THE INVENTION OF ENIAC

by

R. J.. Reiman, Historian

ENIAC was completed in November 1945, 3 months after the surrender of the Japanese in WWII, 200% over the budget of $250,000. So why was it important? It did help detect design flaws in the design of the Hydrogen bomb, but mainly it initiated the computer industry.

How was it invented? The Moore School of Electrical Engineering of the University of Pennsylvania had developed a differential analyzer for the Army Ordinance which was interested in an advanced calculator which could compute ballistic firing tables for gunners to aim their weapons. With onset of WWII, humans computing tables using calculators were falling badly behind the demand. John W. Mauchly, a 35-year-old assistant professor at the Moore School, had proposed the construction of an electronic calculator consisting of vacuum tubes pulsing at the rate of 100,000 beats a second. which could do 1,000 multiplications per second, or compute a single trajectory in 100 seconds. The Ordinance Department was interested.

Mauchly had developed the design concept of a digital calculator from work done by John V. Atanasoff of Iowa State College who had designed and built an electronic calculator which used binary math and Boolean algebra. Arithmetic units employed vacuum tubes and the memory was composed of capacitors for the purpose of storing charges. Input and output was by punch cards. Atanasoff, assisted by Clifford Berry, had later completed the ABC calculator in 1942, but had instructed their patent attorney, Richard R. Trexler to file for a patent in 1941. WWII began in December 1941, the patent was never filed, and the inventors abandoned their efforts. Atanasoff's work lived on through Mauchly who had learned of the details from contacts with Atanasoff from December 1940 to September 1941.

Mauchly was joined by J. Presper Eckert Jr. at Moore and Eckert suggested changes in the design of ENIAC by devising a flip-flop circuit using twenty vacuum tubes for the accumulator. ENIAC was to retain Atanasoff's idea of vacuum tubes as switches and the synchronizing of internal operations with an electronic timer. Vacuum tubes, however, were used for the memory instead of capacitors. Two of Atanasoff's best ideas, the use of binary math and Boolean logic, unfortunately would wait for future machines and their absence made ENIAC unnecessarily complicated.

Project PX, as the military classified ENIAC, started in June 1943. The project team was under terrific pressure to complete their work since the demand for firing tables was increasing. By June 1944, the first two accumulators were completed and were a success. Their number was to be increased from 4 to 20, which extended the time to complete, but made ENIAC considerably more powerful. The fragility of the vacuum tubes were the weakest link in the design and ENIAC had 20,000 of them. Eckert ran them well below their rated voltage and he did extensive testing beforehand to weed out the weak tubes. It took about a year and a half to design ENIAC and a year to build it. ENIAC would initiate the computer industry.