Scanning the Past: A History of Electrical Engineering from the Past

Submitted by Dick Reiman, Historian

Copyright 1992 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. 80, No 5, May 1992

 

George A. Campbell and the Electric Wave Filter

 

It was 75 years ago this month that George A. Campbell was awarded two patents on the electric wave filter. One of the patents covered the principle of low‑pass, high‑pass, and band‑pass filters consisting of inductors and capacitors, while the other concerned application of filters to telephone amplifiers. Wave filters had been used for several years before the basic patents were issued, and the importance of Campbell's discovery was already evident to engineers at the American Telephone and Telegraph Company (AT&T) in May 1917.

 

Campbell was born in 1870 in Hastings, Minnesota. He graduated in civil engineering from the Massachusetts Institute of Technology in 1891 and then went to Harvard University where he received a B.A. in 1892 and an M.A. in 1893. A fellowship enabled him to continue his education in mathematics and physics at several universities in Europe until 1897, at which time he joined the mechanical depart­ment of the American Bell Telephone Company in Boston. Later, in 1901, he received a doctoral degree from Harvard.

 

Campbell's discovery of the electric wave filter grew out of his earlier work on loading coils for telephony. Drawing on his expert knowledge of Oliver Heaviside's transmission line theory, Campbell proposed adding coils at specified intervals along a telephone line as a way to reduce attenuation and distortion of signals. During 1899, he derived a formula for a loaded line which became known as "Campbell's equation." The equation enabled him to calculate the optimum size and spacing of loading coils. Laboratory tests confirmed his analysis by September 1899, and the first use of loading coils for commercial traffic occurred in May 1900. He wrote his doctoral thesis on loading and published a technical paper on loading in 1903. The innovation saved AT&T an estimated one hundred million dollars by 1925. Campbell's priority in the invention of the loading coil was challenged by Michael I. Pupin, a professor at Columbia University, who applied for a patent on the invention in 1900. An interference proceeding eventually awarded the priority to Pupin, who established that he had disclosed the invention two days earlier than Campbell.

 

Campbell's analysis of the loaded line had revealed that it behaved as a low‑pass filter which would reject signals above a critical frequency. In an internal memorandum written in 1909, he discussed band‑pass filters which would transmit a narrow band of frequencies and reject other fre­quencies. In another memorandum written in March 1910, he provided a comprehensive discussion of the electric wave filter along with circuit diagrams and attenuation ver­sus frequency graphs. Campbell's memorandum provided the basis for a patent application, but it was abandoned in 1913 when the AT&T patent attorneys and the patent examiner failed to appreciate the novelty of the invention. However the increasing importance of the filter to the tele­phone company stimulated a renewed effort to obtain patent coverage. This time the application, filed in 1915, proved successful. Campbell published a paper on the theory of the wave filter in 1922 in The Bell System Technical Journal. The editor commented, in an introduction to the paper, that the wave filter already had become "indispensable in many branches of electrical communication." Vannevar Bush later wrote that Campbell's 1922 paper had "marked the advent of a new understanding of the subject."

 

Campbell received the Medal of Honor of the Institute of Radio Engineers in 1936 and the Edison Medal of the American Institute of Electrical Engineers in 1940. He retired from AT&T in 1935 and the company published a book of his collected technical papers and some of his previ­ously unpublished technical memoranda in 1937. He died in November 1954. An obituary characterized him as "a gentle and retiring man" with a "spirit of inquiry that left its endur­ing impact on the character of the Bell Telephone System."

James E. Brittain

School of History, Technology, and Society

Georgia Institute of Technology