Cambridge Instruments Slide-Wire Potentiometer

[David G4EBT]

A while back a neighbour said to me "you're interested in old electronic things - is this any good to you before I take it to the tip?" He'd actually got it off someone who HAD taken it to the tip, and had asked that person if he could have it as he thought 'the wooden box might come in handy'. He told me that his intention was to throw away the ‘workings’ and keep the wooden box, but said he was having a clearout, so didn’t want either the box or the ‘gubbins’ as he put it. Needless to say, I said I'd give it a home!

I can’t begin to imagine how much this instrument would have cost to make, or what use it was ever put to and I can’t find anything about it on internet. It has two sockets to which attach a Weston Cell, (missing) so I assume that by adjusting the various controls, very precise voltages can be obtained. The Weston cell is a wet-chemical cell that produces a highly stable voltage of 1.018638 Volts, suitable as a laboratory standard for calibration of voltmeters. Invented by Edward Weston in 1893, it was adopted as the International Standard for EMF between 1911 and 1990). See:

http://en.wikipedia.org/wiki/Weston_cell

As can be seen from the attached pics, though externally it’s in need of a good clean up, internally it’s beautifully made, with high quality 'switchery' wire-wound pots, many wire-wound resistors, and looks as good as the day it was built. I don’t think it’s as old as it might look at first sight – I'm guessing probably 1950s–1960s rather than 1920s–1930s.

The large dial says 'Volts 0 - 1.7 in 0.1V Steps' and states 1 Div = 0.0002 Volts (yes, two ten thousandths of a Volt per Div!). Errm why, I wonder?

The other controls are:

Fine Rheostat,
Coarse Rheostat,
Standard Cell Voltage 1.0170 - 1.0890V
Push button marked 'Galv Key'

The pairs of terminals are marked:
'Standard Cell' (For externally connecting a Weston Cell?)
Then: 'G', '1', '2', '3', then '2 Volts'

Rather a pity that it can't talk, and tell us what sort of a life it's had, and what use it was put to, maybe in a lab, or at a university? I guess we'll never know. Presumably during manufacture they had to use a precision Wheatstone Bridge to measure the many shunts wound from low resistance wire?

Apart from the R&D, what amazes me is how many intricate components there are, both mechanical and electrical, and the many hours that must have gone into hand making each one of these instruments. I wouldn't like to hazard a guess at the price when new, and I doubt that many were made. I suspect they found little application outside of academia, and I wonder how many are still gathering dust in the labs and store-rooms of universities? Though I haven’t been able to find anything out about this slide-wire potentiometer, I have found a little about Cambridge Instruments - when the business was established, by whom, and for how long it existed, from the Cambridge Museum of Technology website, which I found quite interesting:

See: http://www.museumoftechnology.com/cic.html

Here’s a clip:

When Cambridge University established the Science Tripos in there were no facilities for making apparatus in Cambridge. To meet their requirements for apparatus, the research and teaching staff had to manage as best they could by making their own. The workshop attached to the Department of Mechanism began to make apparatus in 1878 and soon after the Cavendish Laboratory of Physics established its own facilities. It was from this workshop that a mechanic named Robert Fulcher left to set up his own scientific instrument making business, taking advantage of the increasing demand by the University for apparatus. However his design abilities were somewhat limited and Horace Darwin, the youngest surviving son of Charles Darwin the botanist began to take an interest in the firm. Eventually in 1881 Darwin purchased the business in partnership with his friend Dew-Smith so founding the Cambridge Scientific Instrument Company. An example of an instrument from this period is the 1884/85 rocking microtome seen at the museum, together with a later version of the instrument of the 1950s. It was one of Darwin's successful designs that continued to be manufactured until the 1970s.

In 1968 the company was sold to George Kent and since the merger it was organised into four divisions, each being eventually separately sold off. Many ex-company employees either founded other well-known companies or gained important positions in them. William T. Pye, who had joined as foreman in 1880 left in 1898 to form the W.G. Pye Instrument Company with his son, ultimately to branch out into Pye Radio. Charles Foster, a chief draughtsman formed the Foster Instrument Company, E. I. Everett joined with Edgecombe to form Everett Edgecome Ltd, George Whipple who worked at Muswell Hill became Managing Director of E. R. Watts & Son and S.W.J.Stubbens, a foreman formed Unicam Instruments Ltd to name a few examples.

End clip.

I've been advised that that Cambridge Instruments shut down for good in the last few years, there was a bit in the local Cambridge newspaper about them, but I'm told that Cambridge Scientific Instruments are still going. As a corollary, I'm also told that there was a very interesting article in a BVWS Bulletin (don't know which issue) about the instrument maker H.W. Sullivan, who I believe Cambridge S.I. may have bought out.

Haven't a clue as to what its current value is.

Don’t know if anyone can throw any more light on it?

Hope it's of interest.

[vosperd]

Hi David,
Interesting bit of history about Cambridge instruments. I worked for them from 1978 but they had long since moved on from that type of instrument. I heard that Foster Cambridge took over the industrial instrument side of the business.
The medical side went, then the crystal pullers. They merged with Leitz using the Leica name for some years then became Leo, finally to be taken over by Carl Zeiss. I retired a few years ago.
Hope you get more info about the instrument.
Don m5aky

[turretslug]

Yes, a fascinating tale of famous names. A lovely bit of precision instrumentation, too- I shudder to think of how many things of this ilk must have gone to landfill.

