Tag Archives: sumlock

108. Sumlock

This has got to be my geekiest purchase from an antiques and collectors fayre:


It is a working decimal comptometer from the 1960s. To be specific it is a Sumlock model 912/s sterling currency comptometer. It measures 330mm by 315mm by 135mm and was made by the Bell Punch Company. The maximum total it can display is £1,500/17s/11d. This is pre-decimal British currency.


I’m fascinated by old maths and science tools and books. I have an old maths instruction pamphlet from the 19th century with a chapter entitled ‘Mathematics for idiots and women’! So this comptometer is just my kind of thing. When my mother-in-law saw it, she told me about their use in offices. She worked with a comptometer operator in the pay office. Comptometer operators had to be specially trained and earned more than your average ‘office girl’.

I’ve been doing some research and found a rather marvellous website: The Vintage Calculator web museum.

The Sumlock Mechanical Calculator page explains all about this machine. It includes pictures of the internal gear mechanism.

There is also a section on ‘Operating a comptometer’. The processes and algorithms that an operator had to learn were quite complicated and they still had to use mental arithmetic – nothing like modern calculators.

I’m sure there is an arithmetic lesson here somewhere…