Curta
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The Curta is a hand-held mechanical calculator designed by Curt Herzstark. It is known for its extremely compact design: a small cylinder that fits in the palm of the hand. It was affectionately known
Curta Type II mechanical calculator
Curta Type I, on display at the Musée des Arts et Métiers, Paris.
A partially disassembled Curta calculator, showing the digit slides and the stepped drum behind them
Curta Type I calculator, top view
Method of complements
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In mathematics and computing, the method of complements is a technique to encode a symmetric range of positive and negative integers in a way that they can use the same algorithm for addition througho
Curta
…drum replaced the multiple drums, typically around 10 or so, of contemporary calculators, and it enabled not only addition, but subtraction through nines complement math, essentially subtracting by adding. The nines' complement math breakthrough eliminated the significant mechanical complexity created when "borrowing…
Complement numbers on an adding machine c. 1910. The smaller numbers, for use when subtracting, are the nines' complement of the larger numbers, which are used when adding.
Comptometer from the 1920s, with nines' complements marked on each key