Bought as an "early
Galvanometer" that looks as if it needs attention.
This is not all it seems! It
is a concoction. An original very old frequency meter (maybe
1910?) has been dismantled, the heavy metal front has been screwed
to a piece of plywood and fitted to a wooden box, into which
has been screwed the original meter movement.
You can see by the offset of
the needle that something's possibly not quite right. The original
movement , which is located by no less than four hairsprings
still wants to place the pointer at the left of the scale, although
the mechanism has been bent so that it tries to read centre-zero.
Printed very lightly, because
it has been rubbed off are the old dial markings showing that
it once read around 400Hz; the new scale has been marked in black
The large terminals at the top,
perhaps once the original, are now merely dummies..... at the
sides are a pair of terminals connected to one of two meter coils
and a second pair connected to a second coil via a small full-wave
bridge metal rectifier. The meter does work but is more of a
curiosity than a useful piece of test gear. The chap, from whom
I got it, thinks it may have been used in a telephone exchange.
On the off-chance I scoured
through my library of ancient books, and found in a 1910 A-Z,
details of various frequency meters. This then is the "Langsdorf
and Begole" type, which employs a pair of coils mounted
on a common axis. Further reading in a book published in later
years showed this to be an early variety, now obsolete (that
is obsolete in 1912) due to shortcomings that had been overcome
in newer types. Basically, from an AC source, voltage is applied
via a limiting non-inductive resistance to one coil and current,
passed via a capacitor, through the other. The reading will reflect
the frequency of the AC. The pointer position, as can be seen,
indicates not a standard left-zero setting but the position of
the frequency reading at mechanical balance. Mechanical forces
result in reading inaccuracies and have been minimised by virtue
of finely balancing the movement and a lot of care to reduce
The next step is to attempt
to read the old scale markings and try and restore it to at least
its original function.