History question: How were the first voltmeters/ammeters/ohmmeters calibrated? - Page 1
Forum section: electronics / general

author: 4x1md
author_rank: Regular Contributor
timestamp: March 06, 2025, 07:55:11 pm
content: Suppose, you live in late 1800s or early 1900s and want to build a voltmeter/ammeter/ohmmeter. Or you want to significantly improve the precision of an existing meter. How would you calibrate it if you don't have any high-precision resistances and voltage/current sources?How was the measurement equipment calibrated in those days? What was used as a precision voltage/current sources or as precision resistances?

author: tggzzz
author_rank: Super Contributor
timestamp: March 06, 2025, 08:20:22 pm
content: Everything was relative to other measurements on the same equipment.The next step was, of course, relative measurements to an agreed singlestablecentral piece of equipment.More interestingly, the problem of improving equipment and measurements remains unchanged. If you are building the world's fastest scope, what scope and waveform do you use to test it?

author: jfet
author_rank: Contributor
timestamp: March 06, 2025, 08:52:18 pm
content: probably started with Georg Ohm. Do a search...

author: Xena E
author_rank: Frequent Contributor
timestamp: March 06, 2025, 09:27:26 pm
content: Without a starting point reference you wouldn't.The Weston cell was invented in 1893 though, so I'm going to say the Weston cell.🤷🏻‍♀️

author: themadhippy
author_rank: Super Contributor
timestamp: March 06, 2025, 09:32:13 pm
content: For the Amps range depositing silver

author: bdunham7
author_rank: Super Contributor
timestamp: March 06, 2025, 09:35:29 pm
content: Quote from: 4x1md on March 06, 2025, 07:55:11 pmThis has been an issue since electricity was discovered. Typically an association of scientists got together and agreed on certain 'artifact' standards that could be copied and reproduced from the provided specifications. Many of these involved toxic chemicals--at one point the Ohm was defined as a mercury column and the volt was defined by a battery with zinc and mercury. So for your question, you would either build the appropriate reference devices or travel to a place that had them to calibrate your devices.

author: TimFox
author_rank: Super Contributor
timestamp: March 06, 2025, 09:39:12 pm
content: Following the physics, the Ampere can be measured using the law for force between wires, winding appropriate coils in a “current balance” (q.v.)The force is measured in a weighing-like balance against gravity with calibrated weights.

author: tszaboo
author_rank: Super Contributor
timestamp: March 06, 2025, 09:44:59 pm
content: It wasn't. It was 1 amount of current, just because.

author: Gyro
author_rank: Super Contributor
timestamp: March 06, 2025, 09:51:09 pm
content: For a surprisingly long time, the 'International Ohm' was an artifact definition, defined as the resistance of a column of Mercury of fixed diameter, length and temperature. It was only in 1948 that the 'Absolute Ohm' was adopted. Something to be aware of when playing with old resistance standards.

author: TimFox
author_rank: Super Contributor
timestamp: March 06, 2025, 09:59:41 pm
content: Quote from: tszaboo on March 06, 2025, 09:44:59 pmBecause why?

author: Gyro
author_rank: Super Contributor
timestamp: March 06, 2025, 10:01:10 pm
content: Quote from: jfet on March 06, 2025, 08:52:18 pmNope, Charles Bright and Latimer Clark, 1861.

author: 4x1md
author_rank: Regular Contributor
timestamp: March 06, 2025, 10:18:28 pm
content: Quote from: Xena E on March 06, 2025, 09:27:26 pmWow! I've never heard about Weston cell before.

author: Gyro
author_rank: Super Contributor
timestamp: March 06, 2025, 10:23:19 pm
content: Quote from: 4x1md on March 06, 2025, 10:18:28 pmThere are still a few owners around here if you do a forum search.

author: edpalmer42
author_rank: Super Contributor
timestamp: March 06, 2025, 10:24:29 pm
content: This document has some info on the history of voltage standards i.e. the Weston Standard Cell and its predecessors.https://www.ase-museoedelpro.org/Museo_Edelpro/Catalogo/Overview/docs/NBS84.pdf

author: tszaboo
author_rank: Super Contributor
timestamp: March 06, 2025, 10:29:49 pm
content: Quote from: TimFox on March 06, 2025, 09:59:41 pmBecause you had a stone, that was so-so heavy and it became a unit of measurement "1 stone".Some defined current based on electrolysis, some defined it based on it's magnetic properties. With some random magnet's that they had lying around.You actually need to define force and distance before you could define current, in a semi modern way. Which meant, that you needed a 1m long "stick" that you needed to bring other about 1m long sticks" to calibrate to.If you think they could standardize current in 1800, then I have a bridge to sell you.

author: TimFox
author_rank: Super Contributor
timestamp: March 06, 2025, 10:42:45 pm
content: Quote from: tszaboo on March 06, 2025, 10:29:49 pmWhen the metric system was first adoptedin 1799, there were practical standards for length (sticks) and weights. Measuring current with a current balance requires known dimensions (length) and force (weight). This was before real progress in electricity, such as Ampere’s Force Law of 1825, but enabled them to measure stuff against known quantities.

author: David Hess
author_rank: Super Contributor
timestamp: March 07, 2025, 12:49:26 am
content: Quote from: tggzzz on March 06, 2025, 08:20:22 pmThree sampling oscilloscopes can calibrate themselves by measuring their strobe kickouts. Then they can be used to calibrate slower pulse generators to calibrate slower oscilloscopes.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 07, 2025, 04:10:39 am
content: Part of the answer to this comes from recognizing the precision that was desired. We talk fairly casually of eight digits of precision now. But 200 years ago three was a notable accomplishment. Even today much can be done with only two digits of accuracy, and some things can be accomplished with one.

