Plus & Sumlock Mechanical Calculators
Plus Adder DA

Plus Adder DA

Plus Adder DA

Decimal, key-driven ("Comptometer") type, with abbreviated keyboard (only has keys 1 to 5). To enter a number greater than 5 two keys have to be pressed in succession in that column.
The lever on the right is for clearing the total.
The 4 and 5 keys in the left-most column are used for subtraction using the "nines-complement" method.

The numbers in the accumulator, where the result is displayed, have separators for decimal currency calculations, so the photograph above shows a total of 123,456.78.

230 x 153 x 105 mm.. (9" x  6" x 4.1").

Made in England.

This particular example is missing the mechanism in the left-most column.

This is one of the first Bell Punch calculators, which were based on the Petters design that Bell Punch bought from that company in 1936. See the patents listed on the Plus Adder S machine.
The case is unusual in being made of Bakelite. The mechanism of these machines operates on the downwards stroke of the keys and there is no interlocking between the keys to prevent more than one key in a column being inadvertently pressed at the same time.


Close up of the manufacturer's badge to the left of the keyboard.

Cover removed

With cover removed, showing the mechanism.


The mechanism from the rear.


Close up of the plate on the bottom with the model type "D.A.".

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Text & photographs copyright © 2002 - 2023 Nigel Tout, except where noted otherwise.