Forty-two
inches in lengthWhat on earth is it you ask? A drainpipe? Well, according to an Electrical Dictionary printed in 1892, which I have in my library, Cardew's Voltmeter was well known, in those far off days, to odd voltmeter specialists. I suppose it can be classed as a "hot-wire" instrument as it used the expansion of a length of wire to turn a meter movement.
Forty-two
inches in length
The picture below shows the movement inside the wooden box
The dial and pointer are missing
The
wire has also gone from my example... not surprisingly I might
add as it was made predominantly of platinum. The wire went from
the movement, down one tube and round a tiny pulley at the base
then back up to the top.
I think there may have been two types of voltmeter available.. one used a second tube to extend the range to double (say 0-120 and 0-240) and the other to give a standard against which the first wire could be compensated. In the latter case, ambient temperature changes would affect both wires identically and the mechanism would self-adjust to compensate for any differences in wire length which would result. So with the meter pointing to zero at any temperature, the application of a voltage across the main wire would result in a current flow which would heat the wire; the wire would then expand in length and the meter movement would turn a pointer to indicate the measured voltage.
These things were pretty inefficient but would be ideal for measuring, say mains voltage where power was no object.
In fact measurement of AC with a moving coil meter would have been tricky in those days because one couldn't just magic-up a silicon diode or two with which to convert the incoming AC into DC. Remember that rectification, in those days, was a matter for strange tubes filled with iron filings or glass jars filled with noxious subtances and the like!
What I'd really like is a picture of the top with the little door open. Mine has not only lost its platinum wire but also its face and pointer. My dictionary shows the movement... but alas, in order to do that they removed the face! Another thing is those dark tubes. They were intentionally finished like that, rather than polished brass, to improve performance. Fortunately I found this out before I polished them.
The "current" plan is to restring it with thin nichrome wire and make a new face and pointer.
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 ink.
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 friction.
The next step is to attempt to read the old scale markings and try and restore it to at least its original function.
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