The comptometer invented by Dorr Felt is…

The comptometer invented by Dorr Felt is the direct descendant of the key-driven machine of Thomas Hill, patented in the USA in 1857, and of the Pascaline invented by Blaise Pascal in France in 1642.

By just replacing the input wheels of the Pascaline by the columns of keys of Hill's machine, Felt had invented the comptometer.

Addition is performed exactly the same way, and both the Pascaline and the Comptometer make use of the 9's complement method for subtraction, but in the case of the comptometer it is the operator who must choose the right keys for the subtrahend (each key has its 9's complement written in miniature letter next to it).

Dorr Felt was born in Newark, Wisconsin where he grew up on the family farm and which he left at age fourteen to seek employment.

In early 1882, at age twenty, he came to Chicago and worked as foreman of a rolling mill that had a daily output valued at two thousand dollars.

During the US Thanksgiving holidays of 1884, he had decided to build the prototype of the comptometer, a new calculating machine that he had invented.

Because of his limited amount of money, he had used a macaroni box for the outside box, and skewers, staples and rubber bands for the mechanism inside.

It was finished soon after New Year's Day, 1885.

Felt brought his idea to Robert Tarrant, the owner of a Chicago workshop, who gave him a salary of six dollars a week, a bench to work on and what will add up to five thousand dollars to build his first practical machine, which he had finished in the autumn of 1886.

The first two patents are granted to Felt on July 19, 1887 and on October 11, 1887.

By September 1887, eight production machines had been built.

Felt and Tarrant sign a partnership contract on November 28, 1887.