ENIAC: A History of One of the Earliest Computers

The ENIAC was not the first electronic computer to be built, but it was one of the most significant, and had a huge influence on the development of the modern computer industry.


One of ENIAC’s circuit boards on display at the American Computer and Robotics Museum, in Bozeman MT

In 1942, the Second World War was raging, and the US was devoting most of its national resources to the fight. One weapon was mathematics. The Army was producing a variety of new field guns and artillery pieces. Each of them required complex mathematical calculations for proper aiming, which took into account factors like air temperature, wind speed, humidity, elevation, even the temperature of the gunpowder. These calculations were too time-consuming to do on the battlefield, so each artillery gun was accompanied by a set of over 500 different firing tables, which listed all the possible combinations and spelled out the proper gun settings for best accuracy. These firing tables were produced by the Army’s Ballistics Research Laboratory, where a team of 176 people, using mechanical adding machines, did all of the complex calculations by hand. Since all the young men were off fighting the war, most of the people doing the computations were young women. They were called “computers”.

To aid them in their calculations, the “computers” had a machine called a “differential analyzer”, which used a system of gears and wheels to solve differential equations. But the machines broke down often, and were slow to operate. It often took over a month to produce just one firing table. The Army needed something better.

Enter John Mauchly. A professor at the University of Pennsylvania, Mauchly had done some work on electronic circuits based on the ideas of John Atanasoff from Iowa State University, and in 1941 had traveled to Iowa to look at the machine Atanasoff was working on, called the ABC (Atanasoff–Berry Computer). Atanasoff never finished his machine, but Mauchly carried out some experiments using neon tubes (he didn’t have the money to buy vacuum tubes) and in the summer of 1942 wrote an unpublished paper entitled “The Use of High-Speed Vacuum Tube Devices for Calculation”, which was circulated around to colleagues and students. When the Army heard about it, they contacted Mauchly and agreed to fund a project to build an electronic version of the differential analyzer, which would be able to do calculations much faster than the mechanical version. The project became known as ENIAC (Electronic Numerical Integrator and Computer).

In all, the Army spent almost half a million dollars (over $6 million in today’s money) and 200,000 man-hours on the project. The technological challenges were enormous. At its heart, the ENIAC replaced all of the mechanical gears and wheels of the differential analyzer with vacuum tubes and electrical switches. The machine itself weighed thirty tons and filled an entire 30×45 foot room. Since it had some 1500 mechanical relays, it was not fully electronic, but it contained over 17,000 vacuum tubes and required 175kw of electric power. ENIAC’s tubes produced so much heat that two 20-horsepower air blowers were required to keep them cool, but they still tended to burn out rather often, and in general the ENIAC was down for repairs about half the time (the operators had to remove panels and crawl inside the machine to find bad tubes). The computer had a series of “accumulators” for addition and subtraction, and separate assemblies for division, multiplication, and square roots. Each numerical digit required 36 vacuum tubes to store.

Six of the Army’s female “computers” were selected and trained to operate the ENIAC. To produce each ballistics firing table, the operator first had to determine which differential equations needed to be solved, then broke these down into basic addition/subtraction and multiplication/division tasks, which were fed into the ENIAC using punched paper cards. The electronic computer itself then had to be “programmed” by setting a number of switches and connecting particular cables on a series of plugboards. It often took several days to set up the ENIAC for each table.

Once the programming was complete, the ENIAC used a series of relays to store the input in its memory, then performed the necessary mathematical calculations and printed out the results on punched cards. The machine could calculate each ballistics table entry in just 30 seconds, something which took 20 hours to do by hand.

It is somewhat debatable whether ENIAC can be called a true “computer”–it was more like a whole bunch of electronic adding machines wired together. Although it was “programmable” to the extent that it could be set up to solve different types of equations, it was designed solely for calculating ballistic artillery tables. It did however have the ability to moderate its operations according to the results it produced, and it was the most powerful calculating machine of its time.

It took about a year just to design the machine, and then another year and a half to build it. As it turned out, the ENIAC was not up and running until November 1945, a month after the Japanese formally surrendered. The Army declassified the existence of the machine in 1946, and after some upgrades moved it to the Aberdeen Proving Ground in Maryland in 1947. By this time the American atomic bomb program at Los Alamos had already begun using the computer, and John von Neumann made some changes to improve its efficiency, including a rudimentary capability to store programs in memory. When ENIAC was booted up at Aberdeen, its first task was to perform the complex calculations needed during the design of the US’s thermonuclear hydrogen bomb. The computer was finally retired in 1955.

Mauchly, meanwhile, left Penn and formed his own computer company, where he developed the UNIVAC, the first commercially-available mainframe computer. Years later, he would become involved in a patent dispute, which ruled that he had gotten most of his basic ideas from Professor Atanasoff at Iowa State and concluded that the patent for an electronic computer device was in the public domain.

After the ENIAC was dismantled in 1955, most of the modules were donated to the Smithsonian. Today, a piece of the ENIAC machine is on display at the American Computer and Robotics Museum in Bozeman MT, where it is on loan.




2 thoughts on “ENIAC: A History of One of the Earliest Computers”

  1. I think it was built mostly with off-the-shelf parts. The real trick was the intellectual task of figuring out how to put it all together.

    At the same time, the UK was building Colossus to help with code-breaking, but that was highly classified so the folks building ENIAC had no idea it was happening.

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