The first electronic computer, ENIAC, weighed more than 27 tons and took up 1800 square feet.

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ENIAC (Electronic Numerical Integrator and Computer) was the first electronic general-purpose computer. It was Turing-complete, digital, and able to solve "a large class of numerical problems" through reprogramming.

Although ENIAC was designed and primarily used to calculate artillery firing tables for the United States Army's Ballistic Research Laboratory (which later became a part of the Army Research Laboratory), its first program was a study of the feasibility of the thermonuclear weapon.

ENIAC was completed in 1945 and first put to work for practical purposes on December 10, 1945.

ENIAC was formally dedicated at the University of Pennsylvania on February 15, 1946, and was heralded as a "Giant Brain" by the press. It had a speed on the order of one thousand times faster than that of electro-mechanical machines; this computational power, coupled with general-purpose programmability, excited scientists and industrialists alike. The combination of speed and programmability allowed for thousands more calculations for problems. ENIAC calculated a trajectory in 30 seconds that took a human 20 hours (allowing one ENIAC hour to displace 2,400 human hours). The completed machine was announced to the public on February 14, 1946, and formally dedicated the next day at the University of Pennsylvania, having cost almost $500,000 (approximately $6,300,000 today). The U.S. Army Ordnance Corps officially accepted it in July 1946. On November 9, 1946, ENIAC was shut down for a refurbishment and a memory upgrade and was transferred to Aberdeen Proving Ground, Maryland, in 1947. On July 29, 1947, it was turned on and was in continuous operation until 11:45 p.m. on October 2, 1955.