Slide rules are devices used to aid in calculation, and were extremely important tools in science and engineering prior to the development of the hand calculator. Because of their importance instruction in the use of the slide rule was routinely given in Chemistry, Physics, and Engineering classes, frequently using large demonstration rules such as the one seen here. In essence a slide rule works by, 1) adding and subtracting the logarithms of numbers on adjacent sliding scales thereby enabling multiplication and division (log a*b = log a + log b, log a/b = log a - log b), and, 2) comparing various different parallel scales (in essence acting as tables of trigonometric functions, logarithms etc.).
This particular slide rule was the personal property of Dr. Frederick P. Cranston, Jr., Professor of Physics at Humboldt (1962–85).
From Wikipedia article, Slide Rule: "Circular slide rules come in two basic types, one with two cursors [as the one displayed here], and another with a free dish [a second smaller rotating disk and a single cursor ]. The dual cursor versions perform multiplication and division by holding a fast angle between the cursors as they are rotated around the dial. The onefold cursor version operates more like the standard slide rule through the appropriate alignment of the scales.
The basic advantage of a circular slide rule is that the widest dimension of the tool was reduced by a factor of about 3 (i.e. by π). For example, a 10 cm circular would have a maximum precision approximately equal to a 31.4 cm ordinary slide rule. Circular slide rules also eliminate "off-scale" calculations, because the scales were designed to "wrap around"; they never have to be reoriented when results are near 1.0—the rule is always on scale. However, for non-cyclical non-spiral scales such as S, T, and LL's, the scale width is narrowed to make room for end margins." (http://en.wikipedia.org/wiki/Slide_rule#Circular_slide_rules; downloaded 06/27/2012)
The idea of using a logarithmic scale for calculations began with Edmund Gunter when he engraved a scale of logarithms on a piece of wood in around 1620 and used a dividers to add and subtract them to or from each other, and thus multiply and divide. In 1632, William Oughtred divised a pair of sliding scales, which when applied to each other could be used to multiply and divide. It wasn't until 1850, however that the modern form of the slide rule, with one rule sliding between two others, and using a sliding cursor, was divised by Lt. Amédée Mannheim. Circular rules were also developed early, including the first commercial rule, but never achieved the popularity of the linear form.(Hopp, 1999)
The slide rule is constructed of a single 4.25" disk of plastic (celluloid?), graduated on both sides. There is a dual cursor on the obverse (the cursors have a friction fit and may be adjusted relative to each other then rotated as a unit) and a single cursor on the reverse, as seen in this photo.
A similar rule, in aluminum, from Eugene Dietzgen is described in the catalog description above, from: Eugene Dietzgen Co. Dietzgen, Seveteenth Edition General Catalog. Chicago, (1955).
The basic history of the slide rule is outlined in Michael R. Williams, A History of Computing Technology, Prentice-Hall, Inc., 1985, pp. 111-118 and John P. Ellis, The Theory and Operation of the Slide Rule, Dover Pub. Inc., New York, 1961, pp. 247-250. An extensive history of the slide rule is found in A History of the Logarithmic Slide Rule and Allied Instruments by Florian Cajori, The Engineering News Publishing Co., New York (1909). Two more recent and excellent treatises are the books by Dieter von Jezierski, Slide Rules: A Journey Through Three Centurie, Astragal Press, Mendham (2000), and Peter M. Hopp, Slide Rules: Their History, Models, and Makers, Astragal Press, Mendham (1999).