Correntator slide adder

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The Correntator slide adder

Here is a neat "slide adder" called the Correntator. Like most of its kind (there were numerous products built on the same lines), it represents itself as a four-function calculator, but in reality it can do directly only addition and subtraction; to multiply you need to do repeated, shifted additions. You use the stylus clipped to the instrument's right side to move the slides, starting at the digit you want and sliding as far as you can. The result shows in the round windows, and the "carry" - should the result exceed 9 - is done manually by reversing and moving the stylus through the crook at the end of the groove it's in, which increments the next digit by one.
The Correntator was introduced around 1920 by "Continentale Buroreform" (CBR) of Berlin. At 10 cm high and 3 mm thick it is compact, lightweight, and elegant. The all-brass construction extends even to the stylus, which is a hollow brass tube.

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An interesting trick unique to the Correntator is the way it handles the change from addition to subtraction, which require opposite scales and having the "carry" crook at opposite ends of the groove. Most slide adders have linked duplicate mechanisms, one for addition and one for subtraction. The Correntator simply transforms itself from one mode to the other - look carefully at the photos at right.

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The trick is exposed by this photo: there's a grooved metal plate with different scales on each side, that flips on hinges to display the necessary scale and mechanism for either adding or subtracting.
The last photo shows the instrument's cardboard carrying case, which is just as slim and elegant as its content.

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		Exhibit provenance: eBay, from a seller in Holland.

The Correntator slide adder