Invented in 1873, this ingenious mechanism enabled a single wheel to add any digit from 0-9 with just one rotation
Move the slider to extend 0-9 pins from the pinwheel. Each extended pin acts as a gear tooth.
As the crank turns, the extended pins mesh with the counter gear and advance it.
Each extended pin advances the counter by exactly one position. 5 pins = add 5.
One full rotation adds your digit once. Multiple rotations multiply the addition.