This first instruction installment will just deal with two basic functions that can be done with the Curta calculator – addition and subtraction.
Before we do either, let’s clear the Curta calculator. This is done by holding the Curta in your left hand (assuming that you are right-handed) and then using your thumb and forefinger to lift the upper carriage. This spring loaded carriage can be raised about 1/8 inch. While holding the carriage in the raised position, carefully rotate the zero ring once around the Curta in the clockwise direction. I find that the best way to rotate the zero ring is to insert a portion of your right forefinger into the ring and then gently rotate the ring. This action will change all of the numbers on top of the Curta to “0”.
Now for our adding exercise. Rotate the carriage so that carriage position “1” lines up with the arrow above slide entry register position “1”. What we want to do is to calculate 525.47 + 50.82. So the first action is to enter the numbers 52547 in slide entry register positions 5,4,3,2,1. When this is done, rotate the Curta crank one clockwise revolution. This will place the numbers 52547 into the black Response dial. At the same time, the first value in the white Indicating ring will change to “1”. For the purposes of this simple example, that value has no significance, but in a rallying this value will be important, as will be discussed in an upcoming instruction installment.
Now change the numbers in the slide entry register to 5082 in slide entry register positions 4,3,2,1. This positioning will line the columns (decimal point) up properly with the previous entry. Once these numbers are put into the slide entry register rotate the Curta crank once. This will change the numbers in the black Response dial to 57629, which is read as 576.29.
Each time you rotate the Curta crank, the calculator will add what is in the slide entry register to the previous number in the black Response dial. So, in this example, if you rotate the Curta crank again, then the number in the black response dial becomes 627.11. This can be done over and over again. This ability to add the same number over and over again is a key feature that makes the Curta a great tool for rallying.
Subtraction works in a very similar manner. Once again clear or zero the Curta as before. For this example, we want to perform the following calculation: 632.59 – 84.63. Enter 63259 in the slide entry register and rotate the Curta crank one revolution. This will put 63259 into the black response dial. Now put the 8463 in the slide entry register. Then this time, raise or lift the Curta crank up a little to expose the Red Subtraction ring on the central crank handle. With the Curta crank in the raised position, rotate the crank by one revolution. This will subtract the value in the slide entry register from the previous value in the black Response dial. This will give a result in the black Response dial of 547.96. Again, this action can be repeated by successive rotations of the Curta crank.