1930s–1960s: desktop calculator Post–Turing machine Computational models The Curta calculator can also do multiplication and division
By the 1900s, earlier mechanical calculators, cash registers, accounting machines, and so on were redesigned to use electric motors, with gear position as the representation for the state of a variable. The word "computer" was a job title assigned to people who used these calculators to perform mathematical calculations. By the 1920s Lewis Fry Richardson's interest in weather prediction led him to propose human computers and numerical analysis to model the weather; to this day, the most powerful computers on Earth are needed to adequately model its weather using the Navier-Stokes equations. Companies like Friden, Marchant Calculator and Monroe made desktop mechanical calculators from the 1930s that could add, subtract, multiply and divide. During the Manhattan project, future Nobel laureate Richard Feynman was the supervisor of the roomful of human computers, many of them women mathematicians, who understood the differential equations which were being solved for the war effort. Even the renowned Stanisław Ulam was pressed into service to translate the mathematics into computable approximations for the hydrogen bomb, after the war. In 1948, the Curta was introduced. This was a small, portable, mechanical calculator that was about the size of a pepper grinder. Over time, during the 1950s and 1960s a variety of different brands of mechanical calculator appeared on the market. The first all-electronic desktop calculator was the British ANITA Mk.VII, which used a Nixie tube display and 177 subminiature thyratron tubes. In June 1963, Friden introduced the four-function EC-130. It had an all-transistor design, 13-digit capacity on a 5-inch (130 mm) CRT, and introduced reverse Polish notation (RPN) to the calculator market at a price of $2200. The model EC-132 added square root and reciprocal functions. In 1965, Wang Laboratories produced the LOCI-2, a 10-digit transistorized desktop calculator that used a Nixie tube display and could compute logarithms Advanced analog computers Before World War II, mechanical and electrical analog computers were considered the "state of the art", and many thought they were the future of computing. Analog computers take advantage of the strong similarities between the mathematics of small-scale properties—the position and motion of wheels or the voltage and current of electronic components—and the mathematics of other physical phenomena, for example, ballistic trajectories, inertia, resonance, energy transfer, momentum, and so forth. They model physical phenomena with electrical voltages and currents as the analog quantities. Centrally, these analog systems work by creating electrical analogs of other systems, allowing users to predict behavior of the systems of interest by observing the electrical analogs. The most useful of the analogies was the way the small-scale behavior could be represented with integral and differential equations, and could be thus used to solve those equations. An ingenious example of such a machine, using water as the analog quantity, was the water integrator built in 1928; an electrical example is the Mallock machine built in 1941. A planimeter is a device which does integrals, using distance as the analog quantity. Until the 1980s, HVAC systems used air both as the analog quantity and the controlling element. Unlike modern digital computers, analog computers are not very flexible, and need to be reconfigured (i.e., reprogrammed) manually to switch them from working on one problem to another. Analog computers had an advantage over early digital computers in that they could be used to solve complex problems using behavioral analogues while the earliest attempts at digital computers were quite limited. A Smith Chart is a well-known nomogram. Since computers were rare in this era, the solutions were often hard-coded into paper forms such as nomograms, which could then produce analog solutions to these problems, such as the distribution of pressures and temperatures in a heating system. Some of the most widely deployed analog computers included devices for aiming weapons, such as the Norden bombsight and the fire-control systems, such as Arthur Pollen's Argo system for naval vessels. Some stayed in use for decades after WWII; the Mark I Fire Control Computer was deployed by the United States Navy on a variety of ships from destroyers to battleships. Other analog computers included the Heathkit EC-1, and the hydraulic MONIAC Computer which modeled econometric flows. The art of analog computing reached its zenith with the differential analyzer, invented in 1876 by James Thomson and built by H. W. Nieman and Vannevar Bush at MIT starting in 1927. Fewer than a dozen of these devices were ever built; the most powerful was constructed at the University of Pennsylvania's Moore School of Electrical Engineering, where the ENIAC was built. Digital electronic computers like the ENIAC spelled the end for most analog computing machines, but hybrid analog computers, controlled by digital electronics, remained in substantial use into the 1950s and 1960s, and later in some specialized applications. But like all digital devices, the decimal precision of a digital device is a limitation, as compared to an analog device, in which the accuracy is a limitation. As electronics progressed during the twentieth century, its problems of operation at low voltages while maintaining high signal-to-noise ratios were steadily addressed, as shown below, for a digital circuit is a specialized form of analog circuit, intended to operate at standardized settings (continuing in the same vein, logic gates can be realized as forms of digital circuits). But as digital computers have become faster and use larger memory (for example, RAM or internal storage), they have almost entirely displaced analog computers. Computer programming, or coding, has arisen as another human profession.