A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Michael Bradley
Date: 2017 Aug 14, 16:01 -0700
My guess is an early wide range electrical rev counter, which read the voltage from a small wandering generator, mechanically linked to the shaft whose speed required measurement, the generator wired into your panel..
The electrical machine on the panel itself, I guess, is a motor, driven by the aformentioned external generator, itself driven by the shaft whose rotation speed needs measuring.
The disc rotating between fixed magnet is almost certainly an 'eddy current brake' or oscillation damper which provides a light load and a braking force linearly related to the motor's shaft speed. ( See Wikipedia )
The wide range of operation implies a general purpose instrument rather than one for a single purpose. The possible 40 to 1 switchable ratio is the clue to that.
Too many connections and techy bits on the front panel to be left on the bridge of a ship in the hands of a junior navigating watchkeeper .... more likely to be engine room equipment.
The nice linear relationship between shaft speed and generated armature voltage from a generator with constant field magnetisation came as an ideal early instrumentation system once shaft speeds got too fast to measured by 'counting the thumps'.
Only guesses ........ nothing in principle prevents it being part of a log, but ......