Half-Adders

When comptometers were only used for adding, operators were taught that it was more economical in movement to use repeated presses on the low number keys rather than reaching for the high numbers.  In recognition of this fact, comptometer type machines were produced with abbreviated keyboards, such as the examples on this page

Plus Adder

Sumlock Plus Adder Half Keyboard Comptometer

Serial No. 509/D/868.483

Date: 1960s

Price paid (including postage): £22.10

This is a decimal machine dating to the late 1960s.  The grouping of the colour coded decades is interesting – this clearly wasn’t intended for currency use, unless it was for estate agents 😉

Contex Model B – Sterling Model, with Farthings

Contex Model B Sterling Half Adder With Farthings

Serial No. 140858

Date: Before 1960

Price paid (including postage): £14.97

Contex were a Danish company who made a lot of these machines for Sterling currency calculations.  They seem to have been especially popular in Australia. Prior to the introduction of the Australian dollar in 1966, the Australians used the same Pound Sterling as in the UK.

In the rightmost column are the half penny and farthing (quarter penny) keys. The next column to the left is for pennies, and the one after that (in white) is for shillings.  The single key is for entering amounts of ten shillings.  To the left of the 10 shillings key are the four columns for entering pounds.  The total capacity of the machine is £9,999 19s 11d + three farthings.  The reset key is on the left.

The machines have a lightweight and flimsy feel about them, but they wear their years well.

Contex Model B – Sterling with Whole Pennies

Contex Model B Sterling Half Adder

Serial No. 246818

Date: 1960s

Price paid (including postage): £31.99

The difference between this machine and the previous example is that the half penny and farthings keys have been removed.  The opportunity has been taken to increase the number of pounds columns, increasing the capacity of the machine to £99,999 19s 11d.

Contex Model B – Decimal Currency

Contex Model B Decimal Half Adder

Serial No. 945937

Date: 1970s

Price paid (including postage): £29.71

This is a decimal version of the Contex Model B.  The 10 shillings key has been replaced by a pounds column, and the rightmost column counts 10 decimal pennies instead of 12 pre-decimal ones.  It wasn’t long after this machine was built that mechanical calculators became obsolete.