0.5uA detectable using mechanical meter in 1920 ! - Page 1
Forum section: electronics / projects

author: oz2cpu
author_rank: Frequent Contributor
timestamp: October 28, 2022, 08:59:28 am
content: I am a little bit impressed.. it was really possible to detect 0.5uA back in 1920see herehttps://youtu.be/pmxGHhG96q4ok it is not a lot of movement, but it is truly there :-)

author: Gyro
author_rank: Super Contributor
timestamp: October 28, 2022, 09:51:35 am
content: Not so impressive. Mirror galvanometers (with a scale at 1m distance) of the same period, or rather earlier, could easily detect currents in the nA region.

author: jonpaul
author_rank: Super Contributor
timestamp: October 28, 2022, 09:56:38 am
content: check Thomas Edison's "tasimeter" 1878, carbon button heat sensor....https://en.wikipedia.org/wiki/Tasimeterto measure minute temperature changes in heat emitted from the Sun's corona during the July 29 solar eclipse,Jon

author: Gyro
author_rank: Super Contributor
timestamp: October 28, 2022, 10:16:30 am
content: My treasured Robert W Paul Sensitive Galvanometer, pre1900 (he merged with Cambridge Instruments in 1919) and single digits nA, no problem.He was an interesting guy...https://en.wikipedia.org/wiki/Robert_W._PaulEdit: Sorry, I should have dusted it first!

author: oz2cpu
author_rank: Frequent Contributor
timestamp: October 28, 2022, 11:22:33 am
content: wow that is some nice stuff, with that mirror you could focus a light beam and achieve a wild amplification factor,

author: RoGeorge
author_rank: Super Contributor
timestamp: October 28, 2022, 12:50:43 pm
content: You may like this channel then:https://www.youtube.com/c/xofunkox_scientific_experimentsMagnetostriktion, MagnetostrictionFor the fun of it, calculate how much change in length per mm of the light spot displacement on the scale can such an arrangement sense (an 8m length light ray reflected by a mirror glued on a 0.4mm rod turned by the change in length to measure).Make a wild guess before calculating.

author: Gyro
author_rank: Super Contributor
timestamp: October 28, 2022, 01:05:15 pm
content: Quote from: oz2cpu on October 28, 2022, 11:22:33 am"Give me a  lever long enough and a fulcrum on which to place it, and I shall move the world."

author: oz2cpu
author_rank: Frequent Contributor
timestamp: October 28, 2022, 03:02:17 pm
content: cool video there george,but no info about voltage / current windings ? and lightsource and dot distance ? any other mirrors ?

author: Kleinstein
author_rank: Super Contributor
timestamp: October 28, 2022, 03:15:06 pm
content: There usually is only 1 mirror, though in theory a 2nd fixed mirror a bit higher / lower could give a reference to compensate movement of the light souce (usually incandecent lamp).Today one can use a laser pointer, but the mirror galvanometers were more a thing in times before lasers.The low mass light pointer helps to keep the response resonably fast, despite a weak restoring force from a rather thin and long ribbon.The meters with a weaker spring and somewhat larger coil can be quite sensitive, they just get slow. A good magnet (or just brute force, large size as with the Siemens&Halske one) can also help with sensitivity. I remember using a not that special mirror galvanometer with a thermopile, so something in the 1 mV full scale range.

author: Terry Bites
author_rank: Super Contributor
timestamp: October 28, 2022, 04:11:23 pm
content: here's a good read (if you like that kind of thing)

author: RoGeorge
author_rank: Super Contributor
timestamp: October 28, 2022, 05:16:13 pm
content: Quote from: oz2cpu on October 28, 2022, 03:02:17 pmExtra info in the video description.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: October 28, 2022, 08:30:46 pm
content: >Extra info in the video description.thanks but still dont make much sense to me :-)why would this design be any good for any type of measurement ?it also measure room temperature, is ultra critical mecanical, very hard to setup, not practical for anything at all.where would this be implemented into anything real ?

author: coppice
author_rank: Super Contributor
timestamp: October 28, 2022, 08:35:24 pm
content: You do realise that precision watches operating for a couple of days from a tiny coil spring are much older than 1920, don't you? If you can make a mechanism that low power you can use the same jewelled bearing and other techniques to make an ammeter with a full scale reading of a few microamps.

author: TimFox
author_rank: Super Contributor
timestamp: October 28, 2022, 08:51:41 pm
content: 50 years ago, when I was an undergraduate, my physics department used traditional equipment in lab courses to teach us what really happens, rather than what comes up on a digital display, so we had wall-mounted mirror galvanometers with a telescope and arc scale for sensitive measurements.A "ballistic galvanometer" application exploits the mechanical properties (inertia, damping, and natural frequency) of the coil and suspension, giving a maximum excursion proportional to the total charge through the instrument (integrating the current of a pulse).https://en.wikipedia.org/wiki/Ballistic_galvanometerI remember using a ballistic galvanometer to measure the hysteresis curve of an iron toroid ("Rowland ring")https://solitaryroad.com/c1049.htmlAn interesting use of such meters, probably not used in practice anywhere, is described in Vol. 18 of the famous "Radiation Laboratory" series: G E Valley & H Wallman, "Vacuum Tube Amplifiers", McGraw Hill 1948.Section 11-16 (pp 487 - 491) describes a mirror galvanometer where the reflection hits a 920 dual photodiode driving a single high-mu 6SF5 triode that applies feedback through a resistor network to the galvanometer coil.(In the practical example, the minimum full scale was 1 microampere, not too impressive.)The null occurs when the optical spot equally illuminates the two photocathodes.Errors in the simple triode amplifier are negligible for that application.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: October 28, 2022, 09:03:06 pm
content: HA TIM !!I think you just explained exactly how this weight scale workshttps://youtu.be/dwMl0CTE5j4

author: TimFox
author_rank: Super Contributor
timestamp: October 28, 2022, 09:29:43 pm
content: What I liked about the galvanometer circuit I found in the old Rad Lab book was that the galvanometer itself was in the null comparator for a feedback amplifier, and most of the errors from the light source and the rest of the circuit were made negligible by the huge gain (from galvanometer angle to differential voltage into the triode) due to optical leverage.

author: RoGeorge
author_rank: Super Contributor
timestamp: October 28, 2022, 10:26:03 pm
content: Quote from: oz2cpu on October 28, 2022, 08:30:46 pmThat video was about the incredible resolution in measuring a mechanical displacement achieved with only a pin, a mirror and a bit of ingenuity.QuoteThat device measures how much a piece of wire changes its length when a magnetic field is applied. The coil, voltage and current are there only to produce a magnetic field that can be turned on/off. When the magnetic field is applied, the piece of wire going through the core of the coil changes its length a little bit.How would you measure with how much the length of the wire changed?In that apparatus, the piece of wire sits on a 0.4mm diameter pin (rod), and when the wire changes its length, it rotates a little the 0.4mm pin. And on the pin is glued a little mirror that changes its angle because of that little rotation. Then the light deflected by the little rotating mirror travels to a screen that is placed 8 meters away (the spot of light seen on the right side of the video).That very simple mechanical only measurement can sense a change in length of only 25nm (caused by the magnetostriction effect). That's somewhere between the size of a virus (100nm) and a protein (10nm), or how much your hair grows in 7 seconds or so (if I didn't mess the calculations, the only reason I was challenging you to calculate the sensitivity of that instrument).