Cambridge Instruments – Dolezalek Quadrant Electrometer

[Gregg]

What an amazing source of information the EEVBlog is; this was resolved in a couple of hours.  This post was originally posted asking “what is it”.  Forum member chris_leyson came up with the answer. 

It is marked:
   Cambridge Instrument Co. Ltd. England
      C 157555
      Hand engraved: IR9450 
The overall height is about 260mm. Diameter of the removable cylinder with the visual port is 95mm; it is painted flat black inside.

At the top in the center is a pin that moves up and down with a sharp point on the bottom.  It does not have a place to make an easy electrical connection like the other three protrusions on top.  It is electrically insulated and used for hanging a butterfly shaped disc that fits in the hollow of the quadrants below. The stem that is missing should have a small surface silvered mirror that will reflect a light beam through the window in the cylinder.
The other three top terminals are all insulated from each other and the frame; they have holes for wires with thumb screws to fasten them.
The little adjustable whisker is to add or remove a small charge to the hanging butterfly disc.

[Gregg]

More pictures of the Cambridge instrument:

Some provenience:
I became the executor for the estate of a friend that worked as an instrument maker who made lots of neat things for the physics department of a major university.  My friend also collected things that he thought were neat; I’m sure most of us on this forum can relate.  This instrument was probably something that he thought was worth saving when old equipment was being purged and it was given to him.

[T3sl4co1l]

Quadrupole chamber made of hollow 'D' shapes, smells like a cyclotron thing.  Obvious problem, the lack of magnets (and lack of vacuum port too..?).

Tim

[coppercone2]

that looks like an early model of the modern time machine

[coppercone2]

btw there is a wheystone bridge thingy drawn o nit

[T3sl4co1l]

btw there is a wheystone bridge thingy drawn o nit

With a cam in the center.

Get it?  Cam bridge?!!?!....

http://www.richardsradios.co.uk/cambhistory.html
At least they had a sense of humor in their early days...

Tim

[coppercone2]

i thought it was a physic symbol

[Gregg]

btw there is a wheystone bridge thingy drawn o nit

With a cam in the center.

Get it?  Cam bridge?!!?!....

http://www.richardsradios.co.uk/cambhistory.html
At least they had a sense of humor in their early days...

Tim

Tim,
Thanks for finding the link to the logo.  Now we at least know it isn't a schematic of the device.

[chris_leyson]

Apparently it's a quadrant electrometer but missing the quartz suspension fibre, mirror and small vane that sits inside the brass quadrants.
https://physicsmuseum.uq.edu.au/quadrant-electrometer
https://www.orau.org/ptp/collection/electrometers/Cambridgedolzaleck.htm

[Gregg]

Apparently it's a quadrant electrometer but missing the quartz suspension fibre, mirror and small vane that sits inside the brass quadrants.
https://physicsmuseum.uq.edu.au/quadrant-electrometer
https://www.orau.org/ptp/collection/electrometers/Cambridgedolzaleck.htm

Thanks Chris, your google-fu far exceeds mine.  I'll have to dig through the remains of the estate and look for the vane and mirror.  If I find it, I'll certainly post pictures.  Maybe my friend was going to make a vane, since he was an instrument maker.  One of the many projects he never got around to doing.

[Gyro]

Well done to Chris_Leyson, I'm surprised the rest of you guys didn't get it (cyclotron? ), it should have been covered in middle school or high school Physics - electrostatics.

Yes it is a very nice Cambridge Instruments quadrant electrometer (I've been after one for my collection for ages!). It looks in very nice condition, apart from the missing vane and mirror and at least 100 years old. The lacquered brass-work looks really well preserved. The quadrants work as differential pairs, attracting and repelling the leaves of the vane.

Regarding the vane, it might actually still be there - @Gregg, you mentioned that the quadrants aren't quite aligned. ONE of pair of the quadrants should be able to pivot so that the vane can be inserted and removed, you might find that it is still inside. Unfortunately you are missing the tiny circular mirror and suspension fibre but you might be able to fabricate them (if you can find an old galvanometer suspension for instance), look around on the web for an old book on electrostatics. They would probably have been held together with shellac, the vane would be thin Aluminium foil in the shape of a fat flattened 2 vane propeller.