author: Smokey
author_rank: Super Contributor
timestamp: March 07, 2025, 06:44:26 am
content: Quote from: Gyro on March 06, 2025, 10:23:19 pmjust a PSA... When I found out about Weston Cells from here I thought it was super cool and a good idea to buy one off ebay.... Which was a terrible idea. It's full of super nasty stuff and the fact that it went through the USPS means it's probably so shaken up it will never work right again. Plus you need an electrometer to measure it.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 07, 2025, 07:47:17 am
content: Measuring current from the deposition of silver, the mercury column and to a lesser degree the Weston cell are the old method for highest accuracy and metrology labs. For more practical use there were other, more practical references, like precise wire wound resistors, calibrated analog meters and KV divider (or similar) potentiometers (voltmeters). The mercury zink battery also worked reasonable well. As a ready made cell they are not that messy and were used up to the 1990s as coin cells.There was often less fuss about formal calibrations. When relevant, it was common in science to check the meters to a ref. standard like a weston cell as part of the experiment.

author: tggzzz
author_rank: Super Contributor
timestamp: March 07, 2025, 08:06:47 am
content: Quote from: David Hess on March 07, 2025, 12:49:26 amYup, but that requires skill and imagination, not merely brute force application of superior technology.Harmon dividers are similarly "magical".

author: tszaboo
author_rank: Super Contributor
timestamp: March 07, 2025, 09:45:00 am
content: Quote from: TimFox on March 06, 2025, 10:42:45 pmAdopted. You know what this meant? There were some academics in France decided that everyone should use the same system as theirs. It took them 60 years until even France could agree to actually use that system.It's easy to forget that it could take weeks to travel from one side of Europe to the other back then. Nobody could afford to do that just to place the two sticks next to each other and see if they are the same length. There wasn't a quick or easy way to distribute accuracy.

author: EC8010
author_rank: Frequent Contributor
timestamp: March 07, 2025, 10:49:30 am
content: Quote from: tszaboo on March 07, 2025, 09:45:00 amBut it went much further than just deciding on some new units. The real step forward was that the metric system was designed to be a unified system that didn't need scaling factors in physical equations (unlike the Imperial system). That's therealbeauty of metric.I remember doing electrical experiments at school with great big potentiometers and metre rules that would be calibrated by a Weston cell. And I did the current balance experiment with tiny little weights that had to be handled with tweezers. 2 x 10-7N/m stuck in my mind for ever after.

author: tggzzz
author_rank: Super Contributor
timestamp: March 07, 2025, 12:53:22 pm
content: Quote from: EC8010 on March 07, 2025, 10:49:30 amNot quite. There are still "many scaling factors in physical equations", e.g. µ0ee, c, etc.There is not, however, the need to do arithmetic in base 2, 4, 8, 12, 14, 16, 20, 22, 112 and others.QuoteDon't remember doing the current balance, but certainly did use resistance wire plus NiFe cell plus Weston Standard cell to measure voltage to 0.1% with 2% meters.

author: 5U4GB
author_rank: Super Contributor
timestamp: March 07, 2025, 02:24:51 pm
content: Quote from: jfet on March 06, 2025, 08:52:18 pmSo you connected electrodes to him and whatever you measured was 1 ohm?

author: TimFox
author_rank: Super Contributor
timestamp: March 07, 2025, 04:09:57 pm
content: Quote from: tszaboo on March 07, 2025, 09:45:00 amYou underestimate the capabilities of craftsmen and artisans at the turn of the 19th century. Their capabilities of using measuring devices (dividers, calipers, scales, straightedges, etc.) developed since Ancient Egypt in architecture and the Romans in horological, including later construction of precision astronomical instruments up to the time of Newton, is what enabled the academics to develop their new system to replace myriad previous standards. Watchmaking was already a good, if expensive, technology. Goldsmiths did precision weighing back before Imperial Rome. Winding and using a current balance was within 19th Century capabilities, once Ampere developed his equations for the force between wires.The history of technology in Classical times and the Middle Ages is fascinating, even though the early engineers did not write much documentation.

author: donlisms
author_rank: Frequent Contributor
timestamp: March 07, 2025, 08:21:17 pm
content: Quote from: 4x1md on March 06, 2025, 07:55:11 pmThe mechanism then, as now, is to calibrate against a known standard. So the issue was getting "known standards."Measuring devices, per se, weren't the thing yet. The chicken-or-egg problem ultimately meant that they needed the resistance and voltage or current standards, and a way to make comparisons, long before those things got combined into a single convenient machine.QuoteThe most important measurement equipment was the resistance bridge, which does not need calibration, and the potentiometer (the "real" kind, not the modern use of the term), which is essentially calibrated against a voltage standard, quite easily, whenever you get the urge. The display was a galvanometer, and you had it when the needle said "0". With null techniques, several other issues disappear.Defining standards was a long and slow process. It was international, and involved a lot of different techniques and even different ideologies, e.g. resistance was an easily-fixed and managed quantity, but do we use techniques to define current and compute voltage, or define voltage and compute current? And should we just wind a coil and say "This is an ohm," or should we compute the inductance of a coil from its physical properties, and using measurements of time, establish its reactance and define the ohm that way?In any case, the precision of absolute standards lagged behind the precision of comparison techniques by a wide margin. Once resistance standards were fairly firmly established by physical techniques, the ohm was transferred to coils, which were secondary standards being used like primary standards most of the time.Fortunately, all this interesting work was done at a time before people were all so money-minded with regards to scientific literature, so many of the scanned papers are readily available in archives online for free. I started with National Bureau of Standards documents, and kept fanning out through the references given in the texts. There's a lot.

