DECIMAL DIGIT RELAY CODE 0 0 0 1 1 1 0 1 0 0 2 0 1 0 1 3 0 1 1 0 4 0 1 1 1 5 1 0 0 0 6 1 0 0 1 7 1 0 1 0 8 1 0 1 1 9 1 1 0 0 With regular telephone relays and regular telephone company techniques, Dr. Stibitz and Bell Telephone Laboratories designed and constructed the machine. It was called the _Complex Computer_ and was built just for multiplying and dividing complex numbers. Six or eight panels of relays and wires were in one room. Two floors away, some of the girl computers sat in another room, where one of the teletypewriters of the machine was located. When they wished, they could type into the machine’s teletypewriter the numbers to be multiplied or divided. In a few seconds back would come the answer. In fact, there were two more computing rooms where teletypewriters of the machine were stationed. To prevent conflicts between stations, the machine had a circuit like the busy signal from a telephone. In 1940, a demonstration of the Complex Computer took place: the computing panels remained in New York, but the teletypewriter input-output station was set up at Dartmouth College in Hanover, N. H. Mathematicians gave problems to the machine in Dartmouth, it solved them in New York, and it reported the answers in Dartmouth. Special-Purpose Computers With this as a beginning, Bell Laboratories developed another machine for a wide variety of mathematical processes called _interpolating_ (see Supplement 2). Then, during World War II, Bell Laboratories made more special-purpose computing machines. They were used in military laboratories charged with testing the accuracy of instruments for controlling the fire of guns. These computers took in a set of gun-aiming directions put out by the _fire-control instrument_ in some test. They also took in the set of observations that went into the fire-control instrument on that test. Then they computed the differences between the gun-aiming produced by the fire-control instrument and the gun-aiming really required by the observations. Using these differences, the fire-control instrument could be adjusted and corrected. These special-purpose computers were also useful in checking the design of new fire-control instruments and in checking changes due to new types of guns or explosives. Regularly, after each of these special-purpose computers was finished, people began to put other problems on it. It seemed to be fated that, as soon as you had made a machine for one purpose, you wanted to use it for something else. Accordingly, in 1944, two agencies of the U. S. Government together made a contract with Bell Telephone Laboratories for two general-purpose relay computers. These two machines were finished in 1946 and are twins. ORGANIZATION OF THE GENERAL-PURPOSE COMPUTER