Think back to when oscillators were something new to you and exciting and you will probably recall the names of two of the great telephone engineers who worked for the Bell Telephone system in the first half of this century: Colpitts and Hartley. The Colpitts and Hartley oscillators have survived long after their inventors died. Today they are transistorised and can be stabilised by quartz crystals, but they are still recognisable as the fundamental circuits invented during the First World War.
Edwin Henry Colpitts was born on January 9, 1872, at Pointe de Bute in Canada, though he was sometimes described by his contemporaries as an honorary American. A bachelors degree with honours from Mount Allison University, Sackville, New Brunswick, was followed by another at Harvard in 1896, and a masters the next year after completing a postgraduate course in physics, mathematics and engineering. He continued at Harvard for another two years as an assistant to Professor Trowbridge in the Physics Laboratory, until joining the American Bell Telephone Co in 1899.
For the next 38 years he served the Bell System in one capacity or another1, steadily rising through the ranks until he retired in 1937 as Vice-President of Bell Telephone Laboratories. But it was up to about 1920 that most of his efforts were directly involved with hands-on engineering or scientific contributions to telephone engineering. From then until his retirement he held positions which were mostly executive, leaving little time for direct contributions to engineering. When Colpitts began his career with Bell, telephones were no longer new, though much of the science-based work which led to vastly improved performance still lay in the future.
In 1924 Frank Jewett, Chief Engineer at Western Electric, wrote:"Almost from the day he entered the modest laboratories in Boston, Colpitts took a prominent part in the solution of the problems which were to revolutionise telephonic and telegraphic communication." Colpitts, he continued, was a central figure in the fundamental work which lifted the art of telephone engineering to a higher plane and established it as a science on a firm foundation of exact knowledge.
As well as exact knowledge, exact measurements were now required. The methods and instruments which George A Campbell and Colpitts developed for measuring high-frequency alternating currents were to play a big part in the development of telephony. Colpitts himself "made many of the first high-frequency determinations on lines and apparatus".
In this period up to 1907, under Campbell's direction, Colpitts worked long and hard on the development and application of loading coils to open wire and cable circuits, first suggested by Heaviside but patented by Pupin in 1899. "To those of us who were privileged to witness and occasionally to take part in this work there comes to mind a picture of Colpitts in the early morning hours hard at work in some test room or far afield in sunshine or storm2 on a line inspection," wrote Jewett.
Jewett also recorded for us the contribution that Colpitts made to solving, what he called, the threat to the very existence of long distance and even local telephone services in the first few years of this century. That threat was the inductive interference brought about by the introduction of alternating current for the propulsion of trolley buses and trains.
Colpitts threw himself 3 into the work, "sometimes in the laboratory, but now more frequently in rough clothes in the mountains of Pennsylvania or the brush of Georgia4 or in rubber boots in the winter mud of Indiana." In the end solutions to the problems were found by joint collaboration with Westing-house power engineers. Jewett credited Colpitts "in large measure" with helping bring about 5 the closer understanding between communication and power engineers which came about in the first two decades of the century.
In 1907 the Bell System was reorganised and Colpitts moved to the Western Electric Company (part of Bell) in New York. There he began his climb up the administrative ladder6, first as head of the Physical Laboratory, later as Director of the Research Laboratories (1911) and eventually as Assistant Chief Engineer (1917). The technical highlights of that period for Bell were the transcontinental telephone line of 1914, the transatlantic radio telephone experiments of 1915 and of course the introduction of the thermionic valve to experimental and practical engineering. Colpitts was involved with them all.
When he transferred back to AT&T in 1924 as the newly appointed Assistant Vice-President (Development and Research) he was described as possessing a keen analytical ability, the creative imagination of the thoroughly trained physicist, a direct approach, and an integrity of intellect. These personal characteristics received even further career reward in 1934 when Bell's research laboratories merged and he became Vice-President of the Bell Telephone Laboratories.
The Colpitts oscillator is one of the standard circuits of electronics and has been such almost from the day of its invention. Research engineers at Bell began their development work of the new triode (De Forest's audion) and its use in circuitry in 1912 when AT&T paid De Forest $50,000 for the right to use the triode as a telephone repeater. In 1914 they paid another $90,000 for the radio receiver rights to the triode, marking the start of electronic circuits.
