He returned to the United States and was employed at the Research Laboratory of the Western Electric Company.

For this he developed the Hartley oscillator and also a neutralizing circuit to eliminate triode singing resulting from internal coupling.

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[3][4] 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[5] and in Europe by Balthasar van der Pol.

Peterson later got John Manley[12][13][14][15] 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[16][17][18] on the topic of parametric circuits in the mid 1950s.