An abbreviated sterling currency comptometer made by Bell Punch. Pressing a key in each column adds that value to the column's display, with carry rippling through. The lever on the right clears the display when pulled toward the front. Larger comptometers have 9 keys in each column, whereas an abbreviated model requires two presses for numbers greater than 5. The rightmost column display goes up to 11 (12d = 1 shilling) and the next two together go up to 19 (20 shillings to the pound).

A surprising number of different types calulations can be done on this machine, other than addition. If you fancy a go at using a comptometer, a fabulous emulator, by Erez Kaplan, can be found here. Instructions on how to use a comptometer can be found on Nigel Tout's website here. For non-Britons and those too young to remember, an explanation of the old Sterling currency can be found here.

As an example of the power of this machine, let's see how to divide 22 by 7, the approximation of pi (you can follow this example on Erez Kaplan's emulator if you like). The above Sumlock is a sterling comptometer, so we'll only use the leftmost 6 columns of keys. We need to preload 22 into the machine in the far left digits by pressing the 5 key in the leftmost column four times and the 2 key once. Because we are dividing by 7, which has 1 whole number (we'd ignore any decimal values if it had any), the decimal point is 1 place up from the 22 we've preloaded. So we raise the little metal indicator under the display between the two '2' digits (you were wondering what they were for, weren't you?).

We now need the 'nines complement' of our divisor. Basically this means that 1 is subtracted from the number, and each digit of the result subtracted from 9. E.g. 1234 is 9999 - (1234-1) = 8766. On larger comptometers the keys have smaller numbers to the left of the main numbers, which is the key value subtracted from 9, to make it easier. Using the smaller numbers means you only have to remember to subtract 1. (For example, see Nigel Tout's document here.) So in our example, the nine's complement of 7 is 9 - (7 - 1) = 3.

Moving from the left most digit, we select the column where our divisor will first divide in to the dividend. So not above the left most 2 (7 doesn't go into 2), but above the second 2. This is the column we will start from. We press the 9's complement value (3 in this case) for as many times as the column immediately to the left indicates (in our case 2). If that column to the left increases its value then that becomes the new number of times to press the divisor. If it wraps to 0, then an implied 10 is indicated, and we must press that many times etc. Eventually you'll 'catch up' and the column is finished. You then move the divisor a whole column rightward and repeat the process. A step-by-step illustration is shown below.

Column Press Displayed Comment 2200000 Start. Press at least 2 times. 3----- -> 2500000 3----- -> 2800000 Caught up. Move left, press 8 times. -3---- -> 2830000 -3---- -> 2860000 -3---- -> 2890000 -3---- -> 2920000 Catch up column increased, new target 9. -3---- -> 2950000 -3---- -> 2980000 -3---- -> 3010000 Catch up column increased, new target 10. -3---- -> 3040000 -3---- -> 3070000 -3---- -> 3100000 Catch up column increased, new target 11. -3---- -> 3130000 Caught up. Move left, press 3 times. --3--- -> 3133000 --3--- -> 3136000 --3--- -> 3139000 Caught up. Move left, press 9 times. ---3-- -> 3139300 ---3-- -> 3139600 ---3-- -> 3139900 ---3-- -> 3140200 Catch up column increased, new target 10. ---3-- -> 3140500 ---3-- -> 3140800 ---3-- -> 3141100 Catch up column increased, new target 11. ---3-- -> 3141400 ---3-- -> 3141700 ---3-- -> 3142000 Catch up column increased, new target 12. ---3-- -> 3142300 ---3-- -> 3142600 Caught up. Move left, press 6 times. ----3- -> 3142630 ----3- -> 3142660 ----3- -> 3142690 ----3- -> 3142720 Catch up column increased, new target 7. ----3- -> 3142750 ----3- -> 3142780 ----3- -> 3142810 Catch up column increased, new target 8. ----3- -> 3142840 Caught up. Move left, press 4 times. -----3 -> 3142840 -----3 -> 3142846 -----3 -> 3142849 -----3 -> 3142852 Catch up column increased, new target 5. -----3 -> 3142855 Caught up and finished.

So, since we raised the metal indicator between the leftmost digit and second leftmost digit initially, the display now reads 3.142855. Compare this to the actual value (to 6 places) of 3.142857---a difference of less than 0.00007%. Not bad! This may all seem like hard work and error prone compared to an electronic calculator, and in this example it really is. But imagine, say, dividing 4312.87 by 965.78. It is still easier, maybe, on an electronic calculator, but because you can enter the divisor as a whole (9's complement), it is suprising how quickly a result can be obtained with practice with just a few cogs and levers. I have heard just recently (August 2004) from someone who is still using a comptometer for their day-to-day calculations at work, since they find it quicker! Before January 1962, when the first Anitas went on sale, you had very little other choice, though the initial Anita model's keyboards pay homage to their comptometer ancestory (see Nigel Tout's site here).

Last updated 30th October 2005.

© 2003-2005 Simon Southwell. All rights reserved.