Here is a nice specimen of a WWII-era electrical Switchable Tachometer, or Drehzahlanzeiger, as used in versions of the German Luftwaffe jet aircraft Me 262, He 162, and Ar 234. This tachometer received electrical signal from a generator located proximal to the engine drive to indicate engine speed, from 1,000-12,000 RPM.
According to our friends at DeutscheLuftwaffe:
Since the Jumo 004 engines were only reliable up to approx. 9,000 rpm, the displayable measuring range of the speed indicator was taken back to a maximum of 12,000 rpm in the case of later models (other dial). With a push button switch, which was installed in the side bench, could be switched on by means of a double speed sensor (Fl.20233) either the Riedelanlassmotor or the engine itself.
If the Riedelanlassmotor had brought the turbine to a speed of about 2,000 rpm, the engine was started, and the speed indicator switched to the encoder on the engine.
The system consists of a motor-driven speed sensor and a display instrument. The tachometer and speed indicator are connected by an electrical cable.
The speed sensor, a small alternator, is driven by the engine via a short connecting shaft. The magnitude of the voltage generated depends on the drive speed of the motor. The indicator of the electric remote speed measurement system is in principle a calibrated in rpm voltmeter.
The permanent magnetic speed indicator contains a fixed permanent magnet; in the magnetic field of a rotary coil is mounted. The pointer of the instrument is connected to this spin coil. The voltage generated by the encoder (generator) causes an electromagnetic field to form around the rotating coil, causing the coil to rotate against a spring force whose magnitude corresponds to the applied voltage.
It is in very good condition given its age. The glass and face are intact. Measures ~3.25 inches diameter and ~2.5 inches deep.