author: tggzzz
author_rank: Super Contributor
timestamp: March 07, 2025, 08:33:12 pm
content: For many purposes you don't need a common standard. For fundamental research it is often sufficient to demonstrate that if X is doubled then Y is doubled/halved/quadrupled etc.

author: strannik2039
author_rank: Contributor
timestamp: March 08, 2025, 01:15:32 pm
content: London International Conference on Electrical Units and Standards 12-22 October 1908.The issue of establishing one system of electrical measures was first the subject of international discussion at the first, in Paris 1881, congress of electricians, moreover, this congress adopted the system of practical electromagnetic units recommended in the sixties (1861) by the British Association for the Development of Sciences : ohm, ampere, volt, etc.When switching to practical use of this system, it was necessary to attend to the device of standards. Since the standard oma proposed in 1863 by the "British Association" in the form of a coil of plane-silver wire turned out to be almost 1% less than the theoretical oma by the time of the Congress of 1881, the International Commission elected by this Congress (1882-1884) decided to temporarily consider the legitimate ohm (Ohm legal) not the resistance of platinum-silver wire of certain sizes, but the resistance of a mercury column 106 cm long at 0 ° and with the same cross section of 1 square mm.Subsequent more thorough measurements found that the true ohm meets (with an accuracy of 0.01%) the resistance of a longer mercury column, namely 106.3 cm; such a standard, in contrast to the temporary accepted "legal," received the name of the international om. Thus, the foundation was laid for the establishment of international standards for electrical units.In 1893 in Chicago, during the World Exhibition, at the International Congress of Electrical Engineers, representatives of several governments proposed to legalize a new system of international electrical units, not much different from the absolute, but very accurately expressed by means of a mercury ohm, a normal Latimer-Clark galvanic element and an exemplary silver voltameter,arranged according to special specifications. This proposal was unanimously passed by Congress in 1893, and thus the question of establishing general electrical measures seemed to be resolved. Many states have recognized it possible to legalize these electric units:America, England, Canada - in 1894, France - in 1896, Germany - in 1898 and Austria - in 1900.Unfortunately, however, when carefully comparing these laws on electric units, serious disagreements between their main points were soon noticed. While according to the laws of America, England and France, volts, according to the decree of the Chicago Congress, represented 1000/1434 of the electromotive force of a normal Latimer-Clark electromotive cell, according to the German law, volts expressed by ohm and ampere could be equivalent to only 1000/1433 of electromotive force. And such a difference of 0.1% for a unit of electromotive force, which caused an even greater difference - 0.2% for a unit of energy - was generally surprised by the laws of neighboring states (France, England and Germany).This misunderstanding, which occurred due to the insufficiently strict and unclear wording of the decrees of the Chicago Congress, inevitably required the correction of these decrees, even more additional research. At the International Electric Congress at St. Louis in 1904, after an official report on the above disagreements in the existing laws on electric units, it was decided to petition interested governments to establish a permanent international commission to establish uniformity in electric units and standards.In the fall of 1905, the President of the German State Physics-Technical Institute, professor. F. Kohlrausch, invited representatives of England, France, Austria, Belgium and America to come to Berlin for a private meeting on electrical units and standards.At this Meeting, under the chairmanship of Professor Maskar, a work plan was prepared for the next International Conference, then convened by the British Government in October 1908!.

author: TimFox
author_rank: Super Contributor
timestamp: March 08, 2025, 03:45:37 pm
content: As discussed above, there are two important considerations for units definitions:1. For scientific purposes, there are absolute definitions for use in physics equations as electromagnetic theory advanced.2. For commercial purposes, as electrical technology advanced and measurement was required for consistent payment and taxation, as well as production, legal definitions, capable of reproduction in the field and multiple countries, became necessary.For scientific purposes, experimental measurements continued to evaluate the practical legal values in terms of the ideal absolute values.

