Plus & Sumlock Mechanical Calculators
Plus Adder ST

Plus Adder ST

Plus Adder ST

Sterling currency, key-driven ("Comptometer") type, with abbreviated keyboard (only has keys 1 to 5).  To enter a number greater than 5 two or three keys have to be pressed in succession in that column.  The lever on the right is for clearing the total.

The photograph above shows a total of £123,456,789 17s 10d.

305 x 149 x 89 mm. (12" x  5.9" x 3.5").

Made in England.

As with the Bakelite bodied Plus Adders, on the bottom of this calculator there is a list of patents, and the three U.S. ones were taken out by Guy Bazeley Petter, with rights assigned to Petters, Limited, Yeovil, England (the internal combustion engine manufacturer).
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.
The case is unusual in being made of thin sheet metal.  Was this sheet metal casing a war-time economy measure, releasing Bakelite for more important uses, or was it a quick way to meet demand for a 12-column machine without going to the expense of making the tooling for the larger Bakelite moulding?  If you have any information about this please let me know.
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.

Name plate

The "Plus" name plate is on the left of the keys. In this example it is a bit worn.


On the rear of the case is a maker's badge in a recess.


Close up of the maker's badge on the rear.

sheet metal casing

The sheet metal casing has sound-deadening material on the inside.

Cover removed

With cover removed, showing the mechanism.


Side view of the mechanism.

Patents are listed on a label on the bottom. All of the U.K. patents were applied for by Guy Bazeley Petter and Petters Limited of Westland Works, Yeovil, Somerset, from whom Bell Punch acquired them.

The application for the first, 208,232, was submitted on September 14th 1922, and the application for the last, 413,373, was submitted on January 20th 1933.

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