Few people have heard of it, yet many consider John Blankenbaker's KENBAK-1 to be the first commercial personal computer.

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Mechanical Adders: Math without electricity

Addiator

My primary school teachers always told us they used to do math "the hard way" before the introduction of electronic calculators. Little did I know that they were stretching the truth slightly; mechanical adding machines were surprisingly common. Of course, not everyone could afford to own an exquisitely complicated Curta Type I rotary mechanical calculator.

Many people made do with devices like the clever little Faber Castell Addiator slide rule shown in this video. Basic addition and subtraction could be performed quickly using the slide adder mechanism. This particular West German model appeared in the late 1940s, although the original Addiator was created by Frenchman J. L. Troncet in 1889 and sold as the Arithmographe.

Addiators feature three main components: a multi-digit slide mechanism, reset lever and a stylus to manipulate the sliders. Each slider has a series of notches that correspond to the digits. Moving the sliders upwards results in addition, while subtraction is performed in the opposite direction. Carrying digits is performed by sliding the stylus around the top of one column to the next - a surprisingly enjoyable and intuitive action.

Millions of cheap tin and plastic slide calculators eventually flooded the market in the 1960s, but don't confuse these inexpensive imitations with the real thing. Addiators have become collector's items, and desirable models frequently fetch $50 or more on eBay [click here to see some current auctions].

A pageful of beautiful slide adders
Using an Addiator

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