The first general-purpose analog computer was designed in 1930 by American scientist Vannevar Bush (1890–1974), who built a mechanically operated device called a differential analyzer. The first semi-electronic digital computing device was built by mathematician and physicist John Vincent Atanassoff (1903–1995) and one of his graduate students, Clifford E. Berry (1918–1963), between 1937 and 1942. It was created primarily to solve large systems of simultaneous linear equations. It is interesting to note that Atanassoff’s computer was overshadowed by the Electronic Numerical Integrator and Computer (ENIAC; see below), which was once credited as the first computer. In 1973, however, a federal judge recognized Atanassoff’s work and voided Sperry Rand’s patent on the ENIAC, saying it had been derived from Atanassoff’s invention. Today, Atanassoff and Berry get the credit.

The Harvard Mark 1, or the Automatic Sequence Controlled Calculator, was built between 1939 and 1944 by American computer scientist Howard H. Aiken (1900–1973) and his team. It is thought of as the first large-scale automatic digital computer. But there are disagreements about this, with some historians believing that German engineer Konrad Zuse’s Z3 (see above) was the first such machine.

Other early computers were the ENIAC and UNIVAC. The ENIAC (Electronic Numerical Integrator And Calculator) was completed in 1946 at the University of Pennsylvania; it used thousands of vacuum tubes. Until 1973, it was thought of as the first semi-electronic digital computer. That credit was subsequently given to Atanassoff and Berry (see above). The UNIVAC (UNIVersal Automatic Computer) was built in 1951 and was the first computer to handle both numeric and alphabetic data. It also was the first commercially available computer.

Vannevar Bush was an American scientist who invented the differential analyzer, the first general-purpose analog computer.

The third-generation integrated-circuit machines were used primarily during the mid-1960s and 1970s, making the computers smaller, faster (close to a million operations per second), and far more reliable. The first commercial microprocessor was the Intel 4004, which appeared in 1971. It could only add and subtract, and it was used to power one of the first portable electronic calculators. The real push in microprocessors came during the late 1970s to 1990s, allowing for increasingly smaller and more powerful computers. For example, in 1974 the Intel 8080 processor had a clock speed of 2 megahertz (MHz); by 2004 the Pentium 4 (“Prescott”) had a clock speed of 3.6 gigahertz (GHz).