2000 BC. The abacus is first used for computations.

1623. Wilhelm Schickard (1592-1635), of Tubingen, Wurttemberg (now in Germany), makes his "Calculating Clock". This is a 6-digit machine that can add and subtract, and perhaps includes an overflow indicator bell. Mounted on the machine is a set of Napier's Rods, a memory aid facilitating multiplications. The machine and plans are lost and forgotten in the war that is going on. (The plans were rediscovered in 1935, lost again in the war, and re-rediscovered by the same man in 1956! The machine was reconstructed in 1960 and found to be workable.) Schickard was a friend of the astronomer Kepler.

1644-5. Blaise Pascal (1623-1662), of Paris, makes his "Pascaline". This 5-digit machine can only add, and that probably not as reliably as Schickard's, but at least it doesn't get forgotten -- it establishes the computing machine concept in the intellectual community. (Pascal sold about 10-15 of the machines, some supporting as many as 8 digits, and a number of pirated copies were also sold. No patents...) This is the same Pascal who invented the bus.

1674. Gottfriend Wilhelm von Leibniz (1646-1716), of Leipzig, makes his "Stepped Reckoner". This uses a movable carriage so that it can multiply, with operands of up to 5 and 12 digits and a product of up to 16. But its carry mechanism requires user intervention and doesn't really work in all cases anyway. The calculator is powered by a crank. This is the same Leibniz or Leibnitz who co-invented calculus.

1775. Charles, the third Earl Stanhope, of England, makes a successful multiplying calculator similar to Leibniz's.

1770-6. Mathieus Hahn, somewhere in what is now Germany, also makes a successful multiplying calculator.

1786. J. H. Muller, of the Hessian army, conceives the idea of what came to be called a "difference engine". That's a special-purpose calculator for tabulating values of a polynomial, given the differences between certain values so that the polynomial is uniquely specified; it's useful for any function that can be approximated by a polynomial over suitable intervals. Muller's attempt to raise funds fails and the project is forgotten.

1820. Charles Xavier Thomas de Colmar (1785-1870), of France, makes his "Arithmometer", the first mass-produced calculator.

1822. Charles Babbage (1792-1871), of London, having reinvented the difference engine, begins his (government-funded) project to build one by constructing a 6-digit calculator using similar geared technology.

1832. Babbage produces a prototype segment of his difference engine, which operates on 6-digit numbers and 2nd-order differences (i.e. can tabulate quadratic polynomials). The complete engine was to have operated on 20-digit numbers and 6th-order difference, but no more than this prototype piece was ever assembled.

1834. Pehr George Scheutz, Stockholm, produces a small difference engine in wood, after reading a brief description of Babbage's project.

1836. Babbage produces the first design for his "Analytical Engine". Whether this machine, if built, would have been a computer or not depends on how you define "computer". It lacked the "stored-program" concept necessary for implementing a compiler; the program was in read-only memory, specifically in the form of punch cards. In this article such a machine will be called a "program-controlled calculator". The final design had three punch card readers for programs and data. The memory had 50 40-digit words of memory and 2 accumulators. Its programmability included the conditional-jump concept. It also included a form of microcoding: the meaning of instructions depended on the positioning of metal studs in a slotted barrel. It would have done an addition in 3 seconds and a multiplication or division in 2-4 minutes.

1842. Babbage's difference engine project is officially cancelled. (Babbage was spending too much time on the Analytical Engine.)

1843. Scheutz and his son Edvard Scheutz produce a 3rd-order difference engine with printer, and the Swedish government agrees to fund their next development.

1853. To Babbage's delight, Scheutz and Scheutz complete the first really useful difference engine, operating on 15-digit numbers and 4th-order differences, with a printer.

1858. The difference engine of 1853 does its only useful calculation, producing a set of astronomical tables for an observatory in Albany, New York. The person who spent money on it is fired and the machine ends up in the Smithsonian Institute. (The Scheutzes did make a second similar machine, which had a long useful life in the British government.)

1871. Babbage produces a prototype section of the Analytical Engine's "mill" (CPU) and printer. No more is ever assembled.

