The Not So Honorable History of The Wheatstone Bridge

In my last video, on the history of Ohm’s law, I talked about part of how it was made famous was when a man named Charles Wheatstone attributed Ohm’s law to help him create what is now called the “Wheatstone Bridge” in 1843. However, I quickly learned that this “bridge” was actually created 10 years earlier than Wheatstone by a man named Samuel Christie, something that Wheatstone admitted in his paper. This caused my spidey sense to tingle: why did Christie make this bridge and why did it get named after Wheatstone? When I looked into it, I found that Charles Wheatstone skirted the edge of dishonorable behavior and, sometimes, into downright villainy. Did Charles Wheatstone deserve to have his name immortalized with this device?

I would like to begin in August of 1831, when Michael Faraday published his astonishing discovery that changing the magnetic field (or what he called the magnetic lines) in a coil of wire would induce current in that coil. This was big BIG news for the scientific community and Faraday immediately was awarded the Royal Society of London’s highest award the Copley medal “for his discovery of Magneto-Electricity”. Now we turn to a mathematician and a member of the Royal Society named Samuel Hunter Christie. Christie was friendly with Faraday and had actually seen Faraday conduct some of these amazing experiments and felt very strongly that Faraday deserved all of the accolades that he was receiving. However, Faraday had discovered that induction in copper coils was the same as in iron coils and decided that induction was independent of the properties of the conductive wire a conclusion that Christie said he had a, “great difficulty in adopting.”[1] For that reason, in 1833, Christie started a series of delicate experiments to validate or refute Faraday’s claim.

The first idea Christie had was to wind insulated copper wire one direction around a cylinder and then wind insulated iron wire the opposite direction and then connect the two wires and then measure with a compass if the copper or the iron had more current, to which he found that “the intensity of the current from the iron wire was less than half that from the copper.”[2] Now, I must note that despite the fact that this was 6 years after Georg Ohm had published his book about Ohm’s law, Ohm’s law wasn’t known in England or even widely known in Germany so neither Christie nor Faraday had any idea about the difference between current and voltage.

Christie then tried to balance the two induced currents by using less loops of copper wire and more of iron but then wondered about the relationship between the length of wire and the current induced (remember, this is before they knew about Ohm’s law) and he tried the same length of coil of only one material where the current went through a crisscross wires where he varied the length and the thickness of the iron to make it match with the copper by bridging the wires with another wire and adjusting the iron until the current in the bridge was zero. This was the first example of a “Wheatstone Bridge”. However, Christie’s paper was very long and didn’t have much of a conclusion so it had very little impact on scientists but it was well known in the Royal Society.

Now we move to a year after Christie’s talk, in June 1834. That was when 32-year-old Wheatstone displayed a new method to measure the speed of electricity in a wire by looking at a spark at the beginning, the middle, and the end of a long path with a spinning mirror. At the time, they thought that electricity was instantaneous and were amazed with Wheatstone’s discovery that electric signals move at a set speed that depends on the conductor. We now know that there was something wrong with Wheatstone’s experiment as he discovered a speed that was faster than the speed of light,[3] but at the time his experiment was roundly praised and Wheatstone was suddenly considered one of England’s top scientists. By January 1836, Wheatstone was elected as a member of the prestigious Royal Society of London, and became close friends with the secretary of the Royal Society named Peter Roget.[4] 

That was why, in February of the following year, 1837, when Peter Roget was visited by a newly retired soldier named William Cooke about an electric telegraph system, Roget recommended that Cooke go and see Wheatstone. Cooke was delighted to find that Wheatstone had four miles of wire ready for experiment, but was “dismay[ed]” to discover that Wheatstone was also working on telegraphs and had even built two or three.[5] They quickly decided to collaborate, as Cooke wanted to work with a man who had invented a method of “ascertaining the velocity of lightning.”[6]

By June 12, 1837, Cooke and Wheatstone filed for their first patent for a telegraph that used 5 wires and a diamond shape where 2 of the compasses would point to a specific letter to spell out the code. For example, this would be for the letter B. For a short time, both men worked together quite pleasantly and produced more and more efficient and powerful telegraphs. However, Cooke quickly became frustrated that, as a biographer put it, “the glory of Wheatstone’s name and reputation at once overshadowed and eclipsed that of Mr. Cooke.”[7] Cooke complained to Wheatstone, but Cooke never felt that Wheatstone did enough to counter these false statements and, often was the source of these misstatements, Cooke complained, “the invention at once became a subject of public interest; and I found that Mr. Wheatstone was talking about everywhere in the first person singular.”[8]

