Introduction to
Plus & Sumlock Mechanical Calculators

All of the mechanical calculators manufactured by Bell Punch were key-driven machines of the "Comptometer" type.

Introduction to the "Comptometer" type calculator:
The Comptometer was invented by the American Dorr Eugene Felt and was patented in 1887. Manufactured by the Felt and Tarrant Manufacturing Company of Chicago, the Comptometer was the first truly practical and commercially successful adding machine. 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 Bell Punch, 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, and in machines from Sumlock being marked with the Comptometer name.

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 have often kept using them for adding up lists they are quicker than a standard ten-key electronic calculator.

Although these machines were very successful and thousands were sold, only a few companies manufactured the "Comptometer" type of calculator. The main manufacturers were Felt & Tarrant (later Victor Comptometer) and Burroughs in the U.S.A., and Bell Punch in Britain.

There is an operating manual for the Sumlock "Comptometer"-type machine in the Operating Manuals section of this site, with a summary on the Operating a Comptometer page.

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.

Bell Punch Mechanical Calculators

The Bell Punch company did not manufacture any calculators until the 1930s when in 1936 the parent company, Control Systems, acquired the rights to the adding machines of the Comptometer type manufactured by Petters, Limited, of Yeovil, England and sold under the name "Petometer".

Due in part to the use of the old £sd Sterling currency and imperial units of measurement in Britain and the British Commonwealth, Bell Punch manufactured a greater variety of "Comptometer" type calculators than any other company.

The early Petter based models were introduced by Bell Punch in about 1937 under the name "Plus Adder".
These are all abbreviated keyboard models, that is they only have keys 1 to 5 in any column. To add a number higher than 5 then the operator has to press first one key and then another in that column which add up to the required number.

In 1940 a larger, full keyboard (with numbers 1 to 9 in each column), machine under the name "Sumlock" was added to the range, and a separate company, London Computator Corporation, Limited, was incorporated to deal more efficiently with the sales side of the calculators.

In 1950 the name of London Computator Ltd. was changed to Sumlock Limited, and this in turn became Sumlock Comptometer Limited in 1961 when the British operation of Comptometer Corporation was merged.

The magazine of Control Systems Ltd. for Winter 1959 lists the following dates of introduction of Bell Punch mechanical calculators:

• 1939 - Plus [this date may not be accurate since the Plus Adder is listed in the German book "Die Rechenmaschinen" published in about 1937].
• 1940 - Sumlock Mark I.
• 1946 - Sumlock Mark II.
• 1950 - Sumlock Mark III.
• 1952 - Sumlock Mark IV - Figureflow.
• 1953 - Unilectric.
• 1953 - Duolectric.
• 1959 - Sumlomatic.

The last new models of Bell Punch mechanical calculators, the 993 series, were introduced in 1964. Plus models were manufactured by Sumlock Anita Electronics into the 1970s.

Bell Punch were the world's largest producer of "abbreviated" "Comptometer"-type calculators with their "Plus" range. Having columns with keys only up 5, these were cheaper, smaller, and lighter than full-keyboard machines and with little or no speed penalty.

The "Plus" machines are designed purely for adding operations, while the "Sumlock" and "Comptometer" models have keys marked also with the nines-complement numbers and have "prevent carry over" buttons for use with subtraction operations. For details of performing calculations on a Sumlock Comptometer see the article on the sister site to this.

Later Model numbering scheme - The Sumlock and later Plus model numbers are of the form XYY/Z, where X is the number of rows of keys, and YY is the number of columns of keys.
Z is one or two letters to give the type of calculator where:

• C - Meaning not known, machines appear to be decimal calculators.
• D - Decimal calculator.
• F - Sterling (old British £sd) currency calculator with farthings (1/4 penny) keys.
• HD - Decimal calculator with also a half (1/2) key (useful for British decimal currency).
• R - Pre-decimal currency used in the Indian sub-continent (1 Rupee = 16 Annas, 1 Anna = 12 Pies).
• S - Sterling (old British £sd) currency calculator.
• SP - Special ? Found on an Imperial weight calculator (tons, hundredweights, quarters, 7 pounds).
• T - Time calculator (hours and minutes).
• Y - Imperial weight calculator (tons, hundredweights, quarters, pounds).

The Plus calculators changed to a more angular design in early 1958.

For Plusograph see the Calculators made by others section.

Converted machines. When Britain changed from the old Sterling £sd currency to decimal currency on 15th February 1971 the Sterling currency machines could still be used by ignoring the shillings and pennies columns.
However, on machines with few columns to start with, such as the 509/S, this does not leave many useful columns. It appears that to overcome this it was common to modify Sterling currency machines by replacing the shillings and pennies columns with decimal columns, since such modified machines are sometimes seen.
This involves drilling extra holes in the top and fitting the decimal keys and mechanism from another machine. Such machines appear to be decimal, may have some blanked off key holes, have a serial number including /S, and usually still have the yellow lines on the accumulator register which separated the £sd sections. This may not have been an official modification since examples seen are often somewhat crude.

Development of the Plus Adder:

The Sumlock:

In the early 1940's Bell Punch introduced a full-keyboard machine under the Sumlock name which was sold by their London Computator Corporation marketing company. The name refers to the interlocking of the mechanism to prevent incomplete key strokes (the key does not return, and no other key in the same column can be depressed, until it is pressed far enough).
In 1950 the name of the marketing company was changed to Sumlock Ltd.

Sumlock-Comptometer:

In mid-1960 Control Systems Ltd. (the parent of Bell Punch) acquired the English operation of Comptometer Corporation (previously the Felt & Tarrant Manufacturing Company) of Chicago, U.S.A., including the trademark and the right to the "Comptometer" name in England,  and then signed a contract with Control Systems to have all their machines manufactured in England. Sumlock machines then appeared under the Comptometer label.

The Mechanism:

Below is a demonstration model of the later, more complicated, interlocked mechanism. To minimise incomplete key strokes the key does not move the gear wheels on the downwards stroke, but lightly triggers the mechanism to move the gears on the spring-driven return stroke.
Also there is interlocking to prevent inadvertent key presses. Once a key is depressed by a small amount all of the other keys in the same column are locked and cannot be depressed.

References:

1. "News File", Office Magazine, May 1958, p406.

Click here to go to the Vintage Calculators Web Museum

Text & photographs copyright © 2002 - 2013 Nigel Tout, except where noted otherwise.