In terms of perservation, the lacquer on the brass is soluble in most solvents so soft dry cloth clean only. Those nice orange insulators need special care too - they are made of very high insulation resin - fused Sulphur was used at one time, so again, very gentle removal of dust with a soft cloth only. Dust attracts moisture, which gets through the lacquer and causes dark spots.

Thanks for sharing the detailed photos!               


P.S. You can see a 'cheap' educational version here: https://www.ebay.co.uk/itm/A-RARE-ANTIQUE-PHILIP-HARRIS-QUADRANT-MIRROR-ELECTROMETER/323426129823?hash=item4b4db3179f:g:iRoAAOSwjrlayx1e

[Gyro]

I'm reviving this thread as, after years of searching, I'm now the proud owner of a Cambridge Dolezalek Quadrant Electrometer.

My example is a Cambridge Scientific Instruments Company Ltd model, dating from around 1906. Unlike the OP's it has it's inputs brought out on the underside and has a square window. I was also lucky that it came complete with mirror assembly and very luckily, when I opened up the quadrant, I found the original silvered paper vane which had become detached.

The company history is quite interesting, one of the founders was Charles Darwin's son. It was founded in the late 19th century, later getting into electrical measurement instruments, and introducing the quadrant electrometer in 1906. In 1919, the company merged (took over) with the Robert W Paul Instrument company to become the Cambridge and Paul Instrument Company, and then in 1924 changed it's name to the Cambridge Instruments Company, adding the familiar 'bridge' logo seen on the OP's example. Robert W Paul was an interesting character, in addition to his Scientific instruments company, founded in 1891 (I have one of his early High Sensitivity suspended coil Galvanometers, and also one of his Unipiviot ones), he was also the pioneer of cinematography in England, demonstrating moving picture projection in 1896 (around the same time as the Lumiere brothers in France) but after Edison. He had his own film studio.

I've attached some photos of the repaired Electrometer. I've re-fitted the vane, not easy because it is extremely fragile and had been torn from the mirror assembly. Whilst I have replaced the suspension, with a fine piece of glass fiber from a fibre-optic bundle (instead of the piece of cotton that saved the mirror from being lost), this still comes in at around 40um, which is still well short of the 4, 6, or 8um fused quartz suspensions that it originally came with. I've no idea where I would find one of those - I'm sure I have some single mode fibre somewhere, which has a core diameter of 7um which might get me close.

With the correct 6um suspension, these electrometers were capable of measuring below 0.1V potential differences across the quadrant inputs once the vane was charged to around 100V - it has a true differential floating inputs. I had always associated 'passive' electrometers with higher voltage measurements.

I've attached some photos, together with the relevant 1906 pamphlet - it's interesting to see that, at the time, their phone number was 'Cambridge 6'! Hopefully this is of interest to the OP and maybe others.

[Kleinstein]

The single mode fibers may have some 5-8 µm of core diameter, but ther total filter is much thicker, more like 100-200 µm.

[RoGeorge]

In the pdf it says the instrument has a thin 0.004 mm diameter and 60 mm long quartz fibre. 

How do they made a quartz "wire" so thin and so long?  Is that quartz fibre brittle when bent?

[Kleinstein]

Chances are one can make the extra thin quarz fibers just like the normal fibers for optics. So heat a piece if quarz and than pull a fiber. AFAIK the hotter the glass the thinner the fiber and also faster pulling can make it thinner.

The fiber would not be espeically sensitive and surprisingly strong. It would still be brittle and fail with no warning before it fails. With the thin diameter there is a good chance to be free of any defects to start with and thus quite strong.  Bending may allow a radius of possobly down to 100 diamaters or in the 0.5 - 1 mm range.
One might get relatively thin fibers from glass fiber insulation material - not the the same composition, but could be good enough an insulator.

[Gyro]

Thanks Kleinstein. I remember heating and pulling glass tubes to make micro pipettes, as you say, the faster the pull, the thinner the result.

I don't know the relative tensile strength of glass and fused Quartz, I suspect that the latter is probably stronger, allowing smaller diameter. Even 40um fiber is surprisingly 'springy',  it's quite easy to sense when you are about to exceed it's bending limit, I have snapped them at as large as 10mm or so bend radius, which is surprising. Looking closely under the microscope, the old fiber bundle that I have looks to be surprisingly uncontrolled in fibre diameter, so I think it's going to be a matter of experimentation now I have the technique.

