Plus & Sumlock Mechanical Calculators

Plus Adder B


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.

Made in England.

The "Petometer" is the forerunner to the "Plus Adder". Bell Punch bought the design and rights of this machine in 1936 from Petters of Yeovil. It was the basis for the development of the "Plus Adder". Some of the patents listed on the Plus Adder S machine were originally granted to Petters.

Petometer Brochure

An advertising brochure for the Petometer in German from 1931 (scan kindly supplied by Wilfried Denz who has a website featuring old calculators at

A translation is:
"An adding machine, on which you can count after a few hours of exercise, without the need for extended learning."

"Unbeaten on speed and reliability."

Click here to go to the Vintage Calculators Web Museum

Text & photographs copyright © 2002 - 2018 Nigel Tout, except where noted otherwise.