Comptometer 


The fullkeyboard, 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 nonstandard 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 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. The main manufacturers of Comptometer type machines were:
Comptometer type machines do not appear to have been as popular in continental Europe (addlisters, 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 19) 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. 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. 

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. 

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. Comptometertype 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). 


An excellent source of information on Felt & Tarrant Comptometers is at http://www2.cruzio.com/~vagabond/ComptHome.html 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 KeyDriven 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. 


Sumlock 912/S 







Vintage Calculators 

© Text & photographs copyright Nigel Tout 20002015 except where noted otherwise. 