1878. Ramon Verea, living in New York City, invents a calculator with an internal multiplication table; this is much faster than the shifting carriage or other digital methods. He isn't interested in putting it into production; he just wants to show that a Spaniard can invent as well as an American.

1879. A committee investigates the feasibility of completing the Analytical Engine and concludes that it is impossible now that Babbage is dead. The project becomes somewhat forgotten and is unknown to most of the people mentioned in the last part of this chronology.

1885. Dorr E. Felt (1862-1930), of Chicago, makes his "Comptometer". This is the first calculator where numbers are entered by pressing keys as opposed to being dialed in or similar awkward methods.

1889. Felt invents the first printing desk calculator.

1890. US Census results are tabulated for the first time with significant mechanical aid: the punch card tabulators of Herman Hollerith (1860-1929) of MIT, Cambridge, Mass. This is the start of the punch card industry (thus establishing the size of the card, the same as a US $1 bill(then). The cost of the census tabulation rises by 98% from the previous one, in part because of the temptation to use the machines to the fullest and tabulate more data than formerly possible. The use of electricity to read the cards is also significant.

1892. William S. Burroughs (1857-1898), of St. Louis, invents a machine similar to Felt's but more robust, and this is the one that really starts the office calculator industry. (The calculators are still hand powered at this point, but electrified ones follow in not too many years.)

1937. George Stibitz (c.1910-) of Bell Labs, New York City, constructs a demonstration 1-bit binary adder using relays.

1937. Alan M. Turing (1912-1954), of Cambridge University, England, publishes a paper on "computable numbers", which solves a mathematical problem by considering as a mathematical device the theoretical simplified computer that came to be called a Turing machine.

1938. Claude E. Shannon (c.1918-) publishes a paper on the implementation of symbolic logic using relays.

1938. Konrad Zuse (1910-) of Berlin completes a prototype mechanical programmable calculator, later called the "Z1". Its memory used sliding metal parts and stored about 1000 bits. The arithmetic unit was unreliable.

Oct 1939. Stibitz and Samuel Williams complete the "Model I", a calculator using relay logic. It is controlled through modified teletypes and these can be connected through phone lines. Later machines in the series also have some programmability.

Oct 1939. John V. Atanasoff (1903-) and Clifford Berry, of Iowa State College, Ames, Iowa, complete a prototype 16-bit adder. This is the first machine to calculate using vacuum tubes.

c.1940. Zuse completes the "Z2", keeping the mechanical memory but using relay logic. He can't interest anyone in funding him.

Dec 1941. Zuse, having promised to a research institute a special-purpose calculator for their needs, actually builds them the "Z3", which is the first operational program-controlled calculator, and has 64 22-bit words of memory. However, its programmability doesn't include a conditional- jump instruction; Zuse never had that idea. The program is on punched tape. The machine includes 2600 relays, and a multiplication takes 3-5 seconds.

Spring 1942. Atanasoff and Berry complete a special-purpose calculator for solving systems of simultaneous linear equations, later called the "ABC" ("Atanasoff-Berry Computer"). This has 60 50-bit words of memory in the form of capacitors (with refresh circuits) mounted on two revolving drums. The clock speed is 60 Hz, and an addition takes 1 second. For secondary memory it uses punch cards, with the holes being burned rather than punched in them, moved around by the user. (The punch card system's error rate was never reduced beyond 0.001%, which wasn't good enough.) Atanasoff then left Iowa State, and apparently lost all interest in digital computing machines. [The ABC in an article in one of the issues of Scientific American was called the first computer.]

Jan 1943. Howard H. Aiken (1900-1973) and his team at Harvard University, Cambridge, Mass. (with backing from IBM), complete the "ASCC Mark I" ("Automatic Sequence-Controlled Calculator Mark I"). This is the first program-controlled calculator to be widely known: Aiken was to Zuse as Pascal to Schickard. The machine is about 60 feet long and weighs 5 tons; it has 72 accumulators.

