A computer is a general purpose device that can be programmed to carry out

a set of arithmetic or logical operations automatically.

The 'Abacus' was used for arithmetic tasks. What we now call the 'Roman Abacus' was used in Babylonia as early as 2400 BC. Since then, many other forms of reckoning boards or tables have been invented. In a medieval European counting house, a checkered cloth would be placed on a table, and markers moved around on it according to certain rules, as an aid to calculating sums of money.

The 'Antikythera' mechanism is believed to be the earliest mechanical analogue 'computer' and was designed to calculate astronomical positions. It was discovered in 1901 in the Antikythera wreck off the Greek island of Antikythera, between Kythera and Crete, and has been dated to circa 100 BC. Devices of a level of complexity comparable to that of the 'Antikythera'  mechanism would not reappear until a thousand years later.

The 'Slide Rule' was invented around 1620 - 1630, shortly after the publication of the concept of the logarithm. It is a hand-operated analogue computer for doing multiplication and division.

As slide rule development progressed, added scales provided reciprocals, squares and square roots, cubes and cube roots, as well as transcendental functions such as logarithms and exponentials, circular and hyperbolic trigonometry and other functions. Aviation is one of the few fields where slide rules are still in widespread use, particularly for solving time-distance problems in light aircraft. To save space and for ease of reading, these are typically circular devices rather than the classic linear slide rule shape.

Charles Babbage, an English mechanical engineer and polymath, originated the concept of a programmable computer and he invented the first mechanical computer in the early 19th century. After working on his revolutionary difference engine, designed to aid in navigational calculations, in 1833 he realized that a much more general design, an Analytical Engine, was possible. The input of programs and data was to be provided to the machine via punched cards, a method being used at the time to direct mechanical looms such as the Jacquard loom. For output, the machine would have a printer, a curve plotter and a bell. The machine would also be able to punch numbers onto cards to be read in later. The machine was about a century ahead of its time. All the parts for his machine had to be made by hand - this was a major problem for a device with thousands of parts. Eventually, the project was dissolved with the decision of the British Government to cease funding. Babbage's failure to complete the analytical engine can be chiefly attributed to difficulties not only of politics and financing, but also to his desire to develop an increasingly sophisticated computer and to move ahead faster than anyone else could follow. Nevertheless his son, Henry Babbage, completed a simplified version of the analytical engine's computing unit 'The Mill' in 1888. He gave a successful demonstration of its use in computing tables in 1906.

The principle of the modern computer was first applied by Alan Turing, who reformulated Austrian, Kurt Gödel's 1931 arithmetic-based formal language with simple hypothetical devices that became known as Turing machines. He proved that some such machine would be capable of performing any conceivable mathematical computation if it were representable as an algorithm. He also introduced the notion of a 'Universal Machine' (now known as a Universal Turing machine), with the idea that such a machine could perform the tasks of any other machine, or in other words, it is provably capable of computing anything that is computable by executing a program stored on tape, allowing the machine to be programmable. Turing machines are to this day a central object of study in theory of computation. Modern computers are said to be 'Turing-complete', which is to say, they have algorithm execution capability equivalent to a universal Turing machine.

By 1938 the United States Navy had developed an Electromechanical Analog Computer small enough to use aboard a submarine. This was the Torpedo Data Computer, which used trigonometry to solve the problem of firing a torpedo from a boat to a moving target. Early digital computers were electromechanical; electric switches drove mechanical relays to perform the calculation. These devices had a low operating speed and were eventually superseded by much faster all-electric computers, originally using vacuum tubes.

Purely electronic circuit elements soon replaced their mechanical and electromechanical equivalents, at the same time that digital calculation replaced analog. The engineer Tommy Flowers, working at the Post Office Research Station in London in the 1930s, began to explore the possible use of electronics for the telephone exchange. Experimental equipment that he built in 1934 went into operation 5 years later, converting a portion of the telephone exchange network into an electronic data processing system, using thousands of vacuum tubes. In the USA the Atanasoff–Berry Computer (ABC) was produced in 1942 - the first 'Automatic Electronic Digital Computer'. This design was also all electronic and used about 300 vacuum tubes, with capacitors for memory.

'Colossus' was the first electronic digital programmable computing device, and was used to break German ciphers during World War 2. Bletchley Park achieved a number of successes at breaking encrypted German military communications. The German encryption machine, 'Enigma', was first attacked with the help of the electro-mechanical bombes. Tommy Flowers was commissioned to build the 'Colossus'. He spent eleven months from early February 1943 designing and building the first 'Colossus'. After a functional test in December 1943, 'Colossus' was shipped to Bletchley Park, where it was delivered on 18 January 1944 and attacked its first message on 5 February.

Early computing machines had fixed programs and changing its function required the

re-wiring and re-structuring of the machine. A stored-program computer includes by design an instruction set and can store in memory a set of instructions (a program) that details the computation. The theory for the stored-program computer was given by 'Alan Turing' in his 1936 paper. After the war, Turing worked at the National Physical Laboratory, where he produced some of the first designs for a stored-program computer. In 1948 Turing joined Manchester University, where he helped develop the Manchester computers. The Manchester Small-Scale Experimental Machine, nicknamed 'Baby' was the world's first stored-program computer. It was built at the Victoria University of Manchester and ran its first program on 21 June 1948. Although the computer was considered "small and primitive" by the standards of its time, it was the first working machine to contain all of the elements essential to a modern electronic computer.

The 'Bipolar Transistor' was invented in 1947. From 1955 onwards transistors replaced vacuum tubes in computer designs, giving rise to the "second generation" of computers. Compared to vacuum tubes, transistors have many advantages: they are smaller, and require less power than vacuum tubes, so give off less heat. Silicon junction transistors were much more reliable than vacuum tubes and had longer, indefinite, service life. Transistorized computers could contain tens of thousands of binary logic circuits in a relatively compact space.

The next great advance in computing power came with the advent of the integrated circuit. The idea of the integrated circuit was first conceived by a Radar Scientist working for the Royal Radar Establishment of the Ministry of Defence, 'Geoffrey W.A. Dummer'. This new development heralded an explosion in the commercial and personal use of computers and led to the invention of the 'microprocessor'.

With the continued miniaturization of computing resources, and advancements in portable battery life, portable computers grew in popularity in the 2000s. The same developments that spurred the growth of laptop computers and other portable computers allowed manufacturers to integrate computing resources into cellular phones. These so-called Smart Phones and Tablets run on a variety of operating systems and have became the dominant computing device on the market.

The 'Antikythera' mechanism

Slide Rule

Roman Abacus

The Babbage Computer

'Colossus' at Bletchley Park

A replica of 'Baby' at the Museum of Science and Industry in Manchester

A Bipolar Transistor

Photo: © Marsyas • Licensed for reuse under CC BY-SA 3.0

Photo: © Roger McLassus • Licensed for reuse under CC BY-SA 3.0

Photo: Benjamin Herschel Babbage/Wikipedia

Believed to be in the Public Domain (Age - Copyright expired)

Photo: Science & Society Picture Library

Photo: © Tom Jeffs • Licensed for reuse under CC BY-SA 3.0