Drawing on the archives of American learning psychologist Neal E. Miller, this article investigates the role of instrumentation in the expansion and diversification of the behavior therapy domain from the late 1960s to the early 1990s. Through the case of Miller's research on the use of biofeedback to treat idiopathic scoliosis, it argues that the post‐World War II adoption of electronic technology by behavioral psychologists contributed to extending their subject matter to include physiological processes and somatic conditions. It also enabled a technologically‐instrumented move outside the laboratory through the development of portable ambulatory treatment devices. Using the example of the Posture‐Training Device that Miller and his collaborators invented for the behavioral treatment of idiopathic scoliosis, this paper considers how electromechanical psychological instrumentation illustrated a larger and ambiguous strategic shift in behavior therapy from an orientation toward external control to one of self‐control.