# 4 The Collection in Photos

At present, the collection contains a total of 294 slide rules, with 266 linear slide rules, 23 circular rules, as well as 2 and 3 spiral and helical rules, respectively. The “rule” for entrance into the collection is that there must be a logarithmic scale as part of the device. Hence sectors and Gunter’s rules are eligible, for instance, though they are not technically “slide” rules. However, items such as Addiators, Curtas, and other mechanical computational tools are not included.

Perhaps the best way to scan through the slide rule collection is by way of the images in this chapter. The entries are ordered according to Year and sub-ordered according to Maker and Model. Some of the dates for the slide rules are accurately known, but for many the dates are simply honest guesses. For a few further details of each rule of the Collection as well as other information regarding manufacturers, see Slide Rule Makers. Looking for a particular rule, or set of slide rules? A Searchable Table can be used for a quick scan of the entire collection.

The concept of a logarithm was developed by John Napier starting at the end of the 16th century, with his major publication on the subject in 1614. Napier’s tables of logarithms, soon improved upon by his colleague Henry Briggs, could be used to more easily perform multiplication and division of numbers used in astronomy and navigation, for example. By 1620 Edmund Gunter created a scale on a rule that had distances to the numbers on the scale proportional to the logarithms of those numbers, thus allowing quick multiplication without the use of tables. He used calipers or “compasses” to measure distances along the scales and do the additions. Gunter’s scales were of great use in navigation, where bearings were made and trigonometric calculations performed to map out a course. “Gunter’s Rules” had logarithmic scales from 1 to 10 or 1 to 100, as well as logarithmic scales of trigonometric functions. Another instrument often used for navigational calculations, available since the time of Galileo (perhaps its inventor) in the 1500s, was the sector. It had scales that could be used for multiplication and division, finding roots and squares and trigonometric values, and was also used with calipers or compasses. English sectors in the mid-1600s began to include Gunter’s scales on them as well.

Within a short time, William Oughtred showed how two Gunter rules could be slid back and forth next to each other to perform the same task without the use of calipers and the “slide rule” was born. The earliest examples of these instruments are now found in museums, while the proliferation of slide rules within the general public did not begin to take hold until perhaps the middle of the 19th century.

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