05 July 2011

Mechanical adding machine

It is hard to imagine a greater contrast to the mainframe featured in the previous post than the mechanical adding machine pictured below.  It probably dates from around 1970.  I remember in the 1970s South African Post Offices closed at 16:30, but closed for financial transactions at 16:15.  If you walked into the Post Office after 16:15 the lady behind the counter was busy balancing the books using an adding machine like this one.

We got our first electronic calculator in the mid-1970s.  It had an LED display and used horrendously expensive and hard to obtain AAA batteries.  Of course the mechanical model below required no batteries and kept on working even during blackouts.
To use the adding machine one would use the stylus and enter the number by pulling the dials downwards.  To add 5 one would place the stylus into the hole next to the black 5 and pull it until it reached the bottom.  To add tens one would use the dial to the left of the units, and so on.  Every number entered was added to the running total displayed through the holes at the top.
To subtract, one would use the red numbers and 'add' it, and then add 1.  If the reader has not previously encountered tens complement arithmetic, now is the time to figure it out.

To reset the adding machine to 0 one pushed the red 'clear all' lever back and all the dials spun back to 0.