During the 1930's and 40's in Germany, Konrad Zuse was developing a series of automatic calculating machines. They were almost entirely mechanical and had no stored program but they were contolled by punched tape (old film reels hand punched). The first compputer, the Z1, was entirely mechanical, including memory and logic gates, and so data transfer was difficult. The Z2 used relays for gates but was too unreliable for much use. The Z3, in 1941, used relays for memory as well, and had in total 2,600 of them. It was as fast as the Harvard Mark I, produced 2 years later, and could do 3,4 additions in a second. The Z4 was similar except Zuse reverted to a mechanical memory. In 1950, this was the only operational computer in Europe.
A series of computers built around relays was constructed by Bell labs in the 1940's. The main designers were Stiblitz, a mathematician who recognised a connection between binary and relays, and Williams, an engineer. Their first machine was fully designed by 1939 and was then built for the Bell company. It could add, subtract, multiply and divide complex numbers and had approx 450 standard phone relays. It was the first machine to have more than one terminal - 3 teletype machines - and was also the first machine to be remotely used. It was used at a meeting in New Hampshire while it was physically in New York.
The second, third and fourth computers built by Stiblity and Williams were used by the army for ballistics. The fifth model could be split in two to work on two separate problems. It had 9000 relays and weighed 10 tons. It performed arithmetic by looking up tables. (CADET: Can't Add, Doesn't Even Try). However, it could handle floating-point numbers.
Howard Aiken, a professor at Harvard University, needed a computer to perform calculations on non-linear differential equations, so he managed to persuade IBM, in conjunction with the US Navy, to design and build a computer. The finished machine machine had 72 23-decimal digit registers and 60 constant registers. It used fixed point numbers. In total it had 1400 ten-selection wheels and 500 miles of wiring. It was finished in 1943 and was donated to Harvard where it became known as the Harvard Mark I. Data was read from tape and and addition or subtraction took 0.3 seconds due to the mechanical nature of the machine. From 1944 to the end of the war it was used for military use.
The Harvard Mark II was finished in 1947 and was much faster than the Mark I due to the fact that it used relays as opposed to the rotating switches. It had 13,000 special high speed, low power relays. Like the Bell model five, it could be split into two smaller machines. The registers each held 10 digit floating point BCD numbers. For the Mark III and IV, Aiken realised that the greatest increase in productivity would come from making the machine easier to program and use than by increasing the speed. The Mark II finished in 1949 had a stored program and a program counter, allowing conditional execution. It also used vacuum tubes and magnetic memory. The Mark IV was faster as it had a ferrite core memory.
John Atanastoff and a graduate student of his, Clifford Berry together designed and began building the ABC (Atanastoff and Berry Computer) which, though never finished, (as Atanastoff had to join the army in WWII) had an sizeable influence on computing,in that it influenced John Mauchly, one of the key designers of the ENIAC. Atanastoff required a computer to help him solve some complex equations on which he was working. He initially tried modifying a leased IBM tabulating machine, but was prevented from completing this as the machine was leased not owned. He then set about designing his own machine. The story is told of how one night when experiencing difficulty, he decided he wanted a drink so he travelled the 200 miles to the state border (Iowa was then a dry state) to get one. After the relaxing drive and the drinks he then thought of the idea of designing the machine to use binary. By 1939, Atanastoff and Berry had a prototype of the machine built and set about building the final machine. It would use vacuuum tubes, could add and subtract and for memory had capacitors on a rotating drum. The ABC was significant in that it was the first computer to use entirely electronic methods for calculation.
The ENIAC is probably the best known of all the early computers, and it was certainly one of the most influential. It was a large scale computer built to calculate using electronic not mechanical methods. It was built at the Moore School of Engineering for the US Army which required large computing power for ballistic tables. The two principal men involved on the project from the start were J. Presper Eckert and John Mauchly. Mauchly brought ideas of electronic computing from the ABC machine while Eckert is responsible for the ENIAC's reliability. Work started on the ENIAC in 1943 and it was operational by 1945. Eckert decided to use standard cheaper components that were well within design tolerences as opposed to much more expensive ultra-reliable components. Despite this, the project cost nearly $500,000. The ENIAC used 20 10-decimal-digit accumulators, with each digit stored in a ring of 10 vacuum tubes. The ENIAC was in many respects an electronic equivalent of the Harvard Mark I. It used 18,000 vacuum tubes and weighed 30 tons. Addition and subtraction were done by rotating the active vacuum tube in the accumulator digits (similar to the rotating switches in the Mark I) Multiplication was done by tables and shifts.
The ENIAC was continually improved during its useful life. It was given storage and memory units. In 1947 it became a stored program computer with 60 instructions which, though it slowed the machine down, saved much rewiring.
The machine built at Manchester University had undoubtedly got a stored program, yet despite this it is not universally recognised as being the first stored program computer. This is because of its limited usefulness which meant it could only manage the most basic of tasks.
The second of the contenders for the first stored program computer is the EDSAC, built at Cambridge. It, like the Manchester machine, had a stored program, but it was finished slightly later. It was, however, a computer in the true sense of the word as it could handle meaningful tasks. The LEO computers produced for businesses by Lyons were all re-engineered versions of the EDSAC.
The ENIAC was not the only machine built at the Moore School of engineering. Its successor, the EDVAC, was finished in 1952, without the help of any of the three principals on the ENIAC project: Mauchly, Eckert or John Neumann (the mathematician who was involved in the later stages of the ENIAC development). The EDVAC used mercury delay lines, and had two processing units for accuracy.
The very first commercial computer to be sold by a company was the UNIVAC (UNIVersal Automatic Computer) which was delivered to the US Cencus Bureau in 1951 by Remmington-Rand. The company which had started the computer was Electronic Control Company, founded by Mauchly and Eckert, which was bought by Remmington-Rand before the project was finished. 44 UNIVAC's were sold in total.