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awkward to program and maintain. Shannon concluded that its op- erations could be better accomplished with electricity. He had taken a course in Boolean algebra, and in a landmark 1938 paper adapted from his masters thesis,pointed out that a collection of on-off switches arranged according to Boolean principles could carry out logical and mathematical operations. (Shannon was later to say,It just happened that no one else was familiar with both fields at the same time.He went on to Bell Telephone Laboratories, where in 1948 he wrote another seminal paper,A Mathematical Theory of Communicationthat laid the basis for information theory.)

The only drawback to this scheme was that it forced the com- puter to operate with a binary number system rather than the familiar decimal one.That was the birth of the binary digit or bit, which takes on only a value of 0 or 1. Numbers are very long in this system: for instance, the decimal number 31 becomes the binary number 11111. But the advantages of working with a two-state electrical system far outweighed this slight complication, and the computer could always be programmed to deal with input and output in the decimal form favored by humans.

Then it became a matter of engineering to implement Shannons ideas.The first programmable binary calculator was built in 1938 by Konrad Zuse in Berlin, as a mechanical device to illustrate the prin- ciple.This was followed in the 1940s by electric Boolean computers, some of which used electromechanical relays, on-off switches that operate by remote control. An electric current is sent through a coil of wire, producing a magnetic field that pulls a metal finger so that it makes or breaks an electrical circuit.

Relays had been highly developed for telephone networks, which require myriads of choices to route calls, and an early relay-based computer was built at Bell Labs. In 1941, Zuse built an electrical version that worked much faster than a mechanical unit, but in one way, the machine was inferior to Babbages ideal machineit could not perform conditional jumps. The ultimate relay-based computer was the Harvard-IBM Automatic Sequence Controlled Calculator (Mark I), built at Harvard in 1943.This enormous machine, which

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