Dec 1943. Alan Turing and his team at Bletchley Park, near Cambridge, England, complete the first version of the "Colossus". This is a secret, special-purpose decryption machine, not exactly a calculator but close kin. It includes 2400 tubes for logic and reads characters (optically) from 5 long paper tape loops moving at 5000 characters per second.

Nov 1945. John W. Mauchly (pronounced Mawkly; 1907-80) and J. Presper Eckert (1919-) and their team at the Moore School of the University of Pennsylvania, Philadelphia, complete the "ENIAC" ("Electronic Numerator, Integrator, Analyzer, and Computer") for the US Army's Ballistics Research Lab. (Too late for the war and 200% over budget -- problems that would face Eckert and Mauchly again on later projects.) The machine is a secret (until Feb 1946) program-controlled calculator. Its only memory is 20 10-digit accumulators (4 were originally planned). The accumulators and logic use vacuum tubes, 17648 of them altogether. The machine weighs 30 tons, covers about 1000 square feet of floor, and consumes what is either 174 kilowatts (233 horsepower) or 174 hp (130 kW). Its clock speed is 100 kHz; it can do 5000 additions per second, 333 multip- lications per second. It reads data from punch cards, and the program is set up on a plugboard (considered reasonable since the same or similar program would tend to be used for weeks at a time). Mauchly and Eckert apply for a patent. The university disputes this at first, but they settle. The patent is finally granted in 1964, but is overturned in 1973, in part because of the previous work by Atanasoff.

1945-46. John von Neumann (1903-1957) joins the ENIAC team and writes a report describing the future computer eventually built as the "EDVAC" ("Electronic Discrete Variable Automatic Computer" (!)). This report was the first description of the design of a stored-program computer. An early draft which fails to credit other team members such as Eckert and Mauchly gets too-wide distribution, leading to von Neumann getting too much credit, e.g., the term "von Neumann computer" which is derived from this paper.

1945. Grace Murray Hopper, working in a temporary World War I building at Harvard University on the Mark II computer, found the first computer bug beaten to death in the jaws of a relay. She glued it into the logbook of the computer and thereafter when the machine stops (frequently) they tell Howard Aiken that they are "debugging" the computer. The very first bug still exists in the National Museum of American History of the Smithsonian Institution. The word bug and the concept of debugging had been used previously,  perhaps by Edison, but this was probably the first verification that the concept applied to computers.

1947. CSIRAC designed and built by CSIR scientists, was the first stored-memory electronic computer in Australia. In 1947 Maston Beard and Trevor Pearcey (a prominent British radar scientist who had emigrated to Australia after World War II) led a research group at the CSIR (after 1949 called CSIRO) Radiophysics Laboratory which designed and built CSIRAC, the world's fourth stored-memory electronic computer. The first program was run in 1949 and the machine came into full operation in 1951. It embodied many features novel at the time and was able to operate more than 1000 times faster than the best mechanical calculators. In 1955 CSIRAC was transferred to the University of Melbourne where it continued in operation until 1964.

Jan 1948. Wallace Eckert (1902-1971, no relation to Presper Eckert and not mentioned again in this article) of IBM, with his team, completes the "SSEC" ("Selective Sequence Electronic Calculator"). This technological hybrid has vacuum tube logic with 8 20-digit registers, 150 20-digit words of relay memory, and a program that is partly stored but also controlled by a plugboard. [IBM considers it the first computer.]

Jun 1948. Max Newman, F. C. Williams, and their team at Manchester University, Manchester, England, complete a prototype machine called the "Mark I". [This is the first machine that everyone would call a computer], because it's the first with a true stored-program capability. It uses a new type of memory invented by Williams, which uses the residual charges left on the screen of a CRT after the electron beam has been fired at it. (The bits are read by firing another beam through them and reading the voltage at an electrode beyond the screen.) This is a bit unreliable but is fast, relatively cheap, and much more compact (with room for about 1024 or 2048 bits per tube) than any other memory then existing. The Mark I uses six of them, but I don't know of how many bits. Its programs are initially entered in binary on a keyboard, and the output is read in binary from another CRT. Later Turing joins the team and devises a primitive form of assembly language, one of several developed in different places. Newman was the first person shown Turing's 1937 paper in draft form.

