Electronics > Metrology

Passive galvanometer questionnaire.

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Vtile:
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.

Henrik_V:
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)
 

Conrad Hoffman:
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!

Vtile:
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.

Krytron:
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....

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