Spun glass loft insulation may well be a possible source, as long as I can get sufficient length, I considered silk too, but everything I've tried so far is woven from strands that are too short.

Cambridge also offered a fine (80um x 4um) Phosphor Bronze ribbon suspension for permanent, rather than brief, application of bias but I haven't found anything near small enough, and they had lower sensitivity anyway.

I've attached a few other non-pdf images from other (slightly later) catalogue pages.

[RoGeorge]

How sensitive is such an electrometer in practice?

For example, if I connect 2 metal plates at a 9V battery, and keep the plates separated at 9cm apart, between the plates it will be a DC electric field of about 1V/cm.  If I place a 3rd electrode in the air, between the 2 plates, will the electrometer be able to sense the field between the plates?  0.1V would correspond to moving the 3rd electrode with 1mm between the plates, will an electrometer be able to sense that?  (never had access to an electrometer, no idea what it can do in practice)

[Gregg]

Thanks Chris for your contribution to keeping ancient technology alive.  I haven't played with my electrometer in quite some while. 
I did try to separate a single filament from some braided Dyneema fishing line with limited success (I couldn't separate long enough fibers).  Maybe I'll have to try different sizes and brands of line.  If I can get a filament thin enough, Dyneema seems to be almost perfect for the task.  Kevlar is a possible second choice.  Both will allow knots or loops for the ends whereas glass or quartz probably would need to be glued.  Maybe carbon fiber?
Any suggestions for making a new stem for the vane would be appreciated; presently I am using the thinnest aluminum hobby tube I could find and a tiny non magnetic stainless screw at the bottom to hold the vane I made.
Unfortunately I cannot see as well as I used to and my dexterity is also suffering from becoming older. 

[Gyro]

How sensitive is such an electrometer in practice?

For example, if I connect 2 metal plates at a 9V battery, and keep the plates separated at 9cm apart, between the plates it will be a DC electric field of about 1V/cm.  If I place a 3rd electrode in the air, between the 2 plates, will the electrometer be able to sense the field between the plates?  0.1V would correspond to moving the 3rd electrode with 1mm between the plates, will an electrometer be able to sense that?  (never had access to an electrometer, no idea what it can do in practice)

In theory, yes. In practice, it would come down to insulation resistances. The terminals and segment supports are made of "ambroid", which from what I've read is a heated and compressed form of natural Amber. This would have very high insulation resistance if clean. The electrometer quotes 170mm deflection with the scale at 1m distance and a 100V bias on the vane. So it would resolve much smaller than this, eg. 1.7mm/mV.

Without the correct thickness suspension, I can't say at the moment but I think it would have a pretty good try. Of course you also need to consider the quality insulation of the 3rd electrode between the other two and screen against other disturbing electric fields.

[Kleinstein]

For the part holding the mirror, I don't see a need to go extremely low weight. The weight still loads the fiber, but especially if the fiber is a bit thicker this should not be that much of an issue.

Aluminum could be an issue in making good contact. So at least the lower part, up to the contact should be more like a hard more noble metal to make easy contact and with a small diameter (to have low friction when rotating). So more like brass, nickle or bronce wire and than only aluminum for the upper part holding the mirror and hook, that no longer needs to be round. I would go as far as considering some 0.2 mm Manganin (a kind of bronze) wire up to and including the hook, and glue the mirror to it.

[coppercone2]

can you measure capacitance portion of input impedance?

[Gyro]

Thanks Chris for your contribution to keeping ancient technology alive.  I haven't played with my electrometer in quite some while. 
I did try to separate a single filament from some braided Dyneema fishing line with limited success (I couldn't separate long enough fibers).  Maybe I'll have to try different sizes and brands of line.  If I can get a filament thin enough, Dyneema seems to be almost perfect for the task.  Kevlar is a possible second choice.  Both will allow knots or loops for the ends whereas glass or quartz probably would need to be glued.  Maybe carbon fiber?
Any suggestions for making a new stem for the vane would be appreciated; presently I am using the thinnest aluminum hobby tube I could find and a tiny non magnetic stainless screw at the bottom to hold the vane I made.
Unfortunately I cannot see as well as I used to and my dexterity is also suffering from becoming older.

