Why Do We Call It An... Ohm?

THIS IS THE EIGHTH IN A SERIES OF ARTICLES THAT EXPLORE THE HISTORY OF SCIENCE AND ENGINEERING. One way in which this history has been preserved is in the names of the scientific units that we commonly use. Those units will serve as starting points for these articles as we explore “Why do we call it a...?” OHMS LAW DIAGRAM WHY DO WE CALL IT AN... By:OHM Lyle D. Feisel, Ph.D., P.E. (Ret.), Iowa Alpha ’61 When you rst started to learn about this complex journey was the German electricity, what was the rst law or physicist Georg Ohm (1789-1854), for equation you encountered? If I were whom, obviously, the unit of resistance a gambling man, I would wager that is named. And no, that’s not a misprint; it was Ohm’s Law. is is a simple Georg is not supposed to have an e on equation that says the voltage across a the end. resistor, measured in volts, is equal to the Georg Simon Ohm was born on 16 current through the resistor, measured March, 1789, in Erlangen in the King- in amperes, times the resistance of the dom of Bavaria, which is now a part of resistor, measured in ohms. Resistance is Germany. Ohm’s father, Johann, was a determined from the dimensions of the locksmith and apparently had acquired a resistor and the resistivity of the material reasonably good education by essentially from which it is made. educating himself. He provided a basic As we see it today, Ohm’s Law is a background in science and philosophy straightforward linear relationship, for his children, enough for Georg to about as simple as you can get. ere enroll in the Erlangen Gymnasium or were, however, some tortuous twists high school where, apparently, the addi- and turns in the development of that tion to his education was minimal. e equation and there were, at rst, much city was then, and is now, a college town more complicated versions. e driver and in 1805, at the age of 16, Ohm ma- — I almost said “conductor” — through triculated in Erlangen University. His 24 THE BENT WINTER 2021 tenure at the university was brief and —the generation of an electric current then, was the original version of Ohm’s inglorious, lasting only three semesters. when a junction of two dissimilar metals Law. Obviously, this is not the Ohm’s According to one account, his time there is heated — was first reported in 1822. Law we have come to know and love, but was devoted more to dancing, billiards, What was not known, at least not in a it was the best description of the state of and ice skating (!) than the pursuit of quantitative sense, was how an external knowledge at the time. knowledge, and his father said, “Enough circuit affected the output of a source of In 1827, Ohm took leave from the gym- is enough. Out you go.” Of course, he electricity. Ohm, as much a mathemati- nasium in Cologne and spent a year in said it in German. cian as a physicist, set out to explore that Berlin. There he wrote hismagnum opus, While Ohm may not have been a relationship and express it “Mathematical Consideration of The sterling student, he acquired enough mathematically. Galvanic Circuit.” This very mathemat- knowledge — or at least the credentials When talking about the challenges ical treatise (available on Google Books) — to qualify him for a job teaching faced by our scientific forerunners, I presents both mathematical and descrip- at a school in Switzerland where he often say something like, “They couldn’t tive discussions of electric circuits. In it, started in 1806. He spent five years in just go to the laboratory and pull a Ohm reworks some of his earlier data Switzerland, teaching at the school and voltmeter and an ammeter off the shelf.” and derives the equation, V=IR, that we as a private tutor, and then returned to Well, it turns out that in, say, 1822, that know so well. Erlangen University. Apparently, Ohm wasn’t strictly the case. A good labora- One would think that such a simple had spent his spare time in Switzerland tory would have an electroscope; this result would have been accepted and, in- studying the writings of the leading sci- was a device that consisted of two pith deed, celebrated. Such was not the case, entists of the day. In so doing, he learned balls or two gold leaves that repelled however. Ohm had his detractors and enough that he was able, in short order, each other and deflected when attached was subjected to various degrees of crit- to pass the appropriate examinations to a source of charge. The amount of icism and contumely before he and his and be awarded the Doctor of Philoso- deflection was a measure of the electric work were formally recognized. Indeed, phy degree. potential (voltage) of that source. The ever since he received his Ph.D. in 1811, Then, as now, the Ph.D. degree was the lab would also have what later became he sought appointment as a professor in ticket to a university teaching posi- known as a galvanometer, a device that a recognized university but remained tion. It didn’t seem to work for Ohm, measured the force exerted on a mag- frustrated. He achieved some measure however, and while he tried to get a net by a coil of wire connected to an of success in 1833 when he joined the job in a university, he was destined to electrical source. The force, of course, faculty of the Technische Hochschule spend some twenty years teaching at was proportional to what we now call in Nuremburg; still not a university, lesser institutions such as high schools current. Voila! A voltmeter (sort of) and but much more prestigious than his and the German military school. He an ammeter (sort of). previous appointments. The institution was fortunate, however, in that one of For Ohm’s primary and most famous is now named the Nuremburg Institute these schools, the Jesuit Gymnasium in experiment, all he really needed was a of Technology Georg Simon Ohm. Its Cologne where he taught from 1818-26, voltaic pile, some kind of galvanometer, logo, fittingly, is the Greek letter Ω, had excellent laboratory facilities. It and a coil of wire. Using the galvanome- which is, of course, the symbol for the was here where Ohm did much of his ter, he established a base force (current) ohm, the SI unit of resistance. important experimental work and began by essentially shorting the battery with While at Nuremburg, Ohm finally publishing his results. a heavy wire. That current would have received the honors he sought and It is interesting to look at the state of been determined by what we now call deserved. He was awarded the Cop- knowledge of electricity and magnetism the internal resistance of the battery. He ley Medal from the Royal Society of during this period. Prior to 1800, all then inserted into the circuit thin wires London and was made a corresponding experiments had been conducted using (0.025” diameter) of varying length and member of the academies of Berlin, static electricity. The concept of electric measured the reduction in force as the Turin, and Bavaria. current — a constant flow of electrical wire length was increased. When these Finally, in 1849, Ohm was appointed “fluid” — did not exist until Alessan- data were plotted, they produced a curve Full Professor at the University of dro Volta invented the pile or battery that Ohm roughly approximated with Munich where he remained until he in 1800. And while many scientists this equation: died on 6 July, 1854. While many believed there was some relationship V = Klog(1+x) scientists of the 19th century sensed that between electricity and magnetism, the where V is the reduction in force (i.e., there was a relationship between electric effect of electromagnetism current, I continue to re- tension (voltage) and force (current), it was not observed until mind you), K is a constant, was Georg Simon Ohm who, through 1820. Finally, the phenom- and x is the length of the reason, experiment, and mathematical enon of thermoelectricity wire in the circuit. This, analysis, determined the details of that relationship. And that’s why we call an ohm an ohm. Ohms Law Diagram THE BENT WINTER 2021 25 .

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