The Brunsviga Midget (Model M or MB) introduced in 1908
The Brunsviga Midget is a miniaturized pinwheel (or Odhner style) machine and represented the ultimate in portable computing for most of the first half of the twentieth century. This was made in Germany for export to the United States. The finger in the picture above points to an endearing feature of these pinwheel machines, a tiny bell that rings when the mechanism's running total (i.e. the value shown in the 13 place result window at bottom right) crosses from positive to negative numbers or vice versa. As the pinwheel turns, the clickety clack of the advancing result register is punctuated at the end with a resolute Ping!
The lower portion of the mechanism, which shows the 13 digit result, is a carriage that slides left and right up to 7 places. Although there are only 9 number setting levers (at top), by sliding the carriage one can perform power of ten multiplication and work with numbers as large as nine trillion. Alternatively, the user could keep track of the decimal place (via the metal left-right sliders seen on the carriage) to work with smaller numbers with many digits of precision after the decimal.
and how to operate the calculator for multiplication division etc.:
Here are more pictures of the Brusviga M and Brunsviga MB or Midget