Bell Laboratories Relay Computers
The Bell Laboratories relay computers were a series of computing machines built between 1939 and 1949. They were electromechanical, meaning that the actual computations were carried out by electrically-driven moving parts.
The Bell relay computers had their genesis in the mind of mathematician George Stibitz. In late 1937, while working for Bell Labs in New Jersey, Stibitz began researching the design of telephone relays (electrically powered switches). These relays had metal contacts that were opened and shut by a magnetic field generated by a current-bearing coil and were used to route, or "relay," telephone calls. When a relay was closed, an electrical signal (e.g., a telephone call) would be conducted through the closed metal contacts; when the relay was open, no signal could pass.
Stibitz was intrigued by the possibility of using relays to perform calculations. In particular, he noted that the on-and-off switching action of relays could be matched to the two digits (1 and 0) of the binary number system. Stibitz saw that he could construct a binary device to perform addition, subtraction, multiplication, and division. He arbitrarily chose the closed state of a relay to represent 1 and the open state to represent 0.
Towards the end of 1937 Stibitz built a small, crude device to demonstrate the potential of relays to perform binary computations. The prototype, which Stibitz's wife dubbed the "model K" because it was assembled on their kitchen table, was constructed from old coffee cans, plywood, two peanut bulbs (which lit up to display results), and two telephone relays that Stibitz had salvaged from the Bell Labs junk pile. Powered by batteries, the model K successfully added two 1-bit binary numbers to produce a 2-bit output. Stibitz showed the prototype to his colleagues at Bell Labs and suggested a full-blown relay-based calculating machine based upon the model K. Initially, both Stibitz's device and proposal were met with amusement and skepticism. But word of Stibitz's calculating machine spread within Bell Labs, and in the summer of 1938 he was asked to design a more advanced prototype that would be capable of solving complex equations of the sort commonly found in electrical engineering. Working from this more advanced design Bell Labs built the "Bell Telephone Laboratories Model I Complex Calculator."
When completed in late 1939, the Model I could add, subtract, multiply, and divide numbers expressed in binary format. It used 440 electromagnetic relays to perform its calculations. Input to the machine was from a keyboard with keys for the numerals 0 through 9 and for arithmetic operations. A special conversion device took the base 10 keyboard inputs and converted them into binary format for the relay calculations. The Model I could perform ten additions per second. By connecting the computer to similar keyboards in remote locations of Bell Labs, the Model I was established as the world's first "time-share" computer. In September 1940, the Model I was connected via telephone line between its home in New York City and Dartmouth College in Hanover, New Hampshire for a meeting of the American Mathematical Society. The successful demonstration sent keyboard inputs from New Hampshire to the Model I in New York City and received the answers via teletype machine. Another milestone had been achieved: the control of a computer over long-distance telephone lines.
After the Model I, there were to be five additional models of Bell Laboratories relay computers. Bell Labs, however, was not to fund most of them. The $20,000 cost of the Model I had shocked Bell's management and they put on hold any plans for building similar computers. After the advent of World War II the United States government funded further development of Stibitz's machines. (Stibitz served as an unpaid advisor in the development of these subsequent models.)
The Model II was built to test the accuracy of antiaircraft guns. It contained 440 relays to perform binary computations. Input and output to the Model II was via punched paper tape, a technique that would be used on all subsequent models. This worked out well since the speed of input from the tape approximately matched the speed of computation. The Model II contained six "registers" for memory storage, which were used to perform iterative (i.e., repetitive) arithmetic operations. Though built to perform computations related to testing military equipment, the Model II was actually a general-purpose computer that could quickly be turned to wide variety of computational problems. Because of its paper-tape control, the Model II may have been the world's first truly general-purpose computer.
The Model III (completed in 1944) included several improvements over the Model II, such as more computing power (with 1,400 relays), greater memory to retain calculation results, a built-in multiplication table, and the ability to search for stored data while simultaneously performing calculations. The Model IV (completed in 1945) was very similar to the Model III, except that it possessed special circuitry to compute trigonometric functions involving negative angles.
The Model V was the apex of the Bell Labs relay computers. Two Model V computers were built from 1945 to 1947. The Model V was considerably more robust than previous models, with over 9,000 relays and memory space for 30 numbers (Models III and IV could only retain 10 numbers in memory). The Model V also used floating-point arithmetic and had sophisticated error-checking capabilities. Another interesting feature involved the use of several paper tapes, each representing a different problem to be solved. If an error was encountered during the execution of one program-tape, the computer would automatically switch over to another tape and begin computing that program's solution. This feature, which today would be called "multiprogramming," allowed the Model V to calculate unattended during overnight runs, with its operators confident that at least some tapes had been successfully processed. (The earlier Mark III's error-detection circuit had been connected during overnight runs to an electric bell by machine's supervisor's bed.) The last of the Bell Laboratory computers, the Model VI, was built in 1949 for Bell Labs internal use, and was a simplified version of the Model V.
The Bell Laboratories relay computers were reliable machines that incorporated many advanced features for their era, such as binary arithmetic and error-detection capabilities. Later models in the series continued to advance the technology of computing by incorporating floating-point arithmetic and elementary multiprogramming. However, the Bell Labs relay computers incorporated technology that soon became obsolete. Although highly reliable, they were much slower than electronic computing machines. For example, ENIAC (the world's first general-purpose electronic computer) could perform 357 multiplications a second, compared to the roughly 1 multiplication per second of the Model V.
This is the complete article, containing 1,092 words
(approx. 4 pages at 300 words per page).
