Cardew's Patented Voltmeter

 What 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. It's 42 inches or 107cm in length and below is a close-up of its ancient looking label.


 The picture below shows the movement inside the wooden box and unfortunately both dial and pointer are missing. I think this must be viewing the voltmeter from the back... or is it the front?


 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 twin-tube voltmeters 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.


 A more common type of Cardew's Voltmeter used a single tube and below are shown examples.

Below is an example from Italy..




 Left a closer picture of the face of the example found in Italy.

This one was made by Paterson & Cooper of London and is calibrated fairly linearly from 10 to 360 volts.






Below, missing its tube is the dial from an example found in the UK made by Queen & Co.

From the style of the lettering this must be from the 1890s and measures up to 120 volts.


 Below is an advertisement from 1888.


 Above and below are more pictures of my twin-tube example. You'll note that there isn't the name of the maker anywhere on the instrument. Presumably this was on the missing dial? From the residual markings and terminals this example may have had two ranges.. perhaps 100 and 200 volts. On the Cardew label is the serial number 664 and this is pencilled on the rear of the top assembly together with a second number 667, possibly connected in some administrative way to the second of the two voltage ranges?

From the position of the two clamps the instrument would have been used vertically with the wooden box either at the bottom or top. I need to know which way up it was used so I can correctly position a new dial. A clue is the orientation of the Cardew label which seems to indicate the box was at the top, unlike the example shown in the advertisement.
 Is this the front of the voltmeter. If it's the front the spindle to which the pointer would have been fixed may be broken off or may be just pushed in? The position of the mounting brackets on the tubes would however seem to suggest that this is the back. Maybe someone can offer some advice?


 Attached to the lid is a spool for fuse wire. This instrument would require a relatively high current, perhaps an amp or two so would have been used where whatever generator was being used would be supplying at least a magnitude of current greater than that needed to provide the voltage reading.
Click the label to see a description printed in 1891

See a publication from 1888

Langsdorf & Begole Frequency Meter

 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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