Charpentier

Calculimetre

From the 1880's, France, comes a beautiful artifact... the symbol of the Oughtred Society.

Before I ever held one, I hadn't realized that there are only two moving parts: the silver metal (Nickel? A magnet doesn't stick.) disk is one part, and the brass ring - together with the front and back armature - are another.

So the easiest usage is to hold the brass ring and armature steady, and turn the silver disk by its edge.

I've found the engraving to be very accurate. An interesting detail is that the inner rim of the brass ring bears tiny tick marks corresponding to the ends of the tick marks of the outer turn of the steel disk's inner square-root scale. That suggests that the scales were struck on the brass and sliver together, in a single operation, to guarantee alignment.

I imagine that the disk with the ring in its groove, but without any of the armature, was placed face up on the reverse die, the obverse die was laid on top, and pressure was applied - just the way coins are minted. After that, the front armature was riveted to the ring and the back armature was screwed to the front one so as to sandwich the free-spinning silver disk between them. Simple and elegant.