author: strannik2039
author_rank: Contributor
timestamp: March 09, 2025, 10:27:23 am
content: Electrical unitsDuring most of the 19th century. electrical units were still in the making. The work of Gauss (1832) and W. Weber (1851) to create an absolute system of units paved the way for theoretical definitions of electric units, but in practice their values were determined by one or another material construction proposed by any of the scientists. During this period, not primary samples were created based on the maximum approximation to the theoretical definitions of units, but on the contrary, the values of units were determined by certain arbitrarily established samples. In this state of affairs, the main task was to create such samples that would be primarily practically convenient and more or less easily reproducible. In 1838, Lenz proposed to adopt the electrical resistance of copper wire No. 11 with a length of 1 feet at a temperature of 15 ° R as a unit of resistance. In his further works, we meet the values of all three basic units proposed by him. In establishing the values of resistance units and current strength, Lenz proceeded from the parameters of his own devices ("my agometer *," "my multiplier"), but then he gave these values an objective character, referring them to the strictly defined characteristics of some easily reproducible materials and structures. As for the unit of electromotive force, he correctly defined it (based on Ohm's law, which was not yet fully recognized then) as a derivative of the units of resistance and current strength. "The unit of resistance," he wrote in 1842, "of all resistances, except as otherwise stated, is one turn of my Neusilber wire agometer. It corresponds to 6.358 feet of 0.0336 eng copper wire at 15 °. The unit of current is the current deflecting the arrow of my multiplier by 1 °. The electric action of this current unit according to the above is 41.16 cubic meters. centimeters of explosive gas, at 760 mm (temperature 0 °) pressure and at 0 °, per hour... The unit of electromotive force is the force that causes a current of 1 unit at a unit resistance. The electromotive force of one of my Daniel elements in these units... = 47.16. " If we take the electromotive force of the Daniel element equal to 1.1 V, then it is obvious that the unit of the electromotive force of Lenz was approximately 0.023 V.In 1848, Jacobi made and sent to physicists of his time as a standard unit of resistance a copper wire 25 feet long, weighing 345 grains (22.4 g) and a diameter of 0.67 mm; this unit, which amounted to approximately 0.636 Ohm, gained significant distribution in Europe, however, giving way in the 60s to the units of resistance of Siemens and the British Association for Scientific Progress. In addition, Jacobi proposed to take as a unit the strength of an electric current emitting 1 mg of explosive gas in 1 s; this unit was practically convenient and easily reproducible.It should be noted that the scattered efforts of scientists from different countries did not lead to uniform generally recognized results. In the third quarter of the XIX century. in various countries and by various scientists, about 15 units of electrical resistance, 8 units of electromotive force and 5 units of current were used. In addition, the insufficient state of the art and the lack of uniform specifications for manufacture led to the fact that even real samples of units made in the same structural form (Daniel element, Jacobi resistance unit, etc.) more or less significantly diverged in their values.The situation improved only when the issue of electric units became the subject of collective consideration. A great and productive work was carried out by the Committee on Electrical Standards in England (1861-1870). However, an international agreement was already required. The fundamental improvement is associated in its initial stage with the activities of the first International Congress of Electricians (1881), which primarily adopted two systems of absolute electric units - electromagnetic and electrostatic - and gave clear definitions of electric units in absolute measure. However, given their practical inconvenience (too large and too small values), Congress also established an absolute practical system, the units of which were obtained from the previous ones by multiplying their values by 10, in connection with which well-known units appeared: volt, ohm, ampere, pendant, farada and, somewhat later, joule, watt, quadrant (henry). Being based on the absolute Gauss-Weber system of units, all the proposed units were based on the metric system of measures and prefixes of the same system were used to form fractional and multiple electrical units.At the Chicago Congress of 1893, it was decided to base electrical measurements on standards of practical electrical units, rather than their theoretical definitions, that is, to move from an absolute practical system of units to the so-called international. At the 1908 International Conference in London, the numerical values of international units were clarified and accurate specifications were developed for their real reproduction. The following definitions of practical international units were adopted: ohm - the resistance possessed by a column of mercury of constant cross section with a length of 106.300 cm and a mass of 14.4521 g when passing an unchanged electric current at the temperature of melting ice; ampere - the force of an unchanged current, which, passing through an aqueous solution of silver nitrate, releases 0.00111800 g of silver in 1 s;volts - voltage that in a conductor with a resistance of 1 international ohm produces a current of 1 international ampere. Specifically, the voltage unit should be reproduced using a normal element; if the Chicago Congress adopted the Latimer Clark element equal to 1.434 international volts, then the London Conference preferred the normal Weston element = 1.0183 international volts.My posts are a small introduction to the formation of modern units of electrical measurements. But this story is huge and fascinating! What are the collections of works by Ampere, Faraday.

author: TimFox
author_rank: Super Contributor
timestamp: March 09, 2025, 03:00:51 pm
content: Yes, academic and practical experts knew what they were doing before some skeptics on this forum were born.(They built good bridges, too, in the 19th Century.)

author: strannik2039
author_rank: Contributor
timestamp: March 09, 2025, 05:17:49 pm
content: As I said, this is an extensive (huge) topic. And to be honest, there are gaps in it. I could not find answers to the questions:How were the units of electrical quantities specified (derived)?how did the errors of units of electrical quantities decrease ( 1%, 0.1%, 0.01%, .... )? Now, reading collections of works, everything seems logical and natural, but how was it all reproduced at that time, how did it come to that? Indeed, already in 19xx, potentiometers (compensators) powered by zinc batteries had an error of 0.01% - 0.001%!How did Kirchhoff get his laws out?And many other questions. And there are no answers. But if we return to the question in the title of the topic, then I think we need to start with such literature:methods of measuring electrical resistance 1912Electrical instruments and testing 1907A comparison of the Kelvin double bridge and the potentiometer methods of measuring low resistancesThe Galvanic Circuit Investigated Mathemat (Ohm, Georg Simon, 1787-1854)A determination of the international ampere in absolute measureNext, look for literature using links from books. And so dig deeper and deeper.It's a very deep rabbit hole.

author: johansen
author_rank: Super Contributor
timestamp: March 09, 2025, 08:39:46 pm
content: I suspect there is a lot that is simply lost to history.Consider how the inch defaulted to the value Johansson interpolated between the two competing standards at close to half way between the two at 25.4mm, he then starts shipping gauge blocks worldwide, a set of which cost a years wages.Grinding his own gauge blocks in a mostly temperature controlled shop, and making them to an accuracy of a couple parts per million, 120 years ago.So consider the expense of simply fabricating a glass tube with a constant 1mm square cross section.. and shipping it half way around the world for someone else to measure and compare their ohm standard with.... there's a lot that was never written down.I have actually spent an hour waiting for a glass of ice water to stabilize at 0.0c. none of this is easy. you can end up with 4C water on the bottom of the glass while ice is colder than liquid water on top... thinking your thermocouples are accurate... nope!

