Slide RulesCircular Slide Rules
Slide rules that are round offer the length of a 10" rectangular slide rule in a pocket-sized (roughly 3.6") format, since the scales are on the circumference of circles. The scales are also continuous, so there is no need to make adjustments, such as folded and inverse scales, for results of calculations that go off the ends of the scales. Furthermore, these instruments are relatively easy to construct: the scales are printed on one or more disks, and the disks or a single disk and cursor are fastened together with a pin at the center. However, this simple construction is also not very durable, and so circular slide rules may get out of position and thus they lack accuracy, compared to linear slide rules with slides that move along carefully grooved channels.
This collection suggests the diverse appearances and functions of circular slide rules that were manufactured between the mid-19th and late 20th centuries. For example, before Mannheim-type linear slide rules became popular in the late 19th century, American inventors patented a variety of circular designs. Some circular slide rules were made to look like pocket watches, while others were intended to promote particular businesses—Whitehead & Hoag and Perrygraf were especially influential American manufacturers of promotional items. Inventors and makers such as Albert Sexton, Louis Ross, Claire Gilson, Norman Albree, and Ross Pickett wanted their circular slide rules to compete with linear instruments in the engineering and education markets. Other circular slide rules were designed specifically for surveying, such as stadia computers, or for navigation, such as Dalton instruments that may also be seen in the Smithsonian's exhibition, Time and Navigation. Even more specialized in purpose were slide rules for grading earthworks, determining the effects of nuclear bomb explosions, writing efficient computer programs, and betting on horse races.