Addo Multo Model 13 Pinwheel Mechanical Calculator
(c1950)

Image This machine was my introduction to the fascinating world of mechanical calculators, of which I was previously barely aware.

There was a vast variety of mechanical calculators, varying in complexity and going back hundreds of years, however all became obsolete with the development of electro-mechanical and subsequently electronic devices.

The Multo brand is of the ‘pinwheel’ design, so called because of its internal mechanism, and was made by the Addo company of Sweden between about 1949 and 1966. They were designed to be able to be operated by a single (right) hand.

The Model 13 is capable of addition, subtraction, multiplication and division, and features a 10 digit input rotor (and display wheels), an 8 digit counter register (no tens carry) and a 13 digit accumulator register.

There are back transfer function levers and carriage return button, in addition to the usual single step up/down lever, crank, and rotor and register clearing leavers. The accumulator and counter registers can either be cleared together or the counter register only, not the accumulator only. The carriage can also be manually moved up (to the right).

The earlier Model 3, does not have the back transfer, and the later Model 113 has tens carry and automatic reversing in the counter register.

This Multo, serial number 13-5244, was completely seized up when I acquired it, which is apparently a common problem with Multos, due to the complicated design of its single handed operation. However, it was almost completely intact, and thanks to the websites listed and linked on the left I was able to understand how it works, strip it down, clean it and rebuild it, and now it is now working beautifully.

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Count Belmiro © Copyright October 2012
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