He returned to the United States and was employed at the Research Laboratory of the Western Electric Company. In 1915 he was in charge of radio receiver development for the Bell System transatlantic radiotelephone tests. For this he developed the Hartley oscillator and also a neutralizing circuit to eliminate triode singing resulting from internal coupling. A patent for the oscillator was filed on June 1, 1915 and awarded on October 26, 1920.
Following the war he returned to Western Electric. He later worked at Bell Laboratories. He performed research on repeaters and voice and carrier transmission and formulated the law "that the total amount of information that can be transmitted is proportional to frequency range transmitted and the time of the transmission." His 1928 paper is considered as "the single most important prerequisite" for Shannon's theory of information. After about 10 years of illness he returned to Bell Labs in 1939 as a consultant.
In spite of his illness during most of the 1930s, Hartley had formed a theoretical and experimental research group at Bell Laboratories starting in 1929 to investigate nonlinear oscillations and what later became known as parametric amplifiers. This research was mostly parallel to the work being done at the same time in Soviet Russia by Leonid Mandelstam and in Europe by Balthasar van der Pol. A short review and extensive bibliography was published by Mumford in 1960.
The Bell Laboratories work was carried on under Hartley's guidance during the 1930s and 1940s by John Burton and Eugene Peterson (who themselves had begun investigations of nonlinear circuits as far back as 1917 when they had observed unusual characteristics while working with E.F.W. Alexanderson's magnetic modulators, an early form of magnetic amplifier). Peterson later got John Manley
and Harrison Rowe involved in this line of research during the 1940s which culminated in the now famous Manley–Rowe relations and several papers by the latter two authors on the topic of parametric circuits in the mid 1950s.
Hartley, R.V.L., "Information Theory of The Fourier Analysis and Wave Mechanics", August 10, 1955, publication information unknown.
Hartley, R.V.L., "The Mass of a Wave Particle", July 30, 1955, unpublished manuscript, copies available from the Niels Bohr Library & Archives, American Institute of Physics, College Park MD
Hartley, R.V.L., "The Mechanism of Gravitation", January 11, 1956, unpublished manuscript; copies available from the Niels Bohr Library & Archives, American Institute of Physics, College Park MD
Hartley, R.V.L., "The Mechanism of Electricity and Magnetism", June 14, 1956, unpublished manuscript, copies available from the Niels Bohr Library & Archives, American Institute of Physics, College Park MD
^Mandelstam L.I., Papalexi N., Andronov A., Chaikin S., Witt A., "Report on Recent Research on Nonlinear Oscillations", Technical Physics in the USSR, Leningrad, Volume 2, Numbers 2-3, 1935, pp 81 – 134, (NASA Translation Doc TTF-12,678; November 1969)