[60 oldjohn]

Possibly part of a 1800s style multimeter, the other part being a Galvanometer. Are there connections marked G ?


John.

[David G4EBT]

I don't think it's 1800s - Cambridge Instruments was founded in 1881 and as I stated in my post, the Weston Standard Cell was invested in 1893 and seems to have been adopted as an EMF standard from 1911 right up until 1990, so it could conceivably have been very late 1800s or early 1900s. However, looking at the fonts of the engraved lettering it looks to me to be maybe 1930s or even later, but that's just guesswork on my part.

As to the terminals, as I stated above, the six pairs are marked:

'Standard Cell' (For externally connecting a Weston Cell?)
Then: 'G', '1', '2', '3', then '2 Volts'.

I have to marvel at the R&D that must have gone into it, and the fact that it was clearly hand-built - not mass produced, and must have taken countless hours to construct. A joy to behold, but really, just another relic of a bygone era and a sign of how quickly times change, and how soon 'state of the art' equipment soon becomes hopelessly obsolete. I recently visited the Greenwich Museum where John Harrison's clock of 'Longitude' fame resides. A staggering feat of inventiveness and precision engineering, yet a crystal-controlled cheapo watch from a pound shop will keep time just as well if not better.

How times change!

When next you're in Cottingham John, I'll dig it out of the loft so you can cast your eyes over it and take a closer look.

Thanks Don ('Vosperd') for your info about Cambridge Instruments.

[Brian R Pateman]

These wonderfully simple instruments were in use in industrial situations until well into the 1970s.

Any instrument workshop would have at least one as a transfer standard. The portable version (built to the same standard" had the Weston cell and galvanometer built in to the case and were taken out into the field foe checking thermocouples used for temperature measurement.

I have a couple for old times sake.

The Weston cell was used as a reference source in some early electronic instruments as well. I have a memory of finding one in a Solartron digital voltmeter many years ago (at least I think it was a Solartron).

[David G4EBT]

Thanks for reading the post Brian, and for the info. It confirms my original suspicions that it's most likely relatively modern (ie, post WW2) rather than antique. It deserves to be spruced up so that the outside looks as good as the inside, so I must attend to that.

[Jeffreyabb]

My memory is a bit hazy about this, but I seem to remember that the voltage of the Weston cell isn't constant but declines over time at a predictable rate. Also that it was used in some sort of bridge arrangement to that no current is taken from it. I have one tucked away somewhere dating from the 70's or 80's which came out of a process controller. I'm pretty sure that the original voltage & date are on a slip of paper inside its case. Lots of them junked when they became obsolete.
Hope someone can correct me if I'm way out on this.
Jeff

[Brian R Pateman]

The Weston cell provides a constant voltage provided that the current drain is negligible (tends to zero).

There's a simple treatment on Wikipedia;

http://en.wikipedia.org/wiki/Weston_cell

I haven't seen a new one for about thirty years and I would be surprised if they are still commercially available. That said as they are an electrochemical device it shouldn't be too difficult to make one although the chemicals used are moderately hazardous!

[Nickthedentist]

I too think it's postwar because of the PVC insulation used, though the design could well have been 50+ years old by then.

N.

[emeritus]

Here's some info on the Weston cell from a 1927 school physics text book. Appropriately enough, a Weston cell made by Cambridge Scientific Instruments Ltd is illustrated. Temperature variation is 0.0000406 Volts per degree Centigrade, voltage rising with fall of temperature and vice versa relative to 1.0813 Volts at 20 Degrees Centigrade.

[MotorBikeLes]

I don't know that particular model, but we had similar stuff at Tech, back in my day. All our temperature instruments throughout the pottery industry would be regularly checked by the service technicians from the manufacturers (Elliot/electroflo/Bristol, Geo Kent, Ether/Ether Weelco, Foster and others) who all had a potentiometer both for reading thermocouples, and outputting to the instruments for calibration. Mostly "one rung down" from this instrument, but generally by Cambridge. Our 1946-1948 manufactured Kent Multilec controllers were glorified, automatic versions of that, with an electric motor driving a complex, continually rotating camshaft that carried out about ten functions culminating in a fresh indicated temperature. They all included a Weston Cadmium cell, which was used to standardise a large Leclance cell every 50 minutes. It was almost a piece of magic to watch and study it, with its simple tufnol pointer in its suspension wire "pivots", being clamped, then a released clutch being reapplied, rotation of the slidewire, release of the pointer, and after a few seconds, a repeat of the whole cycle.
On one factory, we had Honeywell-Brown controllers. These had an equivalent length (circular) scale of some 28", so very easy to read. Unlike the Mk1 Multilecs, these were fully electronic, and included standard cells for reference. About 1968, I had the failing standard cell of one instrument replaced by a honeywell replacement unit which was a Zener stabilsed setup.
Interesting times, long gone.
Les.