## Mechanical Adding Machines

For the sake of completeness, we mention the predecessors of the electro-mechanical calculators: pure
mechanical "pocket calculators" of the early days. The picture on the left shows a small slide rod calculator
from "Produx". This handy device *Efzet* was intended

for the buisenessman, industrial, craftsman, engineer, student and even the housewife

like the instruction sheet (german only) tells. This device does not seem to be very useful.

## (Electro-) mechanical calculators

The first electronically calculating calculators were gigantic mainframes in the 1940/50s. Until then, calculators worked mechanically. When the handwheel was replaced by an electric engine, the era of mechanical calculators began.

From the first fully automatic machines (1927) until calculators with accumulating memory (1960s), mechanical calculators computed the result indepently after user input. Curta I, The world's first "pocket calculator", has 1/3 from the volume of a Coca Cola-pin and is consequently the smallest four-species machine ever built. Looking at mechanical calculators, one quickly recognize the enormous importance of the positional notation.

**Burroughs Mod. 2.**
The first printing calculating machine was invented in the USA (about 1905)
and was selled worldwide. It features a complete keyboard for 17 digits, a
printing unit and a long carriage for the sheet paper. The device could be
used as a simple electronic accounting machine. At that time the engines
could already be built just compact enough for such a device. Anyway, the
huge engine (for today's circumstances) had to be placed below the device
(on the right hand side in the picture). The sack below the engine is
intended for collecting the leakage oil.

**MADAS**, an electromechanical calculator from 1927
made by the calculating machine factury "Egli AG" in Zurich.
Since multiplying and dividing needs some time, the inventors installed a small
bell (at the upper left) that rings after the calculation has finished.

In 1932, **Rheinmetall** extended an adding machine with
a surface mounting, thus creating a 4-species calculating machine.
The comfortable usability seems to be the reason why they called it "Superautomat".
Calculating is the only ability of this colossus.

**Curta I and Curta II**

These outstanding small pocket calculators were produced almost
unmodified from 1948 to ca. 1971. This is unbelievable for today's
technology products. The Curta could perform all basic arithmetic
operations.
Packing all the complex mechanics into such a small volume was an enormous
challenge. For comparision of the sizes, we put a matchbox from the same
era on the picture.

The Internet is full of literature about the Curta, e.g. the
story about inventor Curt
Herzstark.

**DIEHL VSR-18**, one of many mechanical calculators
built between 1955 and 1965.

The engineers were hardly pressed to design better and better machines, until
the limit of feasibility. So the operating instructions say: "This DIEHL-device VSR
performs outstanding work". That is really true: For example, one were able to
cache results and to transfer back them anytime you want to. With this feature
daily calculations like 25 + 12 x 7 - 17 x 6 could be solved without notating
anything. However, this luxury still had a drawback: Maloperation could damage the
whole device.