oscillator, etc.), but I don’t see how the analog version works. How is the rate of revolution converted into a reliable indication by a needle ? I don’t see any technical info about this on Wikipedia.

I assume a speed indicator on a car or bicycle works the same way, it’s just attached to the wheel instead of the crankshaft.

2

There is a drive cable going into it, and a spinning magnet.

3

I had an analog speed indicator for my bicycle, around 1978, before digital ones were available / affordable. Out of curiosity I opened it. I concluded that it had a permanent magnet, that was mechanically connected to the wheel, so it rotated. The rotating magnetic field would induce an electric current in a metal disk, and consequently a force (Lorentz force) to that disk. That disk was connected to a spiral spring, providing a counter force to limit its rotation, and was also connected to the speedometer’s needle.

ETA: IIRC it was basically as the drawing uploaded by DPRK.

4

You can also make a purely mechanical one (without any electromagnetic parts at all) with a hanging weight attached to a rotating vertical shaft. As the shaft spins, centrifugal force pulls the weight out (and hence up). The angle the weight makes with the vertical will depend on how fast the shaft is spinning.

5

More modern electrical tachs use a frequency to voltage converter and get pulses from the ignition.

There is also a tach often seen on vintage European race cars that moves in steps instead of a continuous sweep. It was called a chronometric tach and works like this: “A tachometer which repeatedly counts the revolutions during a fixed interval of time and presents the average speed during the last timed interval.”

Here is one in the opening of “Grand Prix” at the 4:30 mark:

6

I have a bicycle with an automatic transmission that works on this principle, albeit with three balanced weights working on a horizontal shaft (the wheel axle). Shockingly, it works pretty well despite nigh on 20 years of neglect.

7

So a permanent magnet induces a current, and the needle is deflected by the current. That makes a lot of sense. Thank you all.