The Addometer is a mechanical adder which in terms of use and external appearance resembles Blaise Pascal 's pascaline .

The adder looks like a heavy ruler, about 30 cm long. Circular slots allow you to operate numbered wheels placed inside, through special holes made on the wheel. Each hole corresponded to a number written in large characters on the inside of the slot and its complement to 9, written on the outside in smaller characters. To add, the tip of the stylus was placed in correspondence with the large numberand the disc was rotated clockwise. To subtract, on the other hand, the stylus was placed in correspondence with the external number and performed a counter-clockwise rotation. A much simplified mechanism compared to that of Pascalina allowed the carry over of the tens. The result appeared automatically and it was possible to reset it by pulling the tab on the right of the instrument. 

It was produced from 1928 until the 1960s by Reliable Typewriter and Adding Machine Co .. The numbers were imprinted on perforated and toothed discs. These were very cheap and reliable instruments, so much so that they were covered by a one-year warranty, which was quite unusual for the time.