Calculators, Mechanical and Electromechanical

The widespread use of calculating devices in the twentieth century is intimately linked to the rise of large corporations and to the increasing role of mathematical calculation in science and engineering. In the business setting, calculators were used to efficiently process financial information. In science and engineering, calculators speeded up routine calculations.

At the beginning of the nineteenth century, mechanical calculators were already in widespread use, and by 1822 Charles Babbage was at work on his difference engine (which he never completed because of the mechanical complexity of thousands of brass cogs and gears). Based on technology developed over several centuries, early twentieth century calculators can be divided into two major types: (1) the slide rule; and (2) the adding machine and related devices.

Invented by William Oughtred in the seventeenth century, the slide rule is based on the physical relationship between two logarithmic scales. In the mid-nineteenth century, accurate methods for reproducing such scales on instruments were developed, and the slide rule became much more widely available. By the early twentieth century, slide rules were commonly used by scientists and engineers for calculations involving multiplication, division, and square roots.

During the first half of the twentieth century, slide rules underwent a gradual evolution. Plastic gradually replaced wood and aluminum in construction, and additional scales were added to more expensive slide rules for specialized computation. In addition, a variety of specialized slide rules were developed for particular applications, most notably electrical engineering.

Slide rules had several advantages over manual calculation. First, with practice, calculations could be made much more quickly than by hand. Second, slide rules were compact and light, and so could be used almost anywhere. Finally, calculations could also be carried out with a reasonable degree of accuracy.

However, slide rules also had one important disadvantage—the results of their calculations were approximations rather than exact numbers. This was acceptable for most scientific and engineering calculations, but unacceptable for financial calculations, which had to be exact. As a result, the use of slide rules was limited primarily to scientific and engineering applications.

The second major type of mechanical calculator was the adding machine and related devices. While a variety of adding machines were developed beginning in the seventeenth century, a series of major innovations in the late nineteenth century led to the widespread marketing of machines that were much more reliable, compact, and easy to use.

The combination of increased demand from large business and government organizations, along with improvements in machine tools, materials, and manufacturing techniques, triggered a period of intense innovation that continued until World War I. The key mechanical innovations were all patented before 1900, but putting them into practice took some time, and it was not until the first years of the twentieth century that mechanical calculators began to sell in large numbers (see Computers, Uses and Consequences).

Two key mechanical innovations just before 1900 made this expansion possible. The first, invented almost simultaneously by the American Frank S. Baldwin and the Swede Willgodt Theophil Odhner, consisted of a round disk with moveable radial pins that could be extended beyond the edge of the disk. Input for calculation was a function of varying the number of pins extended by the action of levers, which then meshed with a register mechanism. This design, known both as Baldwin type and Odhner type, proved much more compact and reliable than previous systems.

The second was the introduction of the keyboard for data entry. The first practical machine to use a keyboard for data entry, the Compometer, was invented in 1885 by the American Dorr Eugene Felt, who formed the Felt and Tarrant Manufacturing Company in 1887 to produce his device. The keyboard entry system greatly simplified the operation of calculating machines, and was soon copied by other manufacturers, such as William Seward Burroughs, who founded the American Arithmometer Company (which became the Burroughs Adding Machine Company in 1905). These machines had nine rows of keys, one for each digit (1 to 9). The number was entered by pressing one digit in each column. There was no zero key because zero was represented by the absence of a keystroke in the corresponding column.

During the first half of the twentieth century, the market for mechanical calculators was divided into roughly three categories of machines. The first, and largest in terms of total numbers produced, was adding machines. These machines, produced in large numbers in standard designs that changed little over time, were used primarily for basic accounting by small businesses. Their manufacture was dominated by three large firms: the Burroughs Adding Machine Company and Felt and Tarrant in the U.S. and Brunsviga in Germany, though a variety of smaller firms also competed.

The second type of mechanical calculator was the four-function calculator, which were similar mechanically to adding machines, but performed multiplication and division in addition to addition and subtraction. Such calculations could be carried out on adding machines by trained operators, but four-function calculators were faster and allowed the use of less highly trained operators. Four function machines were used primarily in medium-sized businesses whose volume of calculation did not justify the use of more expensive specialized machines.

They were also used by engineers, which led to the development of machines in the 1950s that included specialized functions such as calculating square roots, such as the Friden SRW model of 1952 (which weighed 19 kilograms). No one firm dominated the market for these machines, and there was considerable competition for market share.

The third type of mechanical calculator was a group of devices known as accounting machines (also called book-keeping machines during the 1920s and 1930s). These devices were used to enter data onto standard forms and then to perform accounting calculations. They could also prepare balances and print the results. Used by larger firms whose volume of calculation could justify the investment in specialized machinery, in some applications accounting machines competed directly with punched card tabulating systems, such as those developed by Herman Hollerith in 1890 and later developed by the successor to Hollerith’s company, International Business Machines (IBM). Accounting machines were also used in specialized niche applications.

All three types of calculator could be either hand or motor driven in operation. Motor-driven mechanical models first appeared just after 1900. By replacing the hand crank with a small electric motor, these machines were less tiring to operate and could reliably perform repeat operations, simplifying the construction and operation of machines that performed multiplication and subtraction. As a result, motor-driven calculators were commonplace on the desks of engineers by the 1940s. Hand-driven machines continued to sell well, however, because they were quieter, lighter, smaller, and less expensive.

Regardless of the type of mechanical calculator, firms found that to be successful they had to provide a high level of service to customers. Calculators were sold, not bought, and firms maintained large sales forces to educate customers as to the capabilities of machines and to anticipate customer’s needs. Calculator manufacturers also had to service their machines, which as mechanical devices need constant maintenance to function reliably, and train operators in the correct operation procedures.

The manufacture and sale of calculators was a widespread industry, with major firms in most industrialized nations. However, the manufacture of mechanical calculators declined very rapidly in the 1970s with the introduction of electronic calculators, and firms either diversified into other product lines or went out of business. By the end of the twentieth century, slide rules, adding machines, and other mechanical calculators were no longer being manufactured.