Now we move to the important actions of a man whose influence has really been lost to time, Moritz Jacobi. Jacobi was a Russian Jewish man who was born in Germany and initially became an architect. Jacobi was skilled at mathematics and language and, in the early 1830s, read about different crude electric motors and decided to try to make a practical one for himself. In this quest, Jacobi read the 1827 book by Georg Ohm and declared, “the theory established by Mr. Ohm… offers so much simplicity, and agrees so well with all the phenomena of the voltaic pile, that I have not hesitated to adopt it.”[9] With the help of this equation, in 1834, Jacobi constructed the first practical electric motor which he published in Germany and then in France. Jacobi’s motor remained the world’s most powerful electric motor until September of 1838, when Jacobi invented an even better motor. It wasn’t until the early 1840s that anyone else caught up to him.[10] Meanwhile, in 1837, Jacobi moved to Saint Petersburg and, with the help of Czar Nicolas, focused on his electricity studies (although he still also worked as an architect) where he often worked with Emil Lenz of Lenz law fame. In October 1838, Jacobi noticed that electrical process would leave deposits of copper and invented electroplating and he and Lenz then used to make some copper copies of medallions! It was in studying how to control the current that Jacobi created machines to control the current by changing the resistance, which he demonstrated to the Academy of Sciences in St, Petersburg in March of 1839.

In 1840, Jacobi went to London and visited Charles Wheatstone whereupon Jacobi was startled to see Wheatstone demonstrate “his” machines to control the current by varying the resistance. Jacobi felt that Wheatstone’s devices were, “founded on exactly the same principles as mine, and with very inconsiderable modifications and differences.”[11] Unfortunately, I cannot find images of Jacobi’s original devices to make my own judgement. But I do know that in 1840, Wheatstone told Jacobi that they had just come up with the same idea independently which Jacobi thought, “quite impossible.” However, Jacobi was mollified when Wheatstone demonstrated that he had expanded the use of these devices from only controlling the current to also using it to measure the resistance of unknown resistors. Although Jacobi thought that Wheatstone, “has shown me, in his unpublished papers, very valuable results which he has obtained by this method,”[12] Wheatstone did not publish his results and methods for a further 3 years. Why not? Well, it seems possible that what caused the delay was that the same month that Jacobi called out Wheatstone for copying Jacobi’s resistance meters, Wheatstone was visited by a young clockmaker named Alexander Bain who demonstrated an electric clock and some electric telegraph equipment that would print the signal. According to Bain, Wheatstone promised to purchase the telegraph equipment but told him that the electric clock was worthless. Then 3 months later, Wheatstone published “his” electric clock and stopped payments on the telegraph equipment. By 1843, a friend of Bain’s published a book about how Bain was the inventor of these items which put Wheatstone at the middle of that controversy. At the same time, Wheatstone was asked to give an important talk to the Royal Academy for something called a Bakerian Lecture. It was possibly for that reason, that in June of 1843, Wheatstone introduced “his” bridge with “his” resistance meters with Ohm’s law to help him.

So, now we finally get to the “Wheatstone” bridge, which was designed to accurately measure resistance which Wheatstone was interested in, “to determine the practicability of communicating signals by means of electric currents to more considerable distances than had hitherto been attempted.”[13] Wheatstone called his device a “differential resistance measurer” for which the current is split into different paths where “the equilibrium having been disturbed by the introduction of a resistance in one of the wires, it may be restored by placing an equal resistance in either of the adjacent wires.”[14] In other words, Wheatstone was suggesting determining the resistance of an unknown system by comparing it to a known resistor and then adjusting the known resistor until no current flowed in the bridge between the two paths (note that neither Wheatstone nor Christie used the term “bridge”). Although I don’t know any details of Jacobi’s variable resistors, I do know Wheatstone’s designs of which he had 2. The first was one where he spun a handle to move wire from a non-insulated cylinder to an insulated cylinder, so that the current only goes through the wire of the insulated cylinder, and thus the length of the resistor is varied. The second method used a large cylinder where a moving connector would change the length of the resistor. Wheatstone called both of these devices “rheostat’s” a name that was used for many years and is sometimes still in use today for a variable resistor that is used to control the current.

In other words, Wheatstone was describing using a Rheostat invented by Jacobi in 1839 with Jacobi’s description of Ohm’s law that was written in 1834 and translated into English in 1838, used a Christie bridge from 1833. Combining different people’s ideas together is a major accomplishment, but it important to acknowledge the contributions of others honestly, so let’s go over how well (or badly) Wheatstone did and the effects of his conclusions.

