I have one from around 1958, and it's quite satisfying to use.
They can multiply and divide as well as adding/subtracting. The above link shows the result of doing 355/113: 3.1415929 with a remainder of 23.
The top left is an accumulator, the top right is a counter, and the lower register is the number you want to add/subtract (entry register). So to do 355/113, the procedure is
- Pull all three 3 metal tabs on the sides to clear all registers
- Enter 355, press the rightmost red arrow button to shoot the entry register number all the way to the left
- Crank forward once. You now have 3550000000000 in the accumulator and "1" in the counter's leftmost position.
- Squeeze the two rightmost chrome handles together to clear both the counter and entry register back to 0
- Enter 113, press the rightmost red arrow button to shoot the number all the way to the left. You're done entering numbers at this point.
- Crank backwards to subtract from the accumulator until it is less than the entry register (takes three times). Don't worry if you overshoot; a bell will ring to indicate underflow and you just add it back. The counter now shows three in the leftmost position. The red dot indicates that it notes you started off subtracting, so it's counting backward cranks as +1 instead of -1.
- Press the right arrow to shift the entry register one position to the right
- Repeat the subtracting process, shifting right until you can't go any more right. You're done.
It sounds more complicated than it is, but really it's just long division. It takes about 20-30 seconds to do that division. That sucker works as well as the day it was built. I've looked inside; it's a mechanical marvel.
Oh yeah, those white slider tabs are for placing the decimal points where you want them