author: TimFox
author_rank: Super Contributor
timestamp: March 09, 2025, 09:06:53 pm
content: As in the posts above, much of the 19th Century work on electrical standards was well documented, since it was published by governmental and non-governmental international organizations.Of course, craftsmanship in precision fabrication was well advanced by then, as in Johansson's work on gauge blocks, following horology and astronomical instruments in the preceding centuries.Besides the difficulties of fabricating tubes, etc., there was also the difficulty in shipping them across oceans without disturbing the guts.So much of the technology of the Roman Empire had to be re-discovered by archaeologists, since Roman engineers did not publish much of their processes (compared with philosophers and historians of that era).

author: strannik2039
author_rank: Contributor
timestamp: March 10, 2025, 05:17:52 pm
content: Franz Ulrich Maria Theodor Aepinus. Theory of electricity and magnetismIn the middle of the XVIII century. The St. Petersburg Academy of Sciences numbered barely a quarter of a century of its existence. Despite such a young age, and maybe that is why, the only scientific institution in the country at that time posed and solved the most pressing problems of our time, among which one of the most prominent places was occupied by the issues of electricity.Being much younger than the Royal Society of London or the Paris Academy of Sciences, the St. Petersburg Academy did not show the routine and inertia from which the named scientific corporations were not free. Characteristic was the attitude of prominent English and French scientists to the then new theory about the electrical nature of thunderstorms. Veniamin Franklin writes in his memoirs that when his first letter was announced at a meeting of the Royal Society (it was addressed to a member of the Royal Society, Collinson), containing a description of the experiments and conclusions that the researcher came to, the latter was laughed at, and Collinson was refused to print the memoir of his correspondent in Philosophical Transactions. The same fate befell another member of the Royal Mitchel Society, to whom Franklin also sent his work. He writes: "I sent one article about the similarity of lightning with electricity to my friend, also a member of the Royal Society, Mitchell. In his response letter, he told me that the article had been read, but caused the experts to laugh "*). It was even worse in France. Translated into French at the initiative of the famous naturalist Buffon, Franklin's composition caused a storm of indignation. New ideas were so alien to ingrained ideas, the prominent French physicist Academician Nollet was then a prominent exponent of which that the latter considered, as Franklin writes, that there is no American Franklin in the world that all this was staged by Nollet's enemies in order to shake the system of his views. And even after Nollet was convinced that Franklin was a real researcher and was constantly conducting his research, the French scientist nevertheless issued a number of open letters to Franklin, "refuting" his views.All this happened in the 50s of the 18th century. This narrative shows the attitude of the then elite of sciences to the unknown new. Perhaps that is why some of the works of the then pioneers were not preserved.

