May 1972 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Georg Simon Ohm's eponymous "law," i.e., Ohm's law, is perhaps the best-known formula in the realm of electricity and electronics. Although Mr. Ohm did not know it at the time, his conclusion holds up in both the macro and micro scale worlds of electron behavior. Voltage is equal to the product of a resistance and the current flowing through it, E = I * R. It is hard to believe that we have only had his result, announced in 1825, at our disposal for less than 200 years. A thorough grasp of Ohm's law is a minimum requirement for entry into the fields of electrical and electronics work; fortunately, only a fundamental grasp of algebra is required. Kirchhoff's law is a relatively easy next step. The big hurdle comes with wanting to get an engineering degree where mastery of Maxwell's equations and the calculus necessary to work with the formulas in their various forms.
The Origin of Ohm's Law
By David L. Heiserman
Today, Ohm's Law stands as one of the most powerful and commonly used laws of electricity and electronics. It states that the amount of current flowing through a conductor (or resistor) is equal to the applied voltage divided by the resistance of the conducting material. In mathematical terms, the equation generally reads I = E/R. What seems simple and obvious today, however, took a great deal of genius, courage and effort to propose for the first time in 1825. Georg Simon Ohm, a German physicist and mathematician, was a man who had the right kind of genius and courage .
Scientists were aware of a "galvanic fluid" (electrical current) that played some mysterious role in their studies; but the elusive and short-lived nature of currents in static electricity made them a difficult subject for any kind of meaningful study.
Alessandro Volta completely changed all this in the early months of 1800 when he formally announced the discovery of his electric generating cell. His "hydro-electric battery," forerunner of modern wet-cell batteries, gave scientists their first source of current that could flow continuously. For nearly twenty years, however, all the studies of galvanic currents suffered from one serious disadvantage - there was no way to measure the amount of current flow.
The breakthrough came in 1820 when Oersted showed that a current passing through a wire produces a magnetic field. A year later, Schweigger and Poggendorff used Oersted's findings to invent the galvanoscope - a crude sort of galvanometer made of hundreds of turns of wire wrapped around an ordinary compass. Current flowing through the wire produced a magnetic field that deflected the compass needle by a proportional amount.
Georg Ohm, then a high school mathematics and physics teacher in Cologne, saw the possibility of combining Volta's hydro-electric battery with a galvanoscope to study the nature of electrical current flow.
Using equipment he constructed himself, Ohm set out to find the exact relationship between applied potential, the length of a conductor, and the amount of deflection of the needle in a galvanoscope. His procedure was to connect the galvanoscope directly to the battery and carefully note the position of the compass needle. This gave him a reference reading. He then inserted a wire of known composition and length into the circuit and noted the new position of the needle. This was his experimental reading. Of course, the resistance of the test wire made the needle show a smaller amount of deflection in the experimental condition.
In 1825, Ohm reported·his first findings in a paper titled "Preliminary Notice of the Law According to which Metals Conduct Contact Electricity." Publishing this paper turned out to be a mistake that plagued Ohm for the next sixteen years.
Technically speaking, the equation Ohm presented in the paper was incorrect. It stated that v = m log (1+x/r); where v was the decrease in the needle's deflection, x represented the length of the conductor, r represented the resistivity of the conducting material, and m stood for the amount of applied potential.
Just before his paper was scheduled to appear in print, Ohm repeated a few of his experiments using a different kind of power source. The results didn't agree with his original findings, and Ohm immediately saw he could develop a much simpler equation that didn't contain a logarithmic term. By the time he contacted the publisher, however, the paper was already in print, and the best he could do was publish a short letter promising to run a new series of experiments. Ohm stated he would show that the amount of current flowing through a circuit goes to zero as the length of the conductor approaches infinity. This bit of mathematical talk constituted his second mistake - a political one in this case. His letter infuriated most scientists of the time because they firmly believed the only proper scientific procedure was to gather mountains of data before playing with any kind of equation.
Ohm's incorrect equation was the result of a widespread lack of knowledge about the basic theory of batteries. After it was too late to stop publication of his paper, Ohm realized he had used an unstable power source - one whose output voltage varied with the amount of loading.
Poggendorff, one of Ohm's few allies in the scientific community, suggested he use a Seebeck thermoelectric battery rather than Volta's hydro-electric battery.
The thermoelectric battery was the first practical device to take advantage of the thermoelectric effect discovered by Seebeck in 1821. The Seebeck effect makes two unlike, tightly bonded conductors produce an electrical potential when one of them is heated. The output voltage is small, but so is the internal resistance. So, Ohm repeated all his experiments using the stable thermo-electric battery and galvanoscope. The equation we now know as Ohm's Law fit the data from his new series of experiments.
In 1826, Ohm was ready to show the world he knew what he was talking about. His second paper was entitled "Determination of the Law According to which Metals Conduct Contact Electricity, Together with the Outlines of a Theory of Volta's Apparatus and the Schweigger Galvanoscope." The corrected equation read, X = a/(b + x); where X represents the amount of current flow through the conductor, a stands for the exciting voltage, x is the resistance of the conductor under test, and b is the combined internal resistance of the power source and galvanoscope.
In the early part of 1827, Ohm published yet a third milestone paper in the history of science called "The Galvanic Battery Treated Mathematically." He then believed he had completely vindicated himself for proposing an incorrect equation and was confident that his colleagues would finally accept his law of electrical conduction.
The scientific community, however, was still not ready to accept Ohm and his works. For one thing, the equation seemed too simple - far too simple to explain a phenomenon that had been challenging the best minds of Europe and America for nearly thirty years. Then, of course, there was Ohm's widely misunderstood statements in the letter following his first paper. Most reputable scientists still considered Ohm a quack. Bitter and disappointed, Ohm returned to his teaching profession.
Six years passed before a few influential scientists began taking serious looks at Ohm's work. The incident that touched off this mild renewal of interest was a paper published by Pouillet in 1831. Pouillet had unwittingly repeated Ohm's work, and he had arrived at exactly the same results. Pouillet believed he was the founder of the law of electrical conduction, and so did most of the scientists of the time. Several scientists, however, noted a strong similarity between Ohm's work and Pouillet's paper.
In 1841, sixteen years after Ohm announced his law of electrical conduction, the British Royal Society presented him the Coply gold medal for "the most conspicuous discovery in the domain of exact investigation." Ohm thus received proper credit for his work, a formal apology for the delay, and a well-deserved round of applause from his peers.
Ohm died in 1854; and, exactly ten years later, the British Association for the Advancement of Science adopted the ohm as the unit of measure for electrical resistance. Thus Ohm (like Ampere and Volta) is now immortalized in the everyday language of modern electrical engineers and technicians everywhere.
Posted October 11, 2017