During the 1920s Vannevar Bush and a team of scientists at Massachusetts Institute of Technology worked on a "differential analyzer" — the first modern analog computer. As their name suggests, analog, computers work by physical analogy. For example, the spaces on a slide, rule, which is a simple analog computer, correspond to numerical values. In contrast a digital computer, of which the abacus is the earliest and simplest example, works by counting discrete units. The modern electronic computers that started coming into use after World War II are digital. Bush needed an analog computer to help him in his research on electric power transmission, which required measurement of continuously varying "electrical currents flowing in a power grid — a time-consuming task because it involved solving high-order differential equations by hand. "Electrically operated, not electronic, the machine Bush and his colleagues completed in 1928 was a complicated mechanical...