author: strannik2039
author_rank: Contributor
timestamp: March 12, 2025, 05:25:16 pm
content: As a result of more than a century of discussion, the scientific community of all countries of the world has concluded that the most appropriate is the International System of Units SI.This system arose as a kind of unification of several systems of units of measurement, used at different times in science and technology, while significant work was carried out to unify previously introduced units, to the greatest extent this applies to units of measurement of electric field values.Let us follow in more detail the sequence of scientific discoveries that predetermined the creation of the system of electrical units. The fine details that accompanied these experiments have already been lost and are difficult to access at the present time. But scientific research, especially in the field of natural sciences, has a peculiar "logical compulsion", and therefore we are able to recreate, with a greater or lesser degree of reliability, the actions of the testers of the distant past.In 1745 Musschenbroek Pieter van(1692-1761), trying to “electrify” water, discovers the ability of a glass beaker with conducting plates to concentrate an electric charge - this is how the first electric capacitor appeared.1785 - year of establishment Charles-Augustin de Coulomb (1736-1806) the fundamental law of electrical interaction, 1789 - the law of interaction of permanent magnets. The terms "quantity of electricity", "magnetic mass", electric and magnetic dipoles enter scientific circulation. Torsion balances used Coulomb for measuring the forces of interaction of charges, are widely known It should only be added that he had previously conducted special studies to determine the rigidity of the suspension thread. The dependence he found on the diameter of the thread and its length allowed him to subsequently introduce an absolute unit of electricity.The establishment of the inverse square law did not cause any difficulties, especially since guesses about it had been made long before Coulomb in the works Daniel Bernoulli (1700-1782), John Boynton Priestley (1733-1804), Henry Cavendish(1731-1810). Determining the dependence of force on the magnitude of charges turned out to be a more difficult task. Before Coulomb the charge was not expressed numerically, it was only known that charges could be “large” or “smaller”.By placing an indefinite charge on a conducting ball fixed to the beam of a torsion balance, Coulomb touched it with exactly the same test ball, located at the end of the insulating rod. It was natural to assume that the initial charge would be distributed equally between them. A new touch with an uncharged ball would reduce the charge by another half. Thus, the unknown charge was divided into parts in the ratio 1/2:1/4:1/8, etc. Now it was possible to determine the dependence of the force on the magnitude of the charge. The guess was confirmed Coulomb, that the force is determined by the product q₁q₂ . The angle of twist of the suspension thread determined the numerical value of this force.In 1791, J. I. Galvani (1737-1798) discovered "animal electricity". A. Volta (1745-1827) objected to him, believing that the appearance of an electric impulse was caused by the use of dissimilar metals as electrodes. Volta confirmed this statement with a direct experiment: when dissimilar metals came into contact, their electrification with opposite charges was observed in the contact area, which was noted by a capacitor electroscope (a metal plate with a varnished upper surface was attached to the head of the electroscope, on which another metal plate with an insulating handle was located. If the lower plate was charged by grounding the upper one, and then, having broken the contact, the upper plate was lifted, then the decrease in the capacity of the lower plate would lead to an increase in its voltage, and, consequently, to a greater divergence of the electroscope leaves). Thus, he discovered the "electrification of dissimilar metals" when they came into contact - the contact potential difference (1795). The effect was small, so Volta tried to assemble a battery by making a chain of different metals, however, the intermediate potential difference always disappeared, the resulting value was only the value corresponding to the contact of the extreme metals.Reproducing Galvani's experiments, Volta replaced the living tissue between the electrodes with a cloth pad soaked in a salt solution. The potential difference between the electrodes increased hundreds of times. By setting up a column of identical contact pairs of metals, oriented in the same way and separated by wet discs of cloth, Volta obtained a voltage proportional to the number of pairs used. Thus, in 1799, the "voltaic pile" appeared - a source of direct current with an adjustable electromotive force. The name of the device reflected its geometric shape: the circles of metals and cloth could only be placed vertically; when placed horizontally, the solution flowed out of the cloth, causing a short circuit of the entire structure. The invention of a permanently operating source of current opened the age of electricity.The action of the first galvanic cells was unstable, the electromotive force quickly decreased with time. N. Gautreau (1753-1803) showed (1802) that the reason for this was the polarization of the element, i.e. the appearance of gas bubbles on the electrodes as a result of reduction-oxidation chemical processes. Only in 1829 Antoine Cesar Becquerel (1788-1848, grandfather of Henri Becquerel) established that manganese peroxide powder could serve as a depolarizer, and only in 1836 J. Daniel (1790-1845) constructed the first stable current source with separate solutions of sulfuric acid and copper sulfate. That is why in his 1826 experiments on the resistance of metals, G. Ohm (1787-1854) used not galvanic cells, but a thermocouple, proposed by T. Seebeck (1770-1831) in 1821. I. Schweigger (1779-1857) had just constructed the first galvanometer, which he called a multiplier (1820), but Ohm was unfamiliar with this invention, and therefore estimated the current strength with the help of a magnetic needle, located, like the wire, in the plane of the magnetic meridian. Under the action of the current, the needle deflected and again returned to its previous position by turning the head of the suspension thread on which the needle was located. Relative values of the current strength were determined by the angle of rotation of the control device.By varying the parameters of the installation, Ohm became convinced that the current in the conductor decreases with the increase of the length of the wire, with the decrease of its cross-sectional area, changes with the change of the substance, having established a series of substances in the order of increasing resistance. The terms "resistance", "voltage", "current" are introduced into science. The following year, Ohm, using an electroscope, makes a comparative assessment of the electromotive forces of various galvanic current sources.In 1820, a series of scientific discoveries followed, associated with the names of G.