Historians disagree about how to categorize the machine pictured above. Was it the first computer? A mammoth mechanical calculator? But there is no question that the differential analyzer-shown with its inventor, MIT’s Vannevar Bush-was a crucial player in the history of computing.

Bush designed the device to solve differential equations in an attempt to model the rapidly expanding power systems of the 1920s. When completed in 1931, the analyzer used electric motors to drive shafts and gears that represented each term in a complicated equation.

The differential analyzer showed the world that machines were suited not just for physical labor, but for mental labor as well. It alerted researchers and funders to the profound possibilities of computers. Bush became director of the Office of Scientific Research and Development-precursor to the National Science Foundation. But though his vision helped shape the government’s attitude toward science, Bush had a blind spot when it came to the digital revolution: He refused to fund early projects in digital computing, including the University of Pennsylvania’s landmark ENIAC. And though the differential analyzer was, in Bush’s words, “the first of the great family of modern analytical machines to appear-the computers, in ordinary parlance,” today’s PCs aren’t direct descendants of his contraption. Bush’s was an analog machine; it represented numbers with physical qualities that vary continuously-distance, rotation and so forth-rather than with the discrete 1s and 0s of digital devices.

Still, the differential analyzer was a critical, if inadvertent, midwife to the birth of digital technology. While laboring over the machine and observing the logic of its action, one of Bush’s students-a mathematician named Claude Shannon -began thinking of new ways to build circuits. Shannon realized that the “true” and “false” of Boolean algebra could be represented by the “on” and “off” positions of an electrical switch-in other words, he came up with the idea of a bit.

Shannon’s work with the differential analyzer led to a thesis published in 1938 that has been called “one of the most important master’s theses ever written.” In it, Shannon laid out the logic upon which all digital circuits are now based.