Synchros and RPM

How does an RPM indicator work?

In a car it is simple.
You only need a flexible shaft to connect a RPM meter to the engine. Inside the meter is a rotating magnet and a aluminum disc on a shaft with a return spring. The magnet spins and induces currents in the disc. These interact with the magnet and the disc begins to move... and so does the pointer.

And in an aircraft?
Well, at least in the Super Connie it is exactly the same principle with one difference. It is not possible to use a flexible shaft since the engines are 10 or more meters away from the cockpit. So they made something clever; they mounted a small generator on the engine and a even smaller electric motor inside the instrument. If the generator turns with 500 rpm for example, then the instrument motor does exactly the same.
And the rest is like in your car.... your old car... without electronic displays.