-H. Oersted (1777-1851), A. Ampere (1775-1836), J. Biot (1774-1862), F. Savart (1791-1841), P. Laplace (1749-1827), who established the laws of the magnetic action of current.Just four months after Oersted's June publication, Biot and Savart reported on their experiments.A small magnetic needle was suspended horizontally near a long vertical wire. The effect of the earth's magnetic field was eliminated by means of an additional magnet. When current was passed through the wire, the needle was always positioned so that its length was perpendicular to the perpendicular dropped from the center of the needle to the wire. If the needle was disturbed from its equilibrium position, it began to oscillate around this position. Following Ampere, Biot and Savart decomposed the action of the entire wire into the sum of actions emanating from the individual elements of the conductor. The scientists assumed that each of these elementary actions was directed in the same way as the general action of the entire conductor. This was not an obvious assumption, but it was justified. Biot and Savart also assumed that the elementary action depended on the distance of the element from the needle and on the angle formed by this distance with the current element. Laplace determined the dependence of this force on distance.The constant c, called the electrodynamic constant, remains numerically unknown. Since the unit of current strength has not yet been determined (all measurements are relative), the constant c can be set equal to one, thereby introducing a magnetic unit of current strength, determined by the Biot and Savart formula.The problem of comparing the magnetic field of the current with some standard field arises. The natural magnetic field of the Earth was chosen as such an absolute field, careful measurements of which were carried out in 1832 with the direct participation of Gauss and Weber.The decade following Oersted's discovery was filled with the work of many scientists on the construction of electrical measuring instruments and magnetometers. In 1820, Ampere discovered "magnetic screws and spirals" - solenoids - which produced the same effects as magnets. Schweigger used Ampere's discovery to construct a sensitive device, which he called a multiplier, in which a magnetic needle suspended on a thread was placed inside a coil of copper wire with a silk winding. Ampere, by fixing two magnetic needles motionless towards each other, creates an astatic (insensitive to the earth's magnetic field) system; In 1825, L. Nobili (1784-1835), having combined the astatic system of Ampere with the Schweigger multiplier, constructed the first sensitive galvanometer; the following year, I. Poggendorf (1796-1877), having attached a mirror to the suspension thread, introduced the mirror method of counting the rotation of the deflecting system.In 1831, W. Weber (1804-1891) designed sine and tangent galvanometers (compass).Using a tangent compass, Weber was the first to determine the value of the resistance standard proposed by B.S. Jacobi (1801-1874) (25 feet of copper wire with a diameter of 2/3 millimeters) in absolute electromagnetic units (1852). Until that time, scientists had limited themselves to comparing the resistances of various standards, using the bridge method of C. Wheatstone (1802-1875), proposed by him in 1843.Galvanic current sources are not applicable for absolute measurements due to their differences. Weber's source was an inductor - a rectangular wire frame suspended in the Earth's magnetic field so that the frame can rotate around a vertical axis. The ends of the winding are connected to a galvanometer. First, the resistance R0 of the galvanometer and inductor is determined. For this purpose, the frame is installed in a plane perpendicular to the plane of the magnetic meridian and then rotated 180°. In this case, in accordance with the phenomenon of electromagnetic induction discovered by M. Faraday (1791-1867) (1831).In 1833, Faraday discovered the laws of electrolysis, on the basis of which he put forward the idea of a design for measuring current strength by the amount of substance deposited on the electrodes. Such a device was called a voltameter; it was this that was used in 1841 by D. Joule (1818-1889) in studying the thermal effect of electric current. The heating element in Joule's experiments consisted of the conductor under study, wound around a thin glass tube. Measurements showed that with the same current strength, two identical resistances immersed in identical calorimeters release twice as much heat, from which followed the conclusion about the proportionality of the released heat to the resistance of the conductor. By changing the current strength, Joule obtained a quadratic dependence for the released heat.In 1832, Weber and K. Gauss (1777-1855) took part in a worldwide magnetic survey of the Earth's magnetic field, undertaken by the Magnetic Union, the first international geophysical organization, in which scientific institutions from almost all the world's largest countries took part.The measurements had to be carried out, if possible, simultaneously, with identical instruments and methods, on numerous, widely scattered land and sea areas. Naturally, two main questions arose: the choice of units of measurement and the development of appropriate measuring instruments and measurement techniques.Gauss was the first to propose expressing the "intensity of the earth's magnetic force" in absolute units based on the units of length, mass, and time. He proposed the millimeter, milligram, and second as such. This system was used until 1870, when, by decision of the Electrical Standards Committee of the British Association for the Advancement of Science, which included W. Thomson (1824-1907), K. Maxwell (1831-1879), W. Siemens (1816-1852), Joule, Wheatstone, and other prominent scientists, it was replaced by the centimeter, gram, and second system of units, since Gauss's system turned out to be inconvenient for engineering calculations.Weber made sensitive magnetometers, the moving element of which were magnets of different masses (depending on the purpose of the study), suspended on elastic threads. A tangent compass was used as an electrical measuring device. He also proposed an absolute magnetic unit of current strength, which was defined as the strength of such a current, which, flowing around the plane of a coil of a unit area, acts at a distance in exactly the same way as a magnet with a single magnetic moment, the axis of which is perpendicular to the plane of the coil.Now galvanometers can be calibrated in the absolute magnetic system of units, and we can begin to determine the electrodynamic constant c.In the experiment with the Coulomb torsion balance, the electrostatic unit of charge q is determined.To determine c, a charge measured in electrostatic units must be passed through a circuit in which a device measuring current in electromagnetic units is included. The task was completed by Weber and R. Kohlrausch (1809-1858) in 1855. The value obtained was c.The magnetic unit of current is approximately 3 x 1010 electrostatic units.The coincidence of the obtained value with the speed of light did not attract the attention of scientists until 1864, when C. Maxwell (1831-1879) put forward a hypothesis about the electromagnetic nature of light.At the same time as these studies, a practical system of units was being developed, largely spontaneously.Even in Faraday's time, there was no specific system of units for physical measurements. Naturally, length, mass, and time were taken as the basic quantities, but their units, except for the unit of time - the second, accepted by all researchers, were completely arbitrary, and were determined more by national, everyday habits than by scientific necessity.For practical use, it is first necessary to find a standard for the required unit. An example is the introduction of the metric system, where the globe was chosen as the standard, one forty-millionth of a doyne of its meridian, and the meter was proposed as the unit of length.In the early days of electrical engineering, the sources of direct voltage were galvanic cells. The electromotive forces of some of them (and they are all almost identical), the most resistant to temperature changes, were proposed as the unit of voltage.First it was the Daniel element, the electromotive force equal to 1.1 V, then the Clark element (1878) was proposed, the electromotive force equal to 1.4 V, and finally - Weston (1893), the electromotive force of which is 1.02 V and practically does not change with temperature changes. If the voltage standard was "proposed" by nature itself, the resistance standard was formed completely arbitrarily. We will give an incomplete table of these values, as well as the sizes of the conductors that make up the resistance.Name Metal Weight Length Diameter ResistanceWheatstone Copper 6.5 g 1 foot 1.74 mm 2.2-10ˉ3 OhmJacobi Copper 22.4 g 25 feet 2/3 mm 0.38 OhmDigney Iron 100 kg 1 km 4 mm 7.80 OhmSiemens Mercury 13.35 g 1 m 1.3 mm(1 mm2) 0.941 OhmOmada 0.984 OhmWeber 1.00 OhmThe most convenient for use "Siemens" was proposed (1864) because mercury was widely used to create reliable electrical contacts. In addition, mercury has the highest specific resistance among