Opening Black's Box: Rethinking Feedback's Myth of Origin

The specific triumph of the technical imagination rested on the ability to dissociate lifting power from the arm and create a crane: to dissociate work from the action of men and animals and create the water-mill: to dissociate light from the combustion of wood and oil and create the electric lamp.

--Lewis Mumford

The engineer who embarks on the design of a feedback amplifier must be a creature of mixed emotions.

--Hendrik Bode

Like any modern episteme worthy of the name, the theory of feedback has a myth of origin. On a sunny August morning in 1927, Harold Black, a twenty-nine-year-old systems engineer, rode the Lackawanna ferry to work at the Bell Telephone Laboratories. Many Bell engineers lived in New Jersey, and on the early morning ferry rides across the Hudson to the Manhattan laboratories they frequently gathered on the forward deck. This morning Black stood alone, staring at the Statue of Liberty, and had an epiphany: "I suddenly realized that if I fed the amplifier output back to the input, in reverse phase, and kept the device from oscillating (singing, as we called it then), I would have exactly what I wanted: a means of canceling out the distortion in the output." ¹ As it happened, the New York Times that day contained a blank page, and Black sketched his idea, "a simple canonical diagram of a negative feedback amplifier plus the equations for the amplification with feedback." He rushed into work, asked a technician to wire up a prototype, and gave birth to a foundational circuit of modern electronics. This story has become enshrined as one of the central "flashes of insight" in electrical engineering in this century, periodically retold as an inspiration for engineers. ² A common textbook on control engineering reprints the story of Black's vision verbatim in the first chapter. ³

At Bell Laboratories from 1927 to 1940, the legend goes, Black, Harry Nyquist, and Hendrik Bode laid the foundations of feedback control that engineers then applied to all types of closed-loop systems, from servomechanisms to thermostats, fire control systems to automatic computers. ⁴ More than other contemporary narratives of control systems such as automatic pilots or servomechanisms, this story of feedback earned a place in engineering legend and college textbooks. It produced design methods and graphical techniques that carry their author's names (the Bode plot, the Nyquist diagram) and earned telephone engineering a claim to priority in feedback history. Feedback theory, moreover, formed the basis of cybernetics, systems theory, and a host of other post-World War II information sciences, so Black's invention is hailed as a foundation of the information age.

Feedback is indeed a fundamental concept in twentieth-century technology, and the Bell Labs feedback theorists did lay critical foundations for it. But the origin myth effaces its sources. It skips over the inventors themselves and the ways in which their backgrounds and prior experience influenced their work. It reveals little about the concrete problems these men worked on when they produced their solutions. The story also removes feedback theory from its engineering culture, that of the telephone network between the world wars. Black's version also does not account for the relationship of his feedback amplifiers to prior traditions of governors and self-regulating machinery.

Thus a reexamination of the sources is in order, retelling Black's legend not as a heroic tale but as the story of an engineer solving the technical problems of a particular place and time and trying to convince others to support his solutions. As it turns out, Black did not understand as much about feedback as he later recalled. To make his idea credible, he needed Nyquist's reformulation of the problem of stability and Bode's analysis outlining the tight constraints that a feedback amplifier had to meet. He also needed the Bell System. Negative-feedback...