Accession No

2390

Brief Description

circular slide rule, by H. C., French, 1/2 20th Century

Origin

France; Paris

Maker

H. C.

Class

calculating

Earliest Date

1900

Latest Date

1950

Inscription Date

Material

glass; paper; metal (steel, white metal)

Dimensions

length 75mm; breadth 53mm; thickness 20mm

Special Collection

Provenance

Purchased from Christie’s, Belgravia, London, England; lot 10, 09/08/1978.

Inscription

‘CALCULIGRAPHE
H.C.
PARIS’ (obverse)
‘MADE IN FRANCE’ (reverse)

Description Notes

Circular slide rule in form of fob watch. White metal case with glass covers over paper dials (one on each side). ‘Winder’ knob moves cursor over scales. Small knob on side of instrument needs to be depressed in order for index arms to be moved.
Obverse: single scale marked ‘ln3’ spread over three circles, divided [1] - [10], numbered 1[.]1, 1[.]2...2[.]2, 2[.]4...4, 4[.]5...9[.]5. Outer scale of equal parts divided [0] - 10, numbered by 1, subdivided to 0.05. Blued steel index arm.
Reverse: inner two rings carry scale marked ‘ln2’, divided [1] - [10], numbered 1[.]2, 1[.]4...2, 2[.]5...5, 6...9. Next ring carries scale marked ‘ln’, divided [1] - [10], numbered as previous scale. Outer ring is scale of sines, divided 6˚ - 90˚, numbered 6, 7...10, 12...20, 25...50, 60...90. Blued steel index arm and cursor.

Condition fair; complete.

References

Events

Description
This instrument is a circular slide rule, a variation on the most common type.

Developed during the seventeenth century, the modern slide rule is based upon the design by William Oughtred (circa 1630). It is one of many calculation devices that is based on the logarithmic scale, a calculation method invented in 1614 by John Napier.

Before the rise of the pocket electronic calculator in the 1970s, the slide rule was the most common tool for calculation used in science and engineering. It was used for multiplication and division, and in some cases also for ‘scientific’ functions like trigonometry, roots and logs, but not usually for addition and subtraction.

A logarithm transforms the operations of multiplication and division to addition and subtraction according to the rules log(xy) = log(x) + log(y) and log(x/y) = log(x) - log(y). The slide rule places movable logarithmic scales side by side so that the logarithms of two numbers can be easily added or subtracted from one another. This much simplifies the alternative process of looking up logs in a table, thus greatly simplifying otherwise challenging multiplication and division. To multiply, for example, you place the start of the second scale at the log of the first number you are multiplying, then find the log of the second number you are multiplying on the second scale, and see what number it is next to on the first scale.

Circular slide rules allow the longest dimension to be reduced by a factor of about 3 (i.e. π).

FM:42530