Felt & Tarrant Model J Comptometer  
Written by AnubisTTP on 2006-11-21  


Comptometer-style mechanical calculators were once among the most common and popular forms of adding machine available. Though Felt & Tarrant originally invented and marketed the actual Comptometer brand calculating device, the word 'comptometer' has since become the vernacular. Comptometers are characterized by their massive array of keys. Unlike a modern calculator which only has a single set of numeric keys and forces the user to enter numbers one digit at a time, comptometers have a column of numeric keys for each digit. This allows a skilled operator to add numbers on the machine at a very fast pace, far faster than on a conventional calculator, by striking every column of keys at once. Comptometers are designed primarily for repetitive addition. Though capable of other mathematical operations, execution is not so straightforward. In fact, most of the entities that manufactured Comptometer-style calculators ran academies which companies could send their employees to, in order to learn how to operate these machines.


This unit was manufactured in 1927 and originally sold for approximately $300, which is a significant sum of money in 1927 economics. The unit shown here has nine digits, eight of which have data entry keys. Units with eleven, thirteen, and even fifteen digits were also made but are less common. There is also a small row of carry suppression buttons for each digit; these are used during subtraction to suppress the automatic carry in the mechanical register. There are also a number of oil holes in the top of the machine, with rows appearing at the top and bottom of the keypad, and several in other locations as well.

The mechanism for clearing the register of the machine is very sophisticated for this type of device: the operator uses their pinkie finger to pull the lever to the right of the machine. A bell sounds and all of the digits reset. The red key at the top of the keypad is the 'controlled' key, a feature many comptometer's lack. In the event of a partial strike of a key that does not advance the register, the keyboard will lock out to alert the user to the keying error. The controlled key is used to unlock the keyboard when this happens. A common source of damaged comptometers is people who have tried to force down a key without pressing the controlled key first to release the keyboard.

Once damaged, a comptometer is rarely repaired. The Model J mechanism is constructed of myriad small thin metal plates, held together by a vast matrix of tiny springs and e-clips. There is no way to make a repair to the machine without removing masses of springs and tiny pins.

Felt & Tarrant Model J Comptometer
Felt & Tarrant Model J, serial number J277278.

Felt & Tarrant Model J Calculator
Family photo of two different Felt & Tarrant Model J comptometers, as well as an original sheet metal comptometer dust cover.

Felt & Tarrant Model J Comptometer
With the cover removed, it can be seen that the Model J's internal layout was apparently designed with the secondary function of convincing the end user that there is no way they could ever possibly repair it on their own.

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