1949-51. Jay W. Forrester and his team at MIT construct the "Whirlwind" for the US Navy's Office of Research and Inventions. The vague date is because it advanced to full-time operational status gradually. Originally it had 3300 tubes and 8900 crystal diodes. It occupied 2500 square feet of floor. Its 2048 16-bit words of CRT memory used up tubes so fast they were costing $32000 per month. This was the first computer designed for real-time work, hence the short word size; it could do 500000 additions or 50000 multiplications per second.

Spring 1949. Forrester conceives the idea of magnetic core memory. The first practical form, 4 years later, will replace the Whirlwind's CRT memory and render all then existing types obsolete. The basic concept for core memory had been patented by An Wang, Harvard University, in 1949, but his technique involved using the cores on single wires to form delay lines. The Whirlwind Project conceived the technique of stringing the cores onto a matrix of wires and thus producing a random access memory.

Jun 1949. Maurice Wilkes (1913-) and his team at Cambridge University complete the "EDSAC" ("Electronic Delay Storage Automatic Computer"), which is closely based on the EDVAC design report from von Neumann's group. This is the first operational stored-program computer that's not a prototype. Its I/O is by paper tape, and it has a sort of mechanical read-only memory for booting, consisting of rotary telephone switches. Its main memory is of another new type, invented by Eckert: the "ultrasonic" or "delay line" memory. In this type, the data is repeatedly converted back and forth between electrical pulses and ultrasonic pulses; only the bits currently in electrical form are accessible. (The ultrasonic pulses were typically fired from one end of a tank of mercury to the other, though other substances were also used.) In the EDSAC, 32 mercury tanks 5 feet long give a total of 256 35-bit words of memory.

Aug 1949. Eckert and Mauchly, having formed their own company, complete the "BINAC" ("Binary Automatic Computer") for the US Air Force. Designed as a first step to in-flight computers, this has dual (redundant) processors each with 700 tubes and 512 31-bit words of memory. Each processor occupies only 4 square feet of floor space and can do 3500 additions or 1000 multiplications per second. The designers are thinking mostly of their forthcoming "UNIVAC" ("Universal Automatic Computer") and don't spend much time making the BINAC as reliable as it should be, but the tandem processors compensate somewhat.

Feb 1951. Ferranti Ltd., of Manchester, England, completes the first commercial computer, also called the "Mark I". 8 of them are sold.

Mar 1951. Eckert and Mauchly, having sold their company to Remington Rand, complete the first UNIVAC, which is the first US commercial computer. It has 1000 12-digit words of ultrasonic memory and can do 8333 additions or 555 multiplications per second; it contains 5000 tubes and covers 200 square feet of floor.

1951. Grace Murray Hopper (1906-), of Remington Rand, invents the modern concept of the compiler.

1951-52. The EDVAC is finally completed. It has 4000 tubes, 10000 crystal diodes, and 1024 44-bit words of ultrasonic memory. Its clock speed is 1 mHz.

1952. The IBM "Defense Calculator", later renamed the "701", the first IBM computer unless you count the SSEC, enters production at Poughkeepsie, New York. (The first one is delivered in March 1953; 19 are sold altogether. The memory is electrostatic and has 4096 36-bit words; It does 2200 multiplications per second.)

1952. Grace Murray Hopper implements the first compiler, the "A-0". (As with "computer", this is a somewhat arbitrary designation.)

1957. An IBM team, led by John Backus, designs the first successful high-level programming language, FORTRAN, for solving engineering and science problems.

1958. The first computer to use the transistor as a switching device, the IBM 7090, is introduced.

1964. The first computer to use integrated circuits, the IBM 360, is announced.

1965. The CTSS (Compatible Time-Sharing System) operating system is introduced. It allows several users simultaneously to use, or share, a single computer.

1970. A first version of the UNIX operating system is running on the DEC PDP-7.

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