Progress was rapid in the early years. The first important circuit was the positive-feedback, or regenerative, amplifier and 1913 saw many claimants to its invention7 in both America and Europe. Patent litigation in America dragged on for 20 years. But Colpitts invented one of the most famous and enduring of electronic circuits - the push-pull amplifier - on November 4, 1912.
Two years later came a circuit for producing and modulating high-frequency oscillations. This was an extension of work performed by G A Campbell, also at Bell, to discover the causes of "singing" in telephone amplifiers. Then, in March 1915, came the Colpitts oscillator, a month after Hartley had revealed the circuit named after him.
In the Second World War he was recalled from retirement to work on submarine warfare8, specifically on echo ranging systems and attack directors as "Head Technical Aide" of the National Defense Research Committee. For this effort, on April 5, 1948, he received the Medal for Merit, the USA's highest civilian award. The citation stated that it was for " outstanding services to the United States from June 1940 to June 1946". It seems ironic that before the war he had received a Japanese award, the Order of the Rising Sun, for a series of lectures he gave in Japan for the Iwadare Foundation.
In 1941 he also found time to accept the position of Director of the Engineering Foundation. This body, a joint agency of four US engineering societies, had been set up in 1914 for "the furtherance of research9 in science and engineering and the advancement of engineering and the good of mankind". It would seem that a man of Colpitts's calibre does not retire easily.
Colpitts died at the age of 77 at his home in Orange, New Jersey, USA, on March 6, 1949, after a lengthy illness and was survived by his second wife, Surah Grace. His first wife, Annie Dove Penney, whom he married in 1899, died in 1940. He was also survived by a son from the first marriage and by three brothers.
But long before his death he had been honoured as a distinguished telephone engineer of the pioneering period of continental and transcontinental wire and radio telephony, a respected administrator, holder of 24 patents, and a member of the relevant engineering and scientific institutes10 of America.
Speak on engineering problems Colpitts had to deal with while working for Bell Telephone Laboratories.
Tell the history of Colpitts’s biggest invention.
Describe Colpitts’s oscillator.
( 1889 – 1970 )
No account of Colpitts's work would be complete without a mention of Ralph Hartley, who invented the complementary oscillator.
Hartley graduated from the University of Utah in 1909 to become a Rhodes Scholar 11 at Oxford, graduating with a BAin 1912 and a BSc in 1913. He joined the laboratories of Western Electric in September 1913 and was in charge of early development of radio receivers for Bell System's radio telephone tests of 1915. By then he must have met Colpitts who was in charge of research there. It was at this time of radio receiver design, in a period of rapid circuit development, that Hartley revealed the oscillator named after him on February 10, 1915.
During the First World War Hartley suggested that the human sense of direction is perceived by the phase difference between sound waves reaching the two ears, one set of waves having to travel further than the other. After the war his interests turned more towards voice and carrier transmission and telephone repeaters, first at Western Electric and then at Bell Telephone Labs.
Hartley is also remembered for his major contributions to Information Theory. He was the first to state the law named after him relating information to bandwidth and time: "The total amount of information which may be transmitted over a system is proportional to the product of the frequency range which it transmits by the time during which it is available for the transmission." This was first published in February 1926 although he had been working on it for several years.
A fuller account, "Transmissions of Information", was given at an international meeting in Italy in 1927 and further published in 1928. Hartley's work on Information Theory followed that of Nyquist and "provided the guiding rules for transmission engineers for 20 years" until the next major advance in 1948 when Claude Shannon included the effects of noise in the system.
In 1929, at the age of 40, illness forced Ralph Hartley to give up work and it was ten years before he could return, as a consultant on transmission problems. During the Second World War he worked on various projects, most notably on servo-mechanisms for radar and fire control systems.
He retired from Bell Laboratories in 1950, holding 72 patents, and lived with his wife at Summit, New Jersey. He died at the ripe old age12of 81 on May 1, 1970.
Tell about Hartley’s contribution to Information Theory.
Explain Hartley’s law.
Speak on Hartley’s scientific interests and inventions.