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Full-Keyboard Sterling Comptometer

Model J, British Sterling currency (£sd), Comptometer.

The full-keyboard, British Sterling currency (£sd), Comptometer illustrated was made by the Felt & Tarrant Manufacturing Co. in the USA in the 1920s or 30s. This is a model J, serial number J331518.

310 mm wide, 375 mm deep, 150 mm high (12.5" x 15" x 6"), 11.5 Kg (25.5 lbs).

Standard decimal Comptometers have all columns identical with 9 keys in each (no key is required for 0).

This example is a Sterling currency version (click here for more details of Sterling currency calculators). It has non-standard numbers of keys in some columns (see photograph below), from left to right :-

The maximum value that can be calculated is 9,999,999,999 Pounds, 19 Shillings, and 11 Pence (ie. £9,999,999,999/19s/11d).

The 2 extra keys required in the Pence column would have caused problems for manufacture on a standard production line.


Standard decimal Comptometers have all columns identical with 9 keys in each (no key is required for 0). Sterling currency machines can be used for decimal calculations by ignoring the rightmost three columns of keys.

Note that there are 2 numbers marked on each key -


The Sterling currency keyboard.
The machine can be used in decimal mode by ignoring the three rightmost columns of keys.

Unusual model J

The machine pictured on the left is also a model J, serial number 297617.
It is unusual because:

  • It only has 10 keys in the pence column (to enter 10 or 11 you have to press one key then another which add up to the required value.
  • It has later model style, thick keys.
  • It has a later model style "Comptometer" badge at the rear of the keys, rather than the usual "Felt and Tarrant" badge.

The Comptometer

The Comptometer was invented by the American Dorr Eugene Felt and was pateented in 1887. Manufactured by the Felt and Tarrant Manufacturing Company  of Chicago, the Comptometer was the first truly practical and commercially successful adding machine, with the first serious manufacture and sales occuring in 1888.

They proved very successful and were developed and manufactured in large numbers into the early 1970s.
"Comptometer" eventually became to be used as a generic name for calculators of this type from other manufacturers, though strictly this is wrong.
However, to complicate matters, in 1957 the Felt and Tarrant Manufacturing Company changed its name to Comptometer Corporation. Then in 1960 Comptometer Corporation sold its UK. operation, including the right to the Comptometer name in Britain, to Control Systems Ltd. (the parent of the Bell Punch Company), which merged it with its own company to form Sumlock-Comptometer. Comptometer Corporation then contracted with Control Systems to have all its machines made by Bell Punch in England and shipped back to the U.S.A. This resulted in all Comptometer production ceasing in Chicago in 1961.
This resulted in machines from Sumlock being marked with the Comptometer name.

The main manufacturers of Comptometer type machines were:

Comptometer type machines do not appear to have been as popular in continental Europe (add-listers, which produce a printed output, were more common there) and there were no significant manufacturers.

The Comptometer was the first succesfull key driven adding and calculating machine. "Key driven" means that just pressing the keys adds the numbers entered to the total - no other action is required - so it is very quick for adding long lists of numbers. The basic function of the Comptometer is addition. There is a column of keys (in general 1-9) for each decade. When a key is pressed, that number is added to that decade, with carry to the next higher decade, if applicable. Pulling the handle forwards clears the total to zero.

Comptometers were very fast in operation when adding up lists, such as required in accounting. Operators were specially trained to make use of the full keyboard and enter each number by pressing all the digits in one go using all fingers, as necessary, at once. In other words all the digits were entered in parallel, the mechanism being able to cope with this.
In contrast a modern electronic calculator only has 10 digit keys so the digits of each number have to be entered one at a time, serially, which is slower.
Comptometers were widely used into the late 1970s and were ousted by advances in the use of computers for accounting rather than the development of electronic calculators. People trained in the proper and swift use of Comptometers often kept using them for many more years since for adding up lists they were quicker than a standard ten-key electronic calculator.

Although designed mainly for addition, it is also possible to perform subtraction, multiplication, and division on Comptometers using special techniques. If you would like to experience the joy (if you are a masochist) of performing the four arithmetic functions on a Comptometer see the article Operating a "Comptometer" in the Collecting Calculators section of this site.

Comptometer Educator

The photograph above shows a Comptometer Educator, which is a dummy abbreviated Comptometer used for training purposes. It is used to practice entering complete numbers in one movement using all the fingers of the hand.

Comptometer school

This photograph is from the journal "Office Magazine" for January 1955 and has the caption "Over 180 girls a year pass through the Liverpool school for Comptometer operators run by Felt & Tarrant Ltd. Course normally lasts three months, but there is a shorter course, provided free, which covers three weeks and deals with one specific application of the Comptometer".

Courses were run in dozens of schools for Comptometer operators (and by the Comptometer's competitors). These courses taught techniques for performing calculations quickly and efficiently.

Comptometer-type machines with abbreviated keyboards were produced, as shown below. These cut down on cost by only having the keys 1 to 5 in each column, but if you need to enter a digit greater than 5 then you have to enter two numbers which add up to the digit required (e.g. for 9 press 5 then 4, for 8 press 4 then 4).

Plus 506

Probably the ultimate in abbreviated "Comptometers" is the Plus 506.

This is small, only 185 x 180 x 127 mm. (7.25" x  7" x 5"), and is almost a pocket "Comptometer".

An excellent source of information on Felt & Tarrant Comptometers is at

There are excellent photographs and descriptions of the mechanism used in the "Plus" machines at John Wolff's Web Museum site. These are accessed from the Comptometers and Key-Driven Calculators section on that site.


Illustrated below are other Comptometer type machines featured on this site:

Click on a picture for more details and more, bigger, pictures.

Plus 506/D
Abbreviated decimal model

Plus 512/S
Abbreviated Sterling currency model

Plus 506
Plus 512

Sumlock 912/S
Sterling currency model

Sumlock 912

Sumlock Comptometer 993m
Sterling currency model

Sumlock Comptometer 993s
Electrically powered model with memory

Comptometer 993m
Sumlock Comptometer 993s

Sumlock 912/Y
Imperial weight model

Sumlock 509/T
Time calculation model

Weight calculator
Time calculator

Vintage Calculators

© Text & photographs copyright Nigel Tout   2000-2015 except where noted otherwise.