Passive galvanometer questionnaire. - Page 1
Forum section: electronics / general

author: Vtile
author_rank: Super Contributor
timestamp: November 05, 2017, 11:13:54 pm
content: Hello, I have had this on my mind some time and spend too many hours on google without any good answer to my interest.The most sensitive electromechanical galvanometers "in use" (that were used). Ie. I do have L&N resistance bridge which do have a galvanometer (I assume D'Arsonval type, but with not standard form factor) stated as 250 ohms (from memory I can check this if anyone is interested) that pretty much outperforms my analog and digital amplified meters (says something about my equipments).Bussoles, D'Arsonvals, Thompson mirror galvos.. What were the specs for the state of the art types of each.

author: Henrik_V
author_rank: Regular Contributor
timestamp: November 06, 2017, 12:28:46 am
content: My galvanometer claim 8nA/mm, passive, only the light bulb wants some power.and I think I read something in the pA range was possible ('amplified' after the mirror)

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: November 06, 2017, 12:44:03 am
content: I'll have to search out my info, but I've got a couple L&N "tophat" style galvos and they're incredibly sensitive. The image in the mirror was viewed with a special telescope and curved scale. I've got the scale, but not the scope that fits into it. They were offered in different resistances. There were also ballistic types. Remember that at null, with the same voltage on both sides, the current through even a low impedance is zero!

author: Vtile
author_rank: Super Contributor
timestamp: November 06, 2017, 12:54:07 am
content: Yes please do! These are fascinating and I kind of feel rootless on my general understanding of measuring technology and this haunts me a bit since every source always just states "with a sensitive galvanometer", but what is sensitive in that technology!!The null is magical indeed as the effective conductance goes to zero (sorry for the term I just did pull it out of my hat).@Henrik_V, Yes they must have reached at least to pico ampere region, since the galvo in my L&N bridge (which is only a cable resistance bridge!!) is (note that I have not verified this in any accurate way) somewhere on the low nA or high pico range (when the movement is fraction of needle width). :O ....And when the full rated external 200 Vdc is applied to the bridge instead of the internal 6Vdc you can go even further in bridge accuracy. ..but that doesn't relate to galvos directly.

author: Krytron
author_rank: Contributor
timestamp: November 06, 2017, 01:03:22 am
content: My galvo is made by B&H Instruments Inc in Fort Worth, TX. P/N BH950A FSN 6625-553-0421. I measured the movement at 8K ohms, 20.7 millivolt FSD at 2.6 microamp FSD. This unit uses a lighted beam from a 6 volt pilot lamp and is has a scale of +/- 15 which the light beam has a point that shines to the scale. It has a mechanical zero,and it can resolve 0.5 unit on the scale. Its large; 3 inches tall, 2 inches wide and 3 1/4 inches deep.Its never been used, I bought it surplus. I thought a long time ago, it would make a good indicator for a bridge application....

author: Vtile
author_rank: Super Contributor
timestamp: November 06, 2017, 01:27:45 am
content: That must be nice peace of instrument.Not too large though as the biggest galvanometers were so huge that the operator were inside the meter itself, while the meter were build top of the huge mass of stone work or preferably directly top of the bedrock

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: November 06, 2017, 04:07:48 pm
content: The Hoffman Technical Reference Library has gotten large enough that I can't find anything. I know somewhere I've got a table of the various L&N galvo data. For the moment, each of my top hats has a fold out tab on the bottom with data hand written in India ink:Sens. 0.46 uV/mmC.D.R.X. 52 ohmsPeriod 4.7 secResist 11.7 ohmsSens. 0.0037 uA/mmC.D.R.X. 350 ohmsPeriod 4.9 secResist 34 ohmsNo doubt the sensitivity is done at whatever the standard distance of the curved scale and telescope is, but regardless it's pretty darn sensitive for a passive instrument. Note that one is in uV and the other is in uA. No idea why, though they are from slightly different time periods. For a combination of old and new, imagine using one of these with a laser pointer and folding the path a couple times with mirrors (doubles the deflection). My old electrical measurement and early engineering books go into unbelievable detail on galvo calculations and bearing design. Think what you could build today with grade 52 neo magnets!