Thanks Gregg. Sorry the info is a case of better late than never, finding the stuff after acquiring mine has been a bit of a detective job. There are very few museum photos and most show battered and incomplete examples. The Smithsonian has quite a good archive of old Cambridge catalogues though, but as I indicated, nothing on the electrometer until 1906.

Thanks for the Dyneema reference, I'll take a look. Yes, kevlar and carbon fibres are possibilities, I don't know how fine they get. In the Carbon case you would need a permanent bias supply for the vane - this would need to be applied relative to both segment connections, which would be inconvenient in a high impedance situation. There is a terminal for this at the top of the center screw, initially I though the knob was just a mechanical stop, then I noticed the cross hole drilled near the top of the screw thread. With an insulating fibre, you only need to charge the vane periodically using the 3rd terminal with the rotating strip at the bottom.

My hands are getting shaky too, attaching the fibre to a couple of wire hooks with cyanocrylate adhesive proved a lot easier than trying to tie knots (not that this is an option with glass fibre) although I did need a magnifier. Presumably Cambridge would have used the same method, probably using shellac.

It sounds as if you had some success with the vane. Hopefully the photos give some idea of the original. As I mentioned, the 1906 model used a silvered paper vane (I don't know if it would have been Mercury amalgam silvered or chemically deposited from Silver Nitrate). I notice the 1908 version mentions an Aluminium vane, so thin foil would do the job. The rest of the assembly is just wire and shellac, the mirror is about 5mm x 0.5mm, front silvered (mine is a bit tarnished but usable). I saw somewhere that it is slightly concave, with a 2m focal length. Looking around, a small disc of reflective window film might make an acceptable substitute - it is available on ebay in 'sample' size.

[Gyro]

can you measure capacitance portion of input impedance?

Tricky, it's going to be very low, and the vane is electrically isolated. One of the documents refers to the electrostatic capacity being "about 80cm", I've no idea how that would translate to modern units, maybe it is an equivalent measure to the distance between two air spaced plates [Edit: or spheres] of standard area?

[Gyro]

For the part holding the mirror, I don't see a need to go extremely low weight. The weight still loads the fiber, but especially if the fiber is a bit thicker this should not be that much of an issue.

Aluminum could be an issue in making good contact. So at least the lower part, up to the contact should be more like a hard more noble metal to make easy contact and with a small diameter (to have low friction when rotating). So more like brass, nickle or bronce wire and than only aluminum for the upper part holding the mirror and hook, that no longer needs to be round. I would go as far as considering some 0.2 mm Manganin (a kind of bronze) wire up to and including the hook, and glue the mirror to it.

Just looking through my electrometer folder. Here are a couple of photos of the later (probably Cambridge instruments era) Aluminium vane assembly...

Edit: Looking at the images, the mirror must be thinner than 0.5mm.

[Gyro]

Kevlar to the rescue... well mostly.

I found some Kevlar sewing thread on ebay that, from the photo, had a nicely splayed end. When it arrived, I found that I was able to fairly easily untwist the thread and liberate individual fiber bundles and then fibers, which my micrometer tells me are 6um. Unlike cotton and silk thread, the individual Kevlar fibers are continuous (for more than the length I need anyway). I needed a stereo microscope and a few attempts to attach a single, nearly invisible, fiber to the hooks, the easiest way was to clamp the first hook, apply cyanoacrylate, and then wave a bundle of loose fibers until one of them caught. The second hook was harder as I was handling a single temperamental fiber.

I've attached a photo of the 40um glass fiber and the 6um Kevlar fiber assemblies for comparison. While the glass fiber is rigid enough to hold the two (30awg) hooks at 90' to each other, the Kevlar is completely 'flexible' but easily strong enough to support the mirror and vane assembly. After leaving overnight for any twist to relax, the mirror position is stable. The original Cambridge booklets indicates a mechanical time constant of 35 seconds and "practically dead beat" with the 6um Quartz thread, the Kevlar gives significantly greater than this, probably around 2-3 minutes, with no overshoot. The 40um glass fiber, by comparison, gives around 2 seconds, with significant oscillation.

