Fraction of an Inch Adding Machine

The Fraction Of an Inch Adding Machine, or Fractron, is simple disc adder. It consists of three discs made of aluminium. The front and the back disc are riveted together. This sandwiches the middle disc though it is still able to rotate because it is really a ring rather than a disc. The moving disc has 64 holes evenly spaced about its rim, representing the fractions of an inch from 0 to 63/64. The stationary front disc shows these fractions in lowest terms and in a decimal representation. Using a stylus you can dial in any number of these fractions, and the total amount is displayed through a small window in the front disc. To reset the display to zero use the stylus on the hole that is marked in black.

The machine does not have a carry mechanism, but there is a notch in the outer disc and a tab on the moving disc. When you hold the disc you can put one finger in the notch. When the addition reaches a whole number of inches, the tab can be felt moving past the notch, alerting you to the fact that the result overflowed.