Very cool!

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My latest hobby is a couple of Monroe Calculating Machines.?ÿ My old one was made in 1921, still works.?ÿ Sometimes there are mistakes but it is always the "nut behind the wheel" (me) so far. The "new" one is circa mid-1930s, it is electric but I haven't got that working yet, it can be used without power.?ÿ The calculating mechanism is fairly simple, just gears and wheels, the electrical switching mechanism is very complicated, a lot going on.

The logic is similar to hp RPN. Enter number on keyboard, add to enter into the register, enter second number, add or subtract as desired.?ÿ It can do addition, subtraction, multiplication, division and square roots.