This Monroe Model G mechanical calculator was made some time between 1918 and 1920, and is the one I used on the cover of my new book. I also featured non-blurry pictures of it as the background image on each chapter, and in those cases left a little easter egg for keen-eyed readers.
@kyle People go crazy with mechanical keyboards, imagine having these...
@Konqi For Comptometers in particular, professionals received training very similar to professional typists. They touch typed without looking at the keys (eyes and other hand was on the document with the figures).
For speed, they not only chorded the keys (an advantage to full-keyboard calculators versus ones that only had a single set of 0-9), they also only pressed 1-5 to keep their hand on "home row" and to make, say, 7, would type 3 then 4. Pros were *very* fast at this.
@kyle Still haven't delved into how to do complex math.
@cavaughan Once you know addition and subtraction, then multiplication is just repeated addition, with shifts to the left for each digit in the multiplier. Division is similar, but starts at the left of the dividend, and you subtract repeatedly until it underflows (the machine will ring a bell!) and then you add one back and shift right. Repeat until you have enough digits of precision to suit you.
@cavaughan I have a Comptometer manual that goes into how to do more advanced things like roots, but it seemed complicated enough that I'd never know if I got the right answer.
@kyle yeah I got all the primary math. My notes are here: https://vintagecomputers.avantguardsystems.com/calculators/arithmometer