pure metals. But the proposed dimensions do not correspond to electrical quantities. This is a tribute to the metric system.Jacobi's standard may have been proposed only because the scientist had at his disposal the necessary coil of thin copper wire in silk insulation."Digney" most likely owes its parameters to researchers involved in the installation of telephone lines, hence the iron, great length, and even weight of 100 kg."Ohmada" is a unit of resistance proposed by the British Association for the Manufacture of Absolute Units of Resistance.W. Weber defined the unit of resistance based on theoretical principles. In modern terminology, it would sound like this: one "weber" is the resistance of a circuit in which a current of 1 A occurs at a rate of change of the flow coupled to the circuit of 1 weber per second. Such a standard is of little use for practical application.The great authority of the Siemens electrical appliance factory led to the widespread use of the Siemens resistance standard.Then begins a series of amazing coincidences that defy logical justification. It was natural to accept as a practical unit of current the current that results from applying a voltage of one weston to a resistance of one siemens. The strength of such a current was 0.10824 magnetic units of current. In the metric system, a quantity can be represented by a series of units constructed according to the principle of decimal multiples, therefore one tenth of a magnetic unit was chosen as a practical unit of current, i.e. one weston per one siemens gives 1.0824 practical units. But it also turned out that one weston squared per one siemens is 1.1024 watts. Naturally, there was a desire to “correct” the standard values so that the ratios would exactly equal one. For now, let us conditionally call the new units volt, ohm, ampere (these names were legalized in 1889).1 Weston / 1 Siemens = 1.0824 amperes1 volt / 1 ohm = 1 ampere(1 Weston)2/ 1 Siemens = 1.1024 Watts(1 volt) / 1 ohm = 1 wattThe joint solution of these equations gives:1 Weston = 1.0185 Volt; 1 Siemens = 0.9409 Ohm:1 ampere = 1/10 magnetic unit of current = 3∙109 electrostatic units; s = 2.9979∙1010 cm∙s-1.The resistance of the standard must be increased by 6.3%, i.e. the length of the mercury column must be increased to 106.3 cm, the voltage created by the Weston element must be decreased by 1.8%. This can be easily done using a potentiometer. In absolute units, the electromotive force of a galvanic element was measured by Poggendorff in 1841 using the compensation method. A circuit is made with an auxiliary source such that its electromotive force E exceeds the electromotive force Ex of the element being studied. The source being studied is connected in parallel to the section of the circuit towards the source E, and a sensitive galvanometer is connected to its circuit. By adjusting the current in the primary circuit with a rheostat, the galvanometer is brought to zero.The practical unit of current strength is the ampere, equal to 0.1 magnetic unit, which is 3∙109 electrostatic units of current strength, and accordingly, one coulomb is equal to 3∙109 electrostatic units of electricity quantity.The experimental substantiation of these relationships was carried out almost until the beginning of the 20th century.Maxwell designed a special machine for determining the constant c. It was a torsion balance of great sensitivity (the entire device was suspended on a thread), in which the forces of attraction of electrostatic and electromagnetic origin were directly compared. This distinguished Maxwell's installation from the design of Weber and Stoletov, where electrostatic and electromagnetic forces were determined separately and only then compared.A silver voltameter was chosen as a physical device for establishing the standard of "ampere": one ampere is taken to be a direct current of such a strength that 1.118 mg of pure silver is released from a silver nitrate solution as a result of electrolysis in one second. This is a very stable standard, but it is too inert: the current strength should not change during the measurement process. The SI system has adopted an equivalent, but different, electromagnetic standard: two sections of parallel conductors located at a distance of one meter from each other are attractedto each other with a force of 2 -10-7 N/m, when the same currents of one ampere flow through them. These forces can be simply "weighed" by attaching the conductor to a scale.Practical units were first approved in 1881 at the First International Congress of Electricians. They were reconfirmed at the congress of scientists on the occasion of the International Industrial Exhibition in Paris in 1900. No major changes were made later, only the units adopted as the main ones were changed: Volt-Ohm was replaced by Volt-Ampere, and so on. In the SI system, the main unit, along with the meter, kilogram, second (sufficient for constructing a system of mechanical units), is the ampere, which allows such a system to embrace electrical phenomena. It remains to include the thermal unit - the kelvin, and the construction is complete.The SI system was formed on the basis of the practical system of electrical quantities, which was improved by the Italian scientist G. Giorgi in 1901. It was he who proposed to rationalize the equations so that the coefficient 4π would be excluded from most of them. He also proposed the ampere as the basic unit. The SI system was adopted in 1960 by the 11th General Conference on Weights and Measures.The system of five physical units is quite sufficient to describe all physical phenomena, but it also includes candela ("candle") - a unit of luminous intensity, and mole - a unit of quantity of substance, but their use is dictated by considerations of ease of use. Thus, luminous intensity, characterizing the power of a light source, also takes into account the peculiarities of light perception by the eye. When introducing mole, the following feature is taken into account: molecular processes are carried out at the level of individual atoms, here it is the number of particles that is important, and not their total mass.The International System of Units is finally built on the basis of seven units (meter - kilogram - second - ampere - kelvin - candela - mole).The question is natural: what would such a system look like if there were no energy coincidences, noted above? Additional coefficients would appear and that's all. Rationalization in such a case would be unnecessary, the system would be incoherent, but this would not interfere with its use.This post is a brief retelling of the articleTO THE HISTORY OF CREATION OF THE SYSTEM OF PHYSICAL UNITS "SI"© B.Z. VinokurovVinokurov B.Z. On the history of developing the Physical Values International System of Units (SI). It was introduced in 1960. It is so natural and compact in its structure that one may think that its appearance was historically inevitable, though the creation of SI had been preceded by a long process of its development. Some facts form the history of the its formation which are now mostly forgotten are discussed in the article.

author: TimFox
author_rank: Super Contributor
timestamp: March 12, 2025, 06:00:45 pm
content: A very important item mentioned in that lengthy post is the relevance ofc, the speed of light in vacuum, to electrostatic and magnetic forces, which was important to the work of Maxwell.This allowed the practical definitions in the evolution of SI to change the emphasis in units definitions from theoretically equivalent definitions of current, voltage, and resistance as practical reproducibility evolved.If theoretical physics were to change that relationship tocin future, it would probably have a negligible effect on the calibration of electrical standards.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: March 28, 2025, 01:47:40 pm
content: Daniell cell was invented in 1836 , copper - zinc reaction produce 1.1018 Vwas very well popular in Europethen was Clark cell 1873 (i think) primary reference for USresistance standard was Siemens mercury column. 0.94 Ohm in Europemore interesting that in 1900 -x they able to measure potential up to 1 microvolt , by some coulometer multiply tests ...