### Thacher’s Calculating Instrument n°4012

No, I’ve not gone to the dark side, to start collecting slide rules. But as the exhibition we set up for 400 years of mechanical calculating drew to an end, I nevertheless developed a healthy appreciation for slide rules, especially very pretty ones like this one. Thacher’s Calculating Instrument is a spelling nightmare to start with. Edwin Thacher, of Pittsburgh, Pennsylvania, developed this special kind of cylindrical slide rule, with 30ft. (or 10m) scales (so good for 4-5 significant figures), in which the second scale is applied onto prismatic brass strips, with two scales on either side of the prism. The scales are cut, not spirals, just like on Loga cylindrical slide rules, and the inner cylinder is smooth. Whereas in the Loga, the outer scale is moveable, in the Thacher cylindrical slide rule, it is the inner scale which can be slid and rotated with respect to the outer prismatic scales.I bought this slide rule at auction, and it came with its original manual. The box was in horrific condition, but my friendly wood restorer managed to bring it back to the beautiful mahogany box it is supposed to be.

The manual specifies there is another model of Thacher’s slide rule to be had, namely model 4013, which comes equipped with a brass slide rail with a 3 in. magnifying glass.

I would have constructed that in a heartbeat, if the base of the cylinder would not have had the model number 4012 inscribed ...

The worst thing about this slide rule is the name of the maker. It is apparently so hard to spell that even on the slide rule itself it is misspelled as Thatcher!

The manual that comes with the calculator was printed in 1917, so about 35 years after the patent. Apart from a description on how to use the slide rule and a set of useful tables, it also contains the pricing information: $40 for model 4012, $47.5 for model 4013 with the magnifying glass - and another $1 for the instruction booklet. Copare thisto the $200-300 for a Peerless stepped drum calculator, which Keufel & Esser also sold at the time.

For a description fo the instrument itself, it may be best to quote directly from the manual: “It consists of a cylindrical slide provided with a knob on each end, and which admits both longitudinal and rotary movement within an open framework or envelope of equidistant bars of triangular section.”

What is interesting next is the arrangement of the three scales that are on the instrument. On the inner cylinder, we have a logarithmic scale A. This is repeated on the outer cylinder on the lower end of the triangles, this is scale B. On the upper part of the triangles there is a scale C, which is quadratic - i.e. with respect to scale B, the numbers on C are the squares, or in reverse, the numbers on B are the square roots of the numbers on C.

Both the cylinder and the “envelope” repeat the same scales on the left and the right part - the cylinder has a double scale, half of which is always hidden under the triangular bars of the envelope, shifted by half the length of the cylinder, scale B on the envelope is always repeated on the top part of one triangular bar on the right, and on the bottom part of the triangular bar on the left. The interesting part is that the cilinder can also be slid out of the envelope and reversed, which makes the relationship between the logarithmic A abd B scales reciprocal.

All in all, this is a very pretty calculating instrument, and I don’t mean to eliminate it from the collection any time soon.