Basically Bob it's this.
All gauges, no matter what they measured used to be voltmeters.
Water Temp, oil pressure, amps... didn't matter they were all voltmeters.
They were high precision devices that were named after their inventor d'arsonval.
Usually with a small d - I don't know why.
There is another more costly way of having a gauge and that is with something called a stepper motor.
A real big version of a stepper motor is what BRP calls the DPS (it steers the Spyder)
Instead of whiring around like a "normal" motor it increments in small steps, Forward and Backward.
The new gauges, as I understand, are now stepper motor type.
More costly, but loads more stable.