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
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 seem 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?