Mechanical Calculator Days


The main mechanical calculators manufactured by Bell Punch were the Plus and the larger Sumlock adding machines.

Plus 509/S Sterling currency adding machine.

Plus 509/S adding machine
Sumlock 912/S adding machine

Sumlock 912/S Sterling currency adding machine.

The Plus and Sumlock are machines of the "Comptometer" type; the Sumlock being the full-keyboard version and the Plus the "abreviated-keyboard" version.  They are intended primarily for addition, but can also be used for subtraction, multiplication and division using learned techniques.

The main feature of the Sumlock machine is that it has a full-keyboard and is "key driven", which means that pressing any key immediately adds the number on that key to the number displayed in that column, with carrying to the next column taking place where necessary.

The machine is designed so that keys in different columns can be pressed simultaneously.  This means to add the cost of several identical items the operator arranges his or her fingers on the required keys and then depresses them all simultaneously the number of times for the number of items.  Working in this way this type of machine is much faster than a 10-key type of machine where each digit has to be entered successively.

The Plus models are "abbreviated-keyboard" machines and only have keys up to 5 in any column, so to enter a number greater than 5 two keys have to be presses one after the other.  Since the fingers do not have to move far from one key to the next, there is not much time penalty in using this type of abbreviated machine.  Bell Punch were the world's largest producer of "abbreviated" "Comptometer"-type calculators with the "Plus" range, which were cheaper, smaller, and lighter than full-keyboard machines.  For further information see the Plus & Sumlock Mechanical Calculators section.

However, to get the most out of the machine it was necessary to learn the techniques at one of the Sumlock training courses, and those run by other calculator companies. These courses taught the most efficient ways of using these full-keyboard adding machines, shortcut methods, and techniques for performing more complicated calculations.

The Plus, Sumlock, and the other models, were very successful and were widely used in accounts and wages departments into the 1970s and beyond.  Due to their speed of use they were displaced more by computerisation rather than the introduction of electronic calculators.

Administration Office of the Italian State Railways

"Sumlocks" in use at the Administration Office of the Italian State Railways in Florence.

The photograph above shows a typical accounting scene before the days of computers, from "Bell Punch news & views" of Spring 1952.

Bell Punch Company and its foreign distributors had stands at many exhibitions and trade fairs, demonstrating the advantages of the Sumlock and Plus adding machines, and the company's ticket issuing machines.  A selection is shown below:

Sumlock exhibitions

Exhibitions and Trade Fairs
Top Left - Brussels International Trade Fair - May 1949
Top Right - Hotel and Catering Exhibition, Manchester - February 1949.
Centre Left - Bristol Youth Exhibition - March 1949.
Centre Right - Philadelphia Fair - May 1949.
Bottom - Velencia Trade Fair - May 1949.


As with all mechanical devices that are well used, the adding machines needed servicing.  This was undertaken at Sumlock service centres around Britain and in the countries around the world where the machines were sold.

Sumlock service centre, London


A photograph of the London service centre for Sumlock and Plus adding machines, from "Bell Punch news & views", Spring 1949.

A selection of the Bell Punch mechanical calculators is illustrated in the Mechanical Calculators section of this site.

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

The Bell Punch Company & the Development of the Anita Calculator
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Text & photographs copyright © 2002 - 2023 Nigel Tout, except where noted otherwise.