Leon Charles Thevenin

(1857-1926)

Thevenin's theorem is known and used throughout the world, but as is usual with such things, little thought is now given to the man whose name it bears. He has been described as a humble man and a model engineer and employee. He was hard working, held strict principles, was scrupulously moral and kind at heart. That alone would make a wonderful epitaph.

He is remembered today almost entirely for one small piece of work. His theorem, published in 1883, was based on his study of Kirchoff”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 Polytechnique 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 [3], 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 leaving the Ecole Superieure [4]. But he did not stay there long. His unusual talents were recognised 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 standardisation of the rules for the erection of overhead power lines stayed in force for many years.

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 Kirchoff’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.

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