May 2009 – History

Scanning the Past: A History of Electrical Engineering from the Past
Submitted by Bob Morrison, Editor

Copyright 1996 IEEE. Reprinted with permission from the IEEE publication, “Scanning the Past” which covers a reprint of an article appearing in the Proceedings of the IEEE Vol. 84, No. 1, January 1996.

Edith Clarke and Power System Stability

Fifty-five years ago this month, Edith Clarke and Seldon B. Crary presented a joint paper on stability problems in power engineering at a meeting of the American Institute of Electrical Engineers (AIEE) in Philadelphia. At the time, both Clarke and Crary were employed as engineers by the General Electric Company (GE) and their paper received a prize as the best AIEE paper published that year. Clarke had overcome many difficulties on her way to becoming a highly respected member of the electrical engineering profession. She specialized in the analysis of electrical power systems and devoted her career to the simplification and mechanization of problem solving in that field.

Clarke was born in Maryland in 1883 and was orphaned at a young age. She used a small inheritance to enable her to enroll at Vassar College in Poughkeepsie, NY, where she studied mathematics and astronomy and received an A.B. degree in 1908. She then taught for a year at a school for girls in San Francisco and taught mathematics for two additional years at Marshall College in Huntington, WV. In the fall of 1911, she enrolled in the civil engineering program at the University of Wisconsin, but left after a year to become a computing assistant to George A. Campbell at the American Telephone and Telegraph Company (AT&T). She learned a great deal about the theory of transmission lines and electric circuits from Campbell before she enrolled in electrical engineering at the Massachusetts Institute of Technology (MIT) in 1918. She earned an M.S. degree ftom MIT in 1919 and was the first woman to receive an electrical engineering degree at the school.

From 1919 to 1921, Clarke worked for GE in Schenectady, NY , where she trained and directed a small group of women computers doing calculations of mechanical stresses in turbine rotors. Despite her credentials, she did not yet enjoy the status or salary of an engineer at GE. In 1921, she filed a successful patent application on a graphical calculator to be used in solving transmission line problems and it was the subject of her first technical paper published in the GE Review in 1923. She left GE in 1921 to accept a one-year teaching job at a women’s college in Turkey, but returned the following year to become a salaried electrical engineer in the Central Station Engineering Department at GE.

In February 1926, Clark became the first woman to present an AIEE paper, later published in the Transactions of the AIEE. In the paper, she explained how equivalent circuits could be used in calculating the stability of power systems. In March 1931, she presented an AIEE paper on the method of symmetrical components. This method, developed earlier by C. L. Fortescue and others, facilitated the analysis of unbalanced three-phase circuits by converting a problem into equivalent symmetrical circuits which were simpler to analyze. In her paper, Clarke extended the method to the analysis of two or more simultaneous faults and she mentioned that a calculating table or an electro-mechanical network analyzer could be used effectively in solving such problems. A discussant of her paper called it “an important step forward in protection engineering.” Clarke went on to contribute to the development of the method of modified symmetrical components and published a tutorial paper on the subject in 1938.

In 1943, Clarke published a book entitled Circuit Analysis of A-C Power Systems, based on her notes for lectures to GE engineers and intended for use as a textbook in engineering schools or as a reference book for power engineers. The principal focus of the book was on the use of symmetrical components to solve problems related to polyphase power generation and transmission. She later published a second volume on the subject in 1950. Clarke retired from GE in 1945 and joined the electrical engineering faculty at the University of Texas in 1947 where she taught until 1956. In 1948, she became the first woman to be elected a Fellow of the AIEE. She returned to her native Maryland in 1956 where she died in 1959 at age 76.

James E. Brittain
School of History , Technology and Society
Georgia Institute of Technology