author: zhtoor
author_rank: Frequent Contributor
timestamp: November 06, 2017, 04:37:21 pm
content: Quote from: Conrad Hoffman on November 06, 2017, 04:07:48 pmHello Sir,this *should* exactly be the way "old" and "new" co-operate in all sphere's of life.to quote "the last samurai": "I BELONG TO THE WARRIOR IN WHOM THE OLD WAYS HAVE JOINED THE NEW"words of wisdom coming from a wise man.respect.

author: TimFox
author_rank: Super Contributor
timestamp: November 06, 2017, 04:40:07 pm
content: Note that the "C.D.R.X" parameter is the critical-damping external resistance. If the movement sees this resistance at its two terminals, the response of the "needle" will be critically-damped, without overshoot. With a higher terminal resistance, the movement will be over-damped and reach its final value more slowly. With a lower resistance, it will be under-damped, with overshoot, and will approach the final value as a damped oscillation.

author: Vtile
author_rank: Super Contributor
timestamp: November 06, 2017, 05:38:11 pm
content: Quote from: Conrad Hoffman on November 06, 2017, 04:07:48 pmA quick reply in coffee break.. Kind of like Ligo and Virgo's detectors.First thing that did came to my mind, although it is bretty bold to say that they would be galvos as they obviously are not, but Lord Thompson idea were pretty similar by using the light beam and distance as amplifier.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: November 06, 2017, 06:14:29 pm
content: OK, here's a question- if you built a galvo using our best stuff, lasers and rare earth magnets, could the design beat the best amplified detectors in terms of noise and resolution? No popcorn noise in a coil (I think). If they were getting those sensitivities way back then, we should be able to do several orders of magnitude better, unless there's some fundamental limit I'm forgetting, like the rotation of the earth or something.

author: Vtile
author_rank: Super Contributor
timestamp: November 06, 2017, 06:43:22 pm
content: Mmmh.. Superconduct levitating magnetic mirror. hmm..

author: zhtoor
author_rank: Frequent Contributor
timestamp: November 06, 2017, 06:45:05 pm
content: Quote from: Conrad Hoffman on November 06, 2017, 06:14:29 pmSir,a watt balance could be done (without beam folding using prisms as you suggested)and a laser length based amplification could be done.the concept is illustrated (especially starting @ 4:27) in the video:-regards.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 06, 2017, 07:39:36 pm
content: Modern rare earth magnet do not give that much advantage in field strength. The advantage is in size - an Alnico magnet can get about as strong, but at something like 100 times the weight. One advantage today are lasers, but the main advantage of a laser is its higher power / brightness. The resolution is more like set by the optics. So the mirror to be moved needs a certain size to avoid too much diffraction. The nice thing about a galvanometer is that it could in theory be all copper conductor - so not much trouble with thermal EMF. And of cause it is low power and thus little self heating. Resolution is likely more limited by the mechanical side, like relaxations in the springs / wires causing the zero to drift. Thus maybe a reason to not go pure copper.Modern extreme null meters are using a superconducting coil and squid to measure the field. The sensitivity is really crazy: enough to compare the mV range voltage from Josphson junctions down to a relative difference in the 10 to the -19 range, so way below the aV. However input impedance is also rather low for a "voltmeter" - something in the micro Henries, no Ohms. It is like waiting until you get a measurable current in the pA's range.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: November 06, 2017, 08:20:07 pm
content: Well, that explains the humongous alnico magnets used in my various old galvanometers. BTW, thanks, as that's a very useful piece of info for some other work I do.On a side note, I also have an old L&N projection unit that looks like a small grey bathtub that fell over on it's face- the ugliest piece of test equipment I own. How they got from those beautiful old top hat units, to that, is beyond my comprehension. Even the simple wood box type were nicer.It never occurred to me what they use to compare the JJ arrays. Figured it wasn't a Fluke 845! A superconducting coil would be cool to experiment with (pun!) but is beyond my modest basement lab.