Cambridge Instruments Slide-Wire Potentiometer

[Yorkie]

A neighbour came across this morning and said 'you're interested in old 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 asked if he could have it as he thought 'the wooden box might come in handy'. He was going to throw away the ‘workings’ and keep the box, but said he was having a clearout, so didn’t want 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 and to buy, or what use it was ever put to. I can’t find anything out about it on t’internet, but it has two sockets to attach a Weston Cell, then I assume that by adjusting the various controls, very precise voltages can be obtained, but to what purpose, I’ve no idea. (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). 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 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 – probably 1950s–1960s rather than 1920s–1930s. Don’t know if anyone can throw any more light on it.

(Comb-jointed wooden cabinets such as this one inspired me to emulate them for my humble home-brew projects!).

Hope it's of interest.

[Paula A]

What a fantastic bit of kit.. high precision for use in bridges and anywhere else an absolute measure of resistance is needed.

[Joe]

It's a thing of beauty whether it has a use or not.

- Joe

[Paula A]

I can think of a use right off the bat.. calibrating Avo's .. it was common to stick the carbon rod resistances across a bridge containing one of these things when they were being filed down (up?) to a precise value. Another use I can think of is calibrating oscillators.

http://www.lhup.edu/~dsimanek/scenario/l...wheats.htm

[Yorkie]

Thanks for reading and for your comments Joe & Paula.

It must have been quite a thing to drive, with so many controls!

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 makred '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? Presumably in a lab, or at the uiniversity? We'll never know.

Presumably they had to use a precision Wheatstone Bridge to measure the many shunts wound from low resistance wire.

I have another Potentiometer made by Cambridge Instruments, which uses an internal Weston Cell (not present), which Colin kindly gave me when we met at the Golborne Swapmeet recently. It brought back fond memories of a 'former life'. During the natural gas conversion era in the late 1960s, for a year I worked in the BG labs designing burners to convert obsolete gas appliances designed for use on coal gas to enable them to work on natural gas. (Had they not been converted, BG would have had to replace the appliances free of charge). We used such an instrument as that which Colin kindly gave me, with a 'black ball thermocouple' suspended in ovens of cookers to check that the oven thermostats were working correctly. The small voltage generated by the thermocouple was balanced with the potentiometer controls against a standard cell and gave an accurrate indication of the oven temperature. I guess that in the forty years that has elapsed since then, that BG thermocouple potentiometer is either in someone's collection, or has long since met its fate as an unloved obsolete relic of a bygone age.

During my apprenticeship with the then 'Gas Board' back in the 1950s gas was still made from coal shovelled into 'retorts' (furnaces), and they used to use 'disappearing filament pyrometers' which was held in front of the burning coal, and adjusted the pyrometer to a point at which the filament was the same degree of redness as the hot coal so the filament 'disappeared', at which point you read off the temperature on the dial. Simple, but clever. If the coal was too hot, they'd reduced the amount of air to it - too cool, increase the air flow. Not just 'stokers' - actually quite a skilled job, as was the work of 'firemen' on the footplates of trains.

At 15, I found it all very scary and was glad when my 3 month stint at the 'gasworks' was over! It was like being in Dante's inferno - flames, noise, heat, smells, hazards everywhere. Mothers used to bring sickly kids into the works to ask if they could breath in the tar fumes to ease their chesty coughs. If it worked at all, I guess the kids stopped coughing out of sheer fear of a return visit! How times change. Back then, the National Health Service was only seven years old - a service that was to be 'free at the point of delivery' (still is), and many if not most people back then, were very reluctant to seek medical help - "I don't hold with Doctors" was a typical comment.

Oh dear, rambling again in 'BOF' mode...sorry about that.

[tiv48]

should of been in a glass case ,a true work of art
rob t

[Shaman]

Thats a fine piece of old craftmanship David. Cambridge Instruments made some pretty fine equipment....regards....Alan.

[Yorkie]

Thats a fine piece of old craftmanship David. Cambridge Instruments made some pretty fine equipment....regards....Alan.

Yes indeed.

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 of universities. I haven’t been able to find anything out about the slide-wire potentiometer, but I have found a little about Cambridge Instruments - when the business was established or for how long it existed, from the Cambridge Museum of Technology website, which I found quite interesting:

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.

So now we know!

[Bushbaby]

Hi David,
If your neighbour has more to throw away like this could it please be in my direction!!
I disagree that its 50s or 60s as I have a feeling it more like 40's although it is very difficult to date equipment of this type. When I was working on the meter section my 2 panel Avo meter I thought that it had been completely rewired as it contained PVC coated cables. I did some research to find that PVC coated cables were indeed in use from the mid to late 30's!!

Great bit of kit thought!

Regards

Rob

[Andrewausfa]

Certainly a lovely instrument David. I bought a Pye Decade Resistance box recently and while relatively modern the workmanship, even on something simple like that, is evident.

I seem to recall Cambridge Instruments shut down for good in the last few years, there was a bit in the local rag about them. Cambridge Scientific Instruments are still going. As a corollary, there was a very interesting article in the BVWS Bulletin about the instrument maker H.W. Sullivan (actually I think Cambridge bought them out??).

I've not been to the Cambridge Museum of Technology yet. I thought it was just the stuff to do with the water pump and hadn't realised they had other interesting things.

Andrew