Bell Telephone Laboratories Model 1 Complex Calculator

George Stibitz

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George Stibitz worked at Bell Labs as a mathematician in the 1930s. In his spare time, he experimented with using telephone relays for electro-mechanical calculation.

"The original notions that led to the series of relay computers had nothing to do with usefulness. I just wondered whether it would be possible to make such simple things as relays do complicated calculations . . .

"I was then a 'mathematical engineer' at the Bell Telephone Labs, and as such I was asked to look into the magnetic circuits of the telephone relay. As you know, a relay is just an electrically-operated switch that opens and closes one or a dozen electrical circuits.

"While looking at the relay's magnetic circuit I naturally noted the piles of contacts that could be closed or opened when the relay operated. I knew that these contacts could be connected in large and complicated meshes, and when so connected they could do very complicated jobs. So, I liberated a pair of relays from the Labs' junk pile and tried out a few circuits.

"Years before in a freshman math course I had learned a little about the binary notation for representing numbers. That notation has digits with only two values, such as zero and one, much as the relay has only two 'values': open and closed.

"It occurred to me that perhaps the two positions of a relay could be used to represent the two values of a binary digit. Then perhaps circuits through the contacts of several relays might represent the two values of a binary digit. I soon found out that this was true-two relays could be wired together to add two binary digits.

"I built an adder of the two relays I had borrowed, a couple of dry cells, two flashlight bulbs, and two strips of metal for keys. My wife named it the K-model, after our kitchen table.

"When I took the K-model to the Labs to show the boys, we speculated on the possibility of building a full-size calculator out of relays. Shortly thereafter the relay computer turned serious."

George R. Stibitz, "Early Computers and Their Uses," presented at Computing and Chili-eating Society, 1981

Around that time, the head of the mathematical engineering group came to Stibitz with a problem. Recent developments in filter and transmission line theory were overloading the desk calculator team with complex number work. Could a large-scale relay calculator handle the work? Bell Labs made Stibitz's relay project official with a budget and circuit designer. The Model I, first in a series of Bell Labs relay calculators and computers, was finished in 1939. Technically, the Model I was not a true computer because it was not controlled by a program. Rather, it was operated directly through a teletype. Although it lacked the speed of the electronic computers that were to appear a few years later, its relays were far less liable to failure than vacuum tubes.

The Bell Labs Model I was the first demonstration of a large-scale digital machine for complex calculation.

"In September 1940, after several months of routine use at the Laboratories, the computer was demonstrated at a meeting of the American Mathematical Society held at Dartmouth College, in Hanover, New Hampshire . . . I gave a short paper on the use and design of the computer after which those attending were invited to transmit problems from a Teletype in McNutt Hall to the computer in New York. Answers returned over the same telegraph connection and were printed out on the Teletype."

George Stibitz, "Early Computers," in A History of Computing in the Twentieth Century, ed. N. Metropolis, J. Howlett, and Gian- Carlo Rota, New York, 1980

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