TavernierGravet

Home Up	Contact Legal Stuff

Slide rules for a new century
Tavernier-Gravet’s evolving slide rule technology

Here is a nice French Mannheim slide rule from the end of the 19th century, nicely presented in a fitted wooden box. It has two scales running 1-100 at the top and two running 1-10 at the bottom, and a cursor that allows moving between them precisely, which enables extraction of squares and square roots.

Click photo to enlarge

When the French artillery officer Amédée Mannheim invented this new kind of slide rule in 1851, he asked the Gravet-Lenoir workshop in Paris to productize the invention. This was a natural choice: Gravet-Lenoir (who would rebrand in 1867 as Tavernier-Gravet) was a leading scientific instrument maker, and well positioned to do justice to Mannheim’s idea. Which was an excellent idea: since their invention two centuries before, slide rules had been made in a plethora of different designs, which Mannheim improved by defining the standard set of scales mentioned above and adding the cursor, a standard feature in all later slide rules. This arrangement remained in use for 100 years, and was the basis for many further improvements.
Over the years I’ve accumulated four Tavernier-Gravet slide rules. Three are straight Mannheims, and the fourth is a “règle Beghin”, a variant with two scales “folded” to place the 1 at the center rather than at the left edge. Each is a nice collectible, but when I recently examined them together I realized that they show an interesting trend in time (and I just love technology trends!): they represent the transition from 19th century to 20th century slide rule technology. Here they are:

Click photo to enlarge

And here is the back side:

Click photo to enlarge

We have here:

A Mannheim made between 1889 and 1895, of wood, with a brass chisel cursor.
A Mannheim made between 1895 and 1900, celluloid on wood, with a brass chisel cursor.
A Mannheim made between 1900 and 1905, wood, with a glass cursor.
A Beghin rule made in 1911, celluloid on wood, with a glass cursor.

(The date ranges are deduced from the various dates present on the backs of the slide rules and other details.)
Beyond the date sequence, when I say this lot represents a transition, I look at two attributes:

The material for the scales -- are they imprinted directly on the wooden body or on a celluloid lamination coating the wood?
The construction of the cursor -- all brass with “chisel” pointers, or glass with a scribed hairline?

The transition from bare wood (boxwood, usually) to a celluloid veneer (first patented by Dennert & Pape, Germany, in 1886) greatly improved readability of the scales; it is not surprising that it was soon adopted by all slide rule makers. The change in cursor, from brass to glass, makes a lot of sense too, since it allows unhindered viewing of the scales. However, the examples shown here are only halfway there: the area around the hairline is glass, but the glass is held in a wide metal holder -- clearly adapted from the older chisel cursor. The transition would be completed a few years later, by T-G and everybody else, with the introduction of all-glass structures like the one seen at the right in the image below.
You can read more about the evolution of the cursor in my article here.

Click photo to enlarge

Tavernier-Gravet began the transitions to celluloid and to glass during the 1890s, and for a number of years were offering both materials and both types of cursors at different price points, until the newer versions became standard. However, slide rule #2 tells of the difficulties they’ve encountered, as often happens with a new technology. On this rule the celluloid layer on the slide has shrunk lengthwise -- by a noticeable 1 mm -- and in so doing it popped right off, detaching itself completely from the underlying wood and curling up at the ends (photo at left, below). This tells us that the company hadn’t yet mastered the art of gluing wood and celluloid, diverse materials with different expansion coefficients in the face of changes in ambient temperature (and perhaps moisture). Evidently they hadn’t yet hit on the correct formulations of glue and other process parameters.
But they did more R&D, and they improved. Slide rule #4 (photo at right)shows us their solution: it has little screws at the ends of the celluloid strips to prevent the fate of rule #2.

Tavernier-Gravet slide rule with peeled celluloid facing

Click a photo to enlarge

These screws were first patented by Nestler in 1901 and are seen in early celluloid-faced slide rules from a number of manufacturers, although they later disappeared once better materials and techniques had made them unnecessary.
And here is a photo of a Tavernier-Gravet rule made in 1942, after they’d solved the peeling problem, dropped the screws, and adopted an all-glass cursor. This one is not mine, and I thank Gonzalo Martin for permitting me to show it here.

Click photo to enlarge

Note the little diagonal notches at the corners of the celluloid strips… another innovation adopted by many manufacturers to reduce the likelihood of chipping or separation of the celluloid.
Technology marches on, improving incrementally but surely!

Exhibit provenance:
Slide rule #1 was an eBay purchase; the other three were acquired from various fellow collectors.

More info:
Early French slide rules on photocalcul site.

Tavernier-Gravet catalogs and ads on the same site.

Acknowledgments:
Many thanks to Jim Bready, Bob de Cesaris, Gonzalo Martin and Marc Thomas for insights and information that made this article better!

Slide rules for a new century

Tavernier-Gravet’s evolving slide rule technology