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 Dividers 
Dividers are one of the earliest and most basic types of mathematical instrument. In their simplest form, dividers consist of a jointed pair of legs, each with a sharp point. They can be used for geometrical operations such as scribing circles but also for taking off and transferring dimensions. They have therefore been a tool not only of mathematicians and designers but also of craftsmen, and these different uses have given rise to their manufacture in different sizes and materials.
Dividers for craft use are often large devices made in iron, sometimes with a screw between the two legs in order to adjust and fix their opening. As a mathematical instrument, dividers are found on a smaller scale, typically made in brass with steel points, though more precious materials such as silver have also been used.
Although ancient examples have been recovered (for example, those found at Pompeii) the major stream of surviving instruments begins in the 16th century. By this period, dividers were available with a socket in one of the legs so that either a divider point or a crayon or pencil holder could be inserted. In the latter configuration, as a device for drawing circles, the dividers became a pair of compasses.
More specialised designs were also available. Singlehanded dividers were used by navigators when plotting course and distance on a sea chart; as their name suggests, they could be manipulated with only one hand, leaving the other free. Ordinary dividers cannot be used effectively on a curved surface such as a globe, and a design with legs whose points curve inwards was developed for this purpose.
Dividers would remain a staple part of the repertoire of the mathematical instrument maker long after 1600. Very often sold as part of a larger set of mathematical instruments, they continue in use today.
M. Hambly, Drawing Instruments, 15801980 (London, 1988).
Stephen Johnston 
