Antique Ammeter

 This is heavy, as its case is made from cast iron. The glass is very thick and the bezel really solid and substantial. It was sold as WW2 vintage, but I suspect it was already very old then.

 I thought something may be missing as there's no hairspring and not a vestige of magnetism to be found, however I'm in the process of changing my mind. What I imagined was a shunt, which is the open wire horizontally crossing the centre, is perhaps a "hot wire" element and as such is possibly platinum?

What looks like part of a broken hairspring is a tensioning device and is pretty well complete.

The pieces of string attached to the needle suspension are probably silk threads.



 Early hot wire ammeters had a silk thread wound around the needle pivot and anchored at one end to a tensioner and the other to the hot wire. A small pulley may have been free to run on the wire element and when the latter heated up from passing current it stretched, allowing the pulley to move toward the needle pivot. This would relax the tension and allow the needle to move across the scale.

The adjusting screw, passing through the lower edge would seem to distort the metal frame in a bow-fashion, causing the hot wire to tighten or slacken, increasing or reducing tension and allowing the pointer to move to read zero.

I should say the meter dates from between 1900 and 1910 but I haven't got a clue. There isn't a date on the back of the scale. The insulator is ebonite so it must date from before 1930, when I think bakelite would have been used.

If there's nothing actually missing it may be interesting to get it working, but is anyone able to provide any information, including a manufacturer? The scale, as you can see reads up to 3 amps which doesn't seeem much for such a large meter (seven inches diameter). Maybe it was used to measure RF current to an aerial although it wouldn't accommodate much in the way of power (around 500 watts). For AC mains it wouldn't be much use for anything substantial either, say 700 watts. Maybe it was used to monitor power from a small generator in an environment demanding a very rugged meter, say a small ship?
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