© by Mark Ollig
Business offices used this device decades before the arrival of electronic calculators.
The design was both innovative and reliable, resulting in increased bookkeeping efficiency.
The Comptometer is a mechanical calculating machine using vertical and horizontal column rows of push-button keys to perform calculations.
This story begins March 18, 1862, with the birth of Dorr E. Felt in Beloit, WI.
One might think naming a baby boy “Dorr” was unusual, and so did I, until I researched and discovered it was a common name given until the late 1920s.
Dorr F. Kellogg was also born in 1862 in Cayuga, NY.
But I digress.
After completing his education in public schools, Dorr Felt began working in the machinist trade.
By 1882, he became skilled as a mechanical draftsman and developed his inventive abilities while working in Chicago at the Ostrander and Huke machine workshop.
In November 1884, Felt was watching a planer metalworking machine controlling the varying depths of a cut which, he said, set him “to scheming on ideas for a machine to simplify the hard grind of the bookkeeper in his day’s calculation of accounts.”
He concluded that the mechanism consisting of a ratchet feeding and key-driven gear motion could be used to create a unique calculating machine.
The same year, Felt began building his prototype calculator within a wooden case, saying, “I went to the grocer’s and selected a box which seemed to me to be about the right size for the casing. It was a macaroni box, so I have always called it the macaroni box model.”
On Thanksgiving Day in 1884, he continued assembling his calculator machine, using makeshift tools, including a jackknife.
“I procured some meat skewers from the butcher around the corner and some staples from a hardware store for the key guides and an assortment of elastic bands to be used for springs,” Felt recalled.
His prototype, completed in 1885, became known as the “macaroni box.”
In 1886, Felt partnered with Robert Tarrant, a Chicago businessman.
Tarrant provided Felt with $5,000 and a weekly salary of $6 to complete building his fully-functional calculating machine, the Comptometer.
By 1887, the completed Comptometer included a numerical keyboard and a series of mechanical registers and dials.
Its operation involved a user pushing numbered button keys to perform the desired calculations and then pulling a lever to see the resulting number on the bottom row.
The Comptometer’s push-button keys were set up in columns and rows, representing the digits zero to nine.
Each key was connected to a gear corresponding to the value of that digit’s place. Pressing a key engages the gear teeth to add or subtract the corresponding value.
The user could reset all the gears to zero after each calculation through a clearing mechanism which improved the efficiency and speed of the computing process.
Dorr Felt was aware of the mechanical arithmetic machine, the Pascaline, created by French mathematician Blaise Pascal in the mid-17th century.
The Comptometer, similar to the Pascaline, utilized the “nine’s complement” approach to calculate the subtraction of decimal numbers, which involved using labeled keys corresponding to the nine’s complement to subtract numbers.
To find the nine’s complement of 1423, we need to subtract each digit from nine, giving us the nine’s complement of 8576.
I envision my third-grade teacher, Mrs. Seymour, smiling and asking if I still recall my multiplication tables.
On July 19, 1887, Dorr Felt obtained US Patent No. 366,945, titled “Adding Machine.”
He later received US Patent 371,496 on an improved version of his adding machine Oct. 11, 1887.
On Oct. 27, 1888, Scientific American wrote of the Comptometer that the “accuracy and durability of the machine have been thoroughly tested in the actuary [insurance] department of the United States Treasury at Washington, where one is in constant use.”
In January 1889, Dorr Felt and Robert Tarrant established the Felt and Tarrant Manufacturing Company in Chicago with an initial investment of $100,000, equivalent to $3.2 million today.
In 1893, the Comptometer was showcased at the Chicago World’s Fair.
Several Comptometer models were built, including Model F, with dimensions of 6 3/16 inches by 9 7/16 inches by 14 3/8 inches. It weighed approximately 21 pounds and was manufactured from 1915 until 1920.
It was necessary to add oil to the 22 small metal holes near the push-button keys to properly lubricate the calculator’s mechanical parts to keep them running smoothly.
Early Comptometer models included a sheet-metal dust cover printed with the following:
Comptometer (Pronounced like thermometer)
Felt & Tarrant MFG. Co. Chicago, USA.
Adds – Divides – Multiplies – Subtracts.
On Aug. 7, 1930, at age 68, Dorr Eugene Felt, the inventor of the Comptometer, died in Chicago.
On July 4, 1937, the Minneapolis Sunday Tribune had a brief article on the Comptometer, saying, “The original model of the first computing machine invented fifty years ago by the late Dorr E. Felt has been placed in the Smithsonian Institution.”
The Comptometer truly revolutionized office efficiency.
(Below are photos I took of my Comptometer)