I'm now able to see significant deflection when applying potentials across the quadrants (I haven't got my scale and light source set up yet). I've been unable to find much on the electrical characteristics of Kevlar fiber, other than it having a moisture content of around 3.5%. This probably makes it resistive in electrostatic terms, so it probably needs constant bias to be applied to the top terminal.

The search for true 6um quartz fiber continues, but in the background now.

[Gregg]

How did you fasten the Kevlar fiber to the hooks?  Tying it seems like an ordeal for an old guy like me.  Maybe a tiny bit of glue. 
It may be possible that they used the quartz filament partly for its torsional properties to always return to its resting state without a charge on the butterfly.
Do you know why the connecting wires between the segments are so long and coiled?
Now you are giving me incentive to take my electrometer out of its box and play with it.   
Thanks for keeping this old technology alive.

[coppercone2]

how about training a spider?

[Gregg]

Spider webs are too elastic for this application.  The gap in the segments where the butterfly shaped armature rotates is a little less than 6mm.  An elastic tether would possibly allow a short circuit.

[coppercone2]

how about this stuff? instead of kevlar. Same method should work for getting the strand as with the kevlar.

https://www.hnshenjiu.com/quartz-fiber-ultra-thin-cloth-product/

[Gyro]

How did you fasten the Kevlar fiber to the hooks?  Tying it seems like an ordeal for an old guy like me.  Maybe a tiny bit of glue.

Hi Gregg, I'm glad you spotted the update. Yes, glue is the way to go. I'm no spring chicken myself, but tying such fine thread would be out of the question. Cambridge supplied their quartz threads already fitted with hooks, probably attached with Shellac at the time. Yes, a tiny quantity of cyanoacrylate super glue (crazy glue in the US?) is the way to go - even the tiny Kevlar thread was immediately 'sucked' into the glue on contact. No problem there, it just requires a few seconds of patience before moving away. I believe cyanoacrylate reacts with some organic items, maybe wool, and possibly silk too. I remember my leg getting really hot once when I accidentally dropped some on my trousers!

It may be possible that they used the quartz filament partly for its torsional properties to always return to its resting state without a charge on the butterfly.

Yes, certainly. Quartz fiber is known for its torsional properties. The same thickness Kevlar has a fairly stable restoring force too, but significantly weaker.

Do you know why the connecting wires between the segments are so long and coiled?

Yes, it's to allow the moveable quadrant half to hinge out without needing to disconnect them. You can see a demonstration in the 4th image in Reply #11. The ends of the wires need to be tweaked so that the two coils don't contact each other, or the segments beneath. Replacing the vane isn't a frequent occurrence but it is handy to be able to open one half to judge when the height adjustment is right.

Now you are giving me incentive to take my electrometer out of its box and play with it.

Go for it, you're not going to get any younger! These scientific antiques deserve to be restored to working condition and preserved. If I can help with the vane construction, just let me know. By the time of your Cambridge Instruments one, they had moved over from 'conductive' paper to thin foil (as reply #23). it ought to be fairly easy to cut out with a scalpel.

Thanks for keeping this old technology alive.

Thank you. At one time I would have said that these things belonged in a museum, but in researching this one, I found the various museum sites around the world had virtually nothing to show, occasionally an image very battered looking specimen with a couple of lines of general text. Often there was just a catalogue number, and no picture. Such a missed opportunity. I used to love the London Science Museum as a kid - the display cases used to be full of magical old instruments. Last time I went, a lot of stuff was no longer there (presumably in storage) to make way for dumbed down 'science experiences'. Maybe we need a Science History Museum to go alongside the British Museum, Natural History Museum and the V&A. I think it's only individuals who can justify the 'no cost' man-hours to properly restore this stuff these days. Pulling information together from obscure references takes time and 'tunnel vision' on today's web too. Maybe they'll end up in a museum when they're truly old enough to be valued again. At least they are getting seen here. [/minor rant]

[Gyro]

Spider webs are too elastic for this application.  The gap in the segments where the butterfly shaped armature rotates is a little less than 6mm.  An elastic tether would possibly allow a short circuit.

