Leon Charles Thevenin was a French telegraph engineer who worked on Ohm's law and extended it to the analysis of complicated electrical networks. He is remembered today almost entirely for one small piece of work. His theorem, published in 1883, was based on his study of Kirchhoff's Laws and is found in every basic textbook on electrical circuits. It has made his name familiar to every student of electrical circuits and to every electrical and electronics engineer.

Leon Charles Thevenin was born at Meaux just outside Paris on March 30, 1857. He graduated from the Ecole Poly technique in 1876 and two years later joined the Corps of Telegraph Engineers. The public telegraph service was to be his working life until his retirement in 1914 on the eve of the first World War. During those 36 years he showed himself to be a great engineer, an excellent administrator and, perhaps first and foremost, a teacher. He continued some of his teaching duties to the end of his life.

At the start of his career, Thevenin joined the department responsible for long-distance underground telegraph lines which was then vastly expanding its service and requiring most of the newly trained young engineers. But he did not stay there long. His unusual talents were recognized and, he moved to the Department of Materials and Construction which had started to tackle the problems involved in the construction of power lines. His standardization of the rules for the erection of overhead power lines stayed in force for many years.

### Thevenin's theorem

Thevenin's theorem states that a linear two-terminal circuit can be replaced
by an equivalent circuit consisting of a voltage source $V_{Th}$ in series with
a resistor $R_{Th}$, where $V_{Th}$ is the open-circuit voltage at the terminals
and $R_{Th}$ is the input or equivalent resistance at the terminals when
the independent sources are turned off

### Teaching

In 1882 Thevenin was asked to take on the job of teaching the young inspectors of the engineering department at the Ecole Superieure. This was the start of his teaching career and his introduction to the work that led to his famous theorem.

He developed an interest in electrical measurement and, with his former teacher Jules Raynaud, he translated a British work on units and physical constants into French. Translation of such foreign publications was part of the routine work of the School. In conjunction with this work, Thevenin made a very careful study of Kirchhoff's Laws and discovered the rule which he then expressed in his theorem, having proved it by a clever application of the already established Superposition Principle.

**Thevenin's Theorem** was published in three separate scientific journals in 1883 in a paper entitled "

*Extension of Ohm's Law to complex electrical circuits*". It was introduced as a "

*new theorem of dynamic electricity*" and gave a simple method of calculating the current that would flow in a new conductor when it was added to an existing network.
Nowadays it is expressed rather differently (in terms of an equivalent circuit consisting of a voltage source and a series resistor) but it is the same theorem. It was Thevenin's first article and appeared in the same year as the publication of the joint translation with Raynaud.

Three further articles followed in that year. The first gave a method of using a galvanometer to measure potential, and made use of the new theorem. The second described a method for measuring resistance, and the third was on the use of the Wheatstone Bridge.

### Launch of Thevenin Theorem

Publication of the theorem in three journals gave it a good launch, but Thevenin also taught it himself in his courses to telegraph engineers in France. By 1889, a century ago, others were already writing of it as the "theorem de Thevenin". It is an early example of practical engineering theory, in this case telegraph theory, being originated by an engineer and taught by an engineering school quite outside the scientific tradition of mathematical physics.

All was not, however, without problems. Thevenin reported his discovery to the French Academy of Sciences but first he disclosed it to another French telegraph engineer whom he deeply admired, A. Vaschy. Vaschy found the concept attractive but thought the theorem was wrong. Others were consulted and controversy grew as to whether it was right or wrong.

Though Thevenin produced a rush of publications3 in 1883, he seems to have published nothing thereafter. Yet his career continued to advance and his teaching skills were sought outside the PTT. In 1885 he was asked to teach a course in industrial tools, and later one on industrial electrical engineering, at a school of commerce. The Institute National Agronomique employed him from 1891 to teach mechanics, and later to lead seminars in applied mathematics. He continued all of these teaching appointments until his death in 1926.

He had already proved himself as head of the Bureau des Lignes (where he improved and unified the construction of lines and personally supervised the implementation of his policies) when in 1896 he was appointed director of the telegraph engineering school. It was a job which brought him immense satisfaction.
Having no ambition to rise further he had almost to be prised out of that position in 1901 to take over as engineer-in-chief of the workshops, a position he held with distinction until his retirement in January 1914.

### A Crucial Theorem

His theorem is now a fundamental part of the theory of electrical engineering and was crucial in developing transmission network theory. It was to prove of immense practical value to engineers. It is now usually taught alongside its complementary theorem, Norton's Theorem, which dates from 1926 - the year Thevenin died. However, both theorems, are said to have been anticipated by the German physicist Helmholtz in l853.

Thevenin remained a bachelor for life, but provided a home for his mother's widowed cousin and her two children. Later he adopted the children.

His favorite recreation was angling and he owned a boat which he used on the River Marne for fishing. His students at the Institute Agronomique nicknamed him The Admiral. He was also a talented violinist but played only in private.

Late in 1926, Thevenin was taken to Paris for medical treatment and it was there that he died on September 21. A kindly man of simple tastes, Thevenin had requested that only his family should attend the cemetery and that a single rose from his garden should decorate his coffin. So it was when he was buried in his home town of Meaux.

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