In 1946 two undergraduates at Harvard University, Theodore A. Kalin and William Burkhart, were taking a course in mathematical logic. They noticed that there were a large number of truth tables to be worked out. To work them out took time and effort and yet was a rather tiresome automatic process not requiring much thinking. They had had some experience with electrical circuits. Knowing of Shannon’s work, they said to each other, “Why not build an electrical machine to calculate truth tables?” They took about two months to decide on the essential design of the machine: 1. The machine would have dial switches in which logical connectives would be entered. 2. It would have dial switches in which the numbers of statements like 1, 2, 3 ··· would be entered. 3. It would scan the proper truth table line by line by sending electrical pulses through the dial switches. 4. It would compute the truth or falsehood of the whole expression. CONSTRUCTION AND COMPLETION OF THE KALIN-BURKHART LOGICAL-TRUTH CALCULATOR With the designs in mind, Kalin and Burkhart bought some war surplus materials, including relays, switches, wires, lights, and a metal box about 30 inches long by 16 inches tall, and 13 inches deep. From March to June, 1947, they constructed a machine in their spare time, assembling and mounting the parts inside the box. The total cost of materials was about $150. In June the machine was demonstrated in Cambridge, Mass., before several logicians and engineers, and in August it was moved for some months to the office of a life insurance company. There some study was made of the possible application of the machine in drafting contracts and rules. GENERAL ORGANIZATION OF THE MACHINE The logical-truth calculator built by Kalin and Burkhart is not giant in size, although giant in capacity. Like other mechanical brains, the machine is made up of many pieces of a rather small number of different kinds of parts. The machine contains about 45 dial switches, 23 snap switches (or two-position switches), 85 relays, 6 push buttons, less than a mile of wire, etc. The lid of the metal box is the front, vertical panel of the machine. UNITS OF THE MACHINE The machine contains 16 units. These units are listed in Table 11, in approximately the order in which they appear on the front panel of the machine—row by row from top to bottom, and from left to right in each row. Table 11 UNITS, THEIR NAMES, AND SIGNIFICANCE UNIT ROW PART NO. MARK NAME SIGNIFICANCE