Actually trained spiders used to be used quite a lot for delicate instrument manufacture, for optical scales etc. I did look at some webs in the garden, the support strands have no adhesive and are very strong (in the aramid fiber region*) but they were too weak for the suspension. I think the torsional restoring force would be even weaker than Kevlar.

^* Expensive silk body armor was a thing in the 19th century!

[Gyro]

how about this stuff? instead of kevlar. Same method should work for getting the strand as with the kevlar.

https://www.hnshenjiu.com/quartz-fiber-ultra-thin-cloth-product/

That looks to be in the ballpark! Taking account of the thickness of the weave, 0.03mm thickness might translate into sub 10um for the individual fibers.

I found another company... https://silicapro.com/quartz-fiber/ that makes 6um quartz fiber, and another one in the US that I can't find now. I haven't yet plucked up the courage to ask for a couple of feet of it. I'm not sure what customs would make of an apparently empty envelope!

[Gregg]

I made a few armatures by plotting the shape in CAD and gluing a print to a stiff aluminum sheet.  I then cut a forming shape much thicker than the part I wanted.  Then with the form on a flat surface I put thin aluminum foil over it and burnished the outline with my fingers along the edges of the form.  I cut the thin foil with an Exacto knife using the form to help keep it in place.  I discovered that the foil was a lot stiffer with all of the edges bent but trying to roll the edges to a smooth bead was not going to work.
After many attempts, the paper seems stiffer and more suited than the heavy duty kitchen type aluminum foil I tried.  I haven’t weighed them to see which is lighter.  I have a gut feeling that flatness may be more important than absolute lightness.
My thoughts; some of which may be wrong: Basically the device is an air dielectric capacitor with one plate free to rotate and indicate a charge differential. Or it could be described as a partial rotational capacitor motor. Nonlinearity of the dielectric gap could lead to poor results.  Proper balance is also critical to keep the plane of the armature parallel within the gaps of the segments.
edit: added pictures

[Gyro]

You have some impressive attempts there, I'm in awe of the rolling of the foil edges! I've had the (possibly) bright  idea of using my scanner as a 1:1 scale camera. I've attached a PDF, scanned at US letter size, so if you print it you should have an actual size template. The top sector is better than the bottom one, which has had some repair (actually both have, but the bottom one more). Don't worry about the white gaps, they are reflections from adhesive rather than actual gaps.

With regard to the foil. The mass will affect the time constant rather than sensitivity - the vane is very light. Thicker foil will take up more of the gap, which might affect sensitivity. Flatness is an issue, but maybe not as much as you think. My reasoning is that for every distortion, if part of the vane is higher, it is closer to the top of the quadrant, so experiences more attraction, but further from the bottom, so experiences correspondingly less. The effect is probably not linear but reduces the importance of absolute flatness.

For the material, the lighter the better (this reduces your out of balance concern), it should end up really fragile! If foil is giving you rigidity problems, I would be tempted to try thin paper, stiffened with cyanoacrylate (which really does strengthen it) and made slightly conductive with either something like a distant spray of the Aluminium/Zinc spray sold to protect car exhausts, or possibly a light rub of powdered Aluminium or Graphite. It really doesn't need very conductive at all, the vane 'load' impedance is pretty much infinite, a bit like the diaphragm of an electrostatic speaker but under much less electrostatic stress.

It's also worth remembering that these electrometers were typically used as null detectors, comparing a very high impedance unknown voltage with low impedance reference voltage from, say, a potentiometer. The information only gives a single data point typical value (150mm deflection at 1m for 100mV, with a 100V vane bias), it doesn't define sensitivity in mV per mm. I would expect the actual readout to become non-linear fairly quickly as the vane moves more into each quadrant. There would also be non-linearity with a rotating mirror reflecting onto a flat scale. For absolute measurements (one input grounded) the user would probably calibrate their individual instrument for scale reading at specific voltages, probably plotting a curve for intermediate values. This would be easy to do using a potentiometer reference. There is no data provided for the two other filament thicknesses or the phosphor bronze one (apart from approx 1/10th of the deflection of the 0.004mm fiber).

I hope these thoughts help anyway. PDF attached.


P.S. I tried to scan an image of the assembly on its side, so that you could see the position of the mirror etc. but the vane held it too far from the scanner platen.