The Curta Calculator