On the way home from work last night, I picked up a package at the Post Office. It was from a friend (Franz M.) in Munich, Germany, and it was full of slide rules. I haven't purchased any slide rules for many months, so for the first time in a long while I had the pleasure of opening up a box with some beautiful old instruments in it.
My favorite of the bunch is a circular “Columbus Calculator”. Mostly it is conventional in design, except that it's made of engraved metal (that is, the scales are engraved). I'm pretty sure that the engraving is photolithographic, and it's very nicely done. The face is plated (with brass, I think) and the plating has worn, exposing parts of the underlying metal – yet the scales are still perfectly readable and usable. I've never seen another like it, nor can I find any reference to it online...
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