Slide rules are great for multiplication, division, powers, roots, and other advanced forms of math, but what about basic addition and subtraction? Slide rules really suck at that. I have one slide rule that can do addition and subtraction, a Jeppesen CR-3, and it's limited to totals between 0 and 360.
That's where a device like the Addiator comes in. To do addition, place the stylus in the hole for each number and drag it towards number window. If the hole is red, you instead drag it away from the window and around the bend, which automatically carries in the next column. Subtraction is done using the lower set of numbers, always moving towards the window unless the hole is red, in which case you move away and around the bend to borrow. To clear the machine, pull the handle at the top all the way up, then push it back in.
The Addiator can handle positive totals up to 9,999.99 (or 999,999). They did make a version that could do negative totals, but they're uncommon.
