Old „Zahlenschieber“ from my grandpa.
They were used until they were replaced by calculators in 1980. On the backside you can use it for subtraction. This one is manufactured by the German company „Addimult“.
The word „Zahlenschieber“ means push/move numbers. You were able to move the rows with the pen attached and then the numbers on the top row change according to what you moved.
But I‘m not sure how I use it to get a result/do a subtraction or addition. Maybe someone of you can enlighten me.
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It was quite simple, to calculate “24+654” you would insert a pen in the far left “4” and pull it down, the insert the pen into “2” on the next column. The top would then read “24” you would then do the same for the next number, “4” and pull down, “5” and pull down then the last digit “6” The top will now read the result of the addition.