Pulse Input Speed Indicator Contruction

Etude

Hello!

Can someone plz explain the construction of a Pulse Input Speed Indicator?

As i understand, it consists of a rotating disc and some kind of reader picks up some sort of magnetic fields from certain spots on the disc as it rotates and the speed depends on the number of such impulses picked up per second.

Am I right ? What exactly are those spots and how does the reader work?

Thank you!