(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.
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 (for today's circumstances) huge engine 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.
The first printing mechanical calculator (1905) is an unicum with an huge exterior engine! From the first fully automatic machines (1927) until these with balancing memory (1960s), mechanical calculators calculated independently after the input of the numbers. The world's first "pocket calculator machine", Curta I, has 1/3 from the volume of a Coca Cola-pin and is consequently the smallest four-species machine ever built. If you look at the picture of Curta I (shown below), you recognize the enormous importance of the positional notation.
Shown above: 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.
1932 Rheinmetall extended an adding machine with an "annex", thus they made a 4-species calculating machine. The comfortable usability seems to be the reason why they called it "Superautomat".
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: You must not run them wrongly,
because they were highly sensitive agains any operating error.