THE CIRCULAR SLIDE RULE

The slide rule allows you to perform mathematical operations such as multiplications and divisions, conversions between kilometers, statute miles and nautical miles, flight plan and navigation calculations such as average speed, distance traveled, fuel consumption or climb/descent rates.

The external white disk on the bezel is mobile, graduated from 10 to 99, and indicates distances, speeds or fuel consumption. The black disk is fixed on the dial and corresponds to times. It features two levels, with the external disk indicating the minutes, and the internal disk indicating the hours (up to nine).

The first slide rule is entirely black and white; it is present on the models until 1967, when red touches were added: the seconds marking (number 36) on the white disk; the kilometer (KM), statute mile (STAT) and nautical mile (NAUT) markings on the black disk; and especially the unit mark (number 10) on both disks is surrounded by a red rectangle, which earned it its nickname “box 10”.

From 1968, the “box 10” was replaced by a new version where the 10 on both disks and the 60 on the white disk are red. Moreover, a red seconds marking (36) was added on the black disk. These colorful accents were designed to ensure easier reading of the markings and the important numbers for the calculations.

The evolution of the slide rule does not concern the scale or the organization of the numbers, but the addition of markings (36) and of colorful red touches to facilitate reading. The slide rule without any red (top left) corresponds to the first generation (1954-1967), followed by the “box 10” (top right) and finally the “red 10/60” (above) since 1968.