Cambridge Instruments – Dolezalek Quadrant Electrometer

[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.