The Rheostat: in his original paper, Wheatstone did add a footnote from Jacobi’s talk in 1840 when Jacobi stated that he had previously invented these variable resistors but only used them for controlling the current, not measuring the resistance. Wheatstone also added that *his* rheostat was easier to use saying that Jacobi’s devices “differs in mechanical construction from either of mine and is less convenient to manipulate” without any details as to why.[15] After this talk, Wheatstone was memorialized as the originator of the name and, often, the inventor of the rheostat itself! As far as I can tell, that is because Jacobi didn’t give his invention a name and Wheatstone (and this particular talk) was so famous as to obscure Jacobi’s influence. Also, Wheatstone’s paper had lovely pictures and Jacobi’s original paper was in Russian and I can’t find it, even with modern technology (and one later in German didn’t include a picture).

Promoting Ohm’s Law: Wheatstone had actually been promoting the validity of Ohm’s law since 1840 (around when he started using Jacobi’s rheostat to measure resistance) but continually failed to mention how he learned about Ohm from Jacobi. Because of this Ohm’s work was pretty popular in England by 1843 (the Royal Society of London had given Ohm an award in 1841) but in his 1843 talk Wheatstone siad that Ohm’s law was “not yet generally understood and admitted[16]” and, after many years, Wheatstone became known as *the* person who discovered and promoted Ohm with no mention of Jacobi, so much so that in his obituary he was called, “one of the first, if not the very first in this country to appreciate the importance of Ohm’s beautiful and simple law.”[17] (An Englishman named William Sturgeon published the English translation of Jacobi’s paper on Ohm’s law in 1837, and, soon many people in England were Ohm admirers, including a young James Joule).

The Bridge: Wheatstone’s original paper and his talk had no mention of Christie and his bridge, and the preprints that were sent to friends and colleagues also didn’t mention Christie. However, it seems like someone must have complained about Christie because by the time the paper was printed formally it contained a footnote that said, “Mr. Christie… has described a differential arrangement of which the principle is the same as that on which the instruments described in this section have been devised. To Mr. Christie must, therefore, be attributed the first idea of this useful and accurate method of measuring resistances.”[18] However, after this footnote was added, Wheatstone had his paper translated into French and German without the footnote and, as Christie’s original 1833 talk was in English and not very popular, most scientists outside of England had no idea about Christie’s input.

Meanwhile, Wheatstone’s influence continued to grow. In January, 1845, a year after Wheatstone’s talk, a Quaker man named John Tawell killed his mistress and a person of his description had been found to have purchased a ticket to London. The police then used the newly installed Cooke-Wheatstone telegraph to warn the police to look out for him (amusingly telegraphing quaker as “kwaker” as there was no Q in their telegraph), and Tawell was arrested and then, in March, hanged for his crime.[19] This, the first capture of a murderer by telegraph, made Wheatstone’s reputation, and soon more and more people became interested in electric telegraphs. By September 2, 1845, one of the world’s first public electric telegraph company was formed called the “Electric Telegraph Company” and Wheatstone received 30,000 pounds (equivalent to around $5.5 million dollars today) for his contributions and Wheatstone was hired as a “technical advisor.” However, according to Cooke, when they tried to incorporate the company the following year, Alexander Bain filed a petition that Wheatstone had stolen his electric clock and electric printing telegraph and a member of the House of Lords decided that Bain’s position had so much merit that the company had to pay him off which they did as well as hiring Bain. The company also hired a man named Henry Mapple who also claimed that Wheatstone stole from him and Wheatstone quit the company in a huff.[20] Despite all of this, Wheatstone was *still* known as the inventor of the telegraph and his reputation continued to grow, up to 1866 when he was awarded the knighthood for “his” discovery of the electric telegraph (after complaining Cooke also received a knighthood for the telegraph in 1868).

Six years before Wheatstone’s knighthood, in 1860, an engineer and founder of the Siemens company (who was then working on telegraphs in Germany) named Werner Siemens wrote an article about using columns of mercury as a standard measure of resistance as he felt that the standards created by Jacobi in 1839 (which, by the way, were similar to Wheatstone’s standard resistance from 1843 as being a length of copper wire of a certain length) were inconsistent, which he demonstrated by using what Siemens called “Wheatstone’s Bridge.”[21] (As Siemens was German he probably didn’t know about Christie). Within a month, this article was translated into English and, as far as I can tell, this was the first mention in English of a “Wheatstone bridge.” Although Siemens’ plan for standardizing resistance with mercury was eventually dropped for defining an Ohm by 1 volt over 1 amp, Siemens’ article made the Wheatstone bridge popular, both as a name and a device. By June, of 1861, a Scottish professor named William Thomson (later Lord Kelvin) wrote about his new bridge or balance for very short wires, he totally skipped Christie’s or Jacobi’s influence and stated that, “the beautiful arrangement first invented by Professor Wheatstone… is frequently called ‘Wheatstone’s Bridge,’ especially by German writers.”[22] (Thomson preferred the term, ‘Wheatstone’s balance’)

In the 19th century the “Wheatstone” bridge was used by everyone in telegraphs and the burgeoning field of electrical generators and light bulbs in the same way that Wheatstone (and Christie) did, by adjusting the resistance of a rheostat until two branches are balanced. However, by the 20th century, many people started using bridges with 3 constant known resistors and using the difference in voltage to get the unknown resistor. In addition, it is often given as a challenging physics problem with 5 resistors just to give a problem where it isn’t easily changed into a simple parallel or series circuit, although, as far as I can tell, it is almost never actually used in that way.

So that is a history of Wheatstone’s bridge. By the way, part of the reason that people were so invested in the “Wheatstone Bridge” is because, starting in 1861, the British Association for the Advancement of Science formed a committee (including Charles Wheatstone and William Thompson) to device standard units for electricity, and the names went through various ideas before setting down to their current names. For example, in 1861, volts were suggested as a measure of resistance, ohma for electric potential and farads for electric charge. However, by 1864, the definition of volts and ohma were switched, by 1867, ohmas became ohms, and by 1873 charge was changed to Coulombs and Farads ended up as the units of capacitance. Then in 1882, Werner Siemens’s little brother Wilhelm Siemens was nominated as the president of the British Association and immediately added another name to the list: Joules as a measure of heat energy, as Joule did so much with electricity to discover the laws of heat. And that story and the story of James Prescott Joule is next time on the lightning tamers!

[1] “great difficulty in adopting” Christie, S “The Bakerian Lecture” (Feb 28, 1833) Proceedings of the Royal Society of London Vol. 123 (1833) p. 95

[2] “the intensity of the current” Christie, S “The Bakerian Lecture” (Feb 28, 1833) Proceedings of the Royal Society of London Vol. 123 (1833) p. 98

[3] “nearly 288,000 miles” Wheatstone, C “An account of some experiments to measure the velocity of electricity” Proceedings of the Royal Society of London vol 3 (Dec 1837) p. 300

[4] “elected as a member of the prestigious Royal Society of London” Proceedings of the Royal Society vol. 3 (Dec 1837) p. 366

[5] “imagine my dismay..” Letter XIV: William Cooke to his mother (Feb 27, 1837) Extracts (1895) p. 19

[6] “the velocity of lightning” ibid

[7] “the glory of Wheatstone’s name…” Clark, L “Memoir of Sir William Fothergill Cooke” (1879) found in Extracts (1895) p. 72

[8] “I found that Mr. Wheatstone…” Cooke quoted in Extracts (1895) p. 890

[9] Jacobi, M “On the application of Electro-magnetism to the moving of Machines” (April 1835) The Annals of Electricity, Magnetism, and Chemistry (Oct 1837) p. 422

[10] “Jacobi’s motor remained the world’s most powerful electric motor..” According to “Jacobi’s Motor” from the Electrotechnisches Institut https://www.eti.kit.edu/english/1382.php

[11] “founded on exactly the same principles…” Jacobi, M “On the Principles of Electro-Magnetical Machines” (1840) 10th Meeting of the British Association (1841) p. 21

[12] “has shown me, in his unpublished papers…” Jacobi, M “On the Principles of Electro-Magnetical Machines” (1840) 10th Meeting of the British Association (1841) p. 21

[13] “to determine the practicability…” Wheatstone, C “An Account of Several New Instruments…” Proceedings of the Royal Society vol. 133 p. 303

[14] “differential resistance measurer” Wheatstone, C “An Account of Several New Instruments…” Proceedings of the Royal Society vol. 133 p. 323-4

[15] “differs in mechanical construction…” ibid

[16] “not yet generally understood and admitted” Wheatstone, C “An Account of Several New Instruments…” Proceedings of the Royal Society vol. 133 p. 303

[17] “one of the first…to appreciate the importance of Ohm’s” “Obituary Notices: Charles Wheatstone” Proceedings of the Royal
vol. 24 (1876) p.xxiv

[18] “Mr. Christie… has described…” Wheatstone, C “An Account of Several New Instruments…” Proceedings of the Royal Society vol. 133 p. 325

[19]“kwaker” according to Stubley, P Calendar of Crime (2014)

[20] “Electric Telegraph Company… quit” Cooke, W The Electric Telegraph: Was it Invented by Professor Wheatstone? (1854) p. 44-8

[21] “Wheatstone’s bridge” Siemens, W “Proposal for a new reproducible Standard Measure of Resistance” (1860) translated and found in The London, Edinburgh and Dublin Philosophical Magazine Forth Series (Jan, 1861) p. 31

[22] “the beautiful arrangement first invented…” Thomson, W “On the Measurement of Electric Resistance” (June 6, 1861) Proceedings of the Royal Society vol. 11 (1862) p. 313

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