Trademark of the Felt and Tarrant Manufacturing Company, Comptometer is a smashup between computing and measure. This is not quite a computer as we understand it today, but we are getting pretty close. I would even risk describing this descendant of the Pascaline as an arithmetic unit of a one-register CPU. OK, it may be far-stretched, but as I wrote, this is a very personal view. Nevertheless, the optional sheet metal cover of the device tells it all: with this comptometer, you can add, subtract, multiply and even divide! And if you don’t know how to pronounce the mane of this devil’s machine, just read the box: pronounced like a thermometer. Now you know! My Comptometer is a decimal model F with eight columns – of nine keys. With a serial number of 185201, it was possibly manufactured circa 1918. The Felt and Tarrant company, established in Chicago, produced shoebox comptometers from 1886 thru 1933. These machines were used anywhere computing was necessary: banks, factories, offices, etc. They were able – when operated by some well-trained personnel – to add, subtract, multiply and divide at a lightning-fast speed. It is that it is hard to imagine a faster way to carry out an addition. Indeed, by pressing a key, it immediately adds to the current value to the register in its column (well, since there is only one register, we can call it the result). If the sum in the column is over nine, then the column is zeroed and a carry is added to the column to the left. Nothing like the + = keys to press with our modern calculators. With this approach, you may press – if you have the required dexterity – all the digits of the value you want to add at once. Therefore, you can add in theory two eight-digit numbers in a record number of “keystroke”. One of the videos attached to the post demonstrates this characteristic adding 12 to 21. In the video, I press 1 and 2 at the same time, followed by 2 and 1, pressed at the same time as well. For the subtraction, it is a bit more complicated.

The comptometer uses 9’s complement operations. This is why each keycap has, in lower size, the 9’s complement of the key: 2 for 7, for 1 for 8, etc. The operator must simply pick the right key. My way of performing multiplications is by repeating additions. But I am sure that there are more elaborated methods. And I have no clue regarding the division, but I doubt that comptometers were heavily used to perform divisions. The crank lever is used to zero the register. And this is all you really need to use one. I removed my comptometer from its case, so you can gauge the complexity of the mechanics. Although I could eliminate a lot of gunk during the operation, thorough cleaning of the machine is required. The main issue with these systems – besides the bending of the minute mechanism by some barbaric user smashing the keyboard –, is the drying and solidification of the lubricant. In my comptometer, I believe that the issues I experience with the fifth column failing to reset properly are caused by a locking mechanism by dried-out oil (hey, this is almost a 100-year-old machine). Since I do not feel able to re-assemble such complex machines, I will not engage in a full restoration which would require a full disassembling at this point. Maybe when I’ll retire. I hope that you will agree that it is a beautiful machine. Enjoy the pics and videos!