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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 at the large number and the disc was rotated clockwise. To subtract, on the other hand, the stylus was placed in correspondence with the external number and rotated counterclockwise. 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 sixties 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.

The main models on the market were:

- for the calculation with fractions

- decimal calculation, the ruler is graduated in inches

- the last three wheels are for feet, inches and eighths of an inch

- same as model B, but with the ruler in centimeters

- for Brazilian currency units, with the centimeter ruler

- English currency: 5 reels for pounds, 2 for shillings and one for pennies

In all models the wheels were colored appropriately to easily distinguish their function. For example, cents, units (dollars) and thousands had different colors.

In the United States, adders of this type were enormously popular. On the contrary, in Europe, where they had been introduced earlier, rack and pinion additives were preferred, such as the Addiator. The first small adder to have wide diffusion in America was the Calculator, patented by Bonham and Schram in 1905, which was produced and marketed by various companies. One of these, in 1924, introduced an improved version (the Lightning), followed by the Addometer and other more or less similar machines. Addometer and Lightning were the most successful products.

Other firms include Sterling. This produced small plastic adders. In particular, he created a famous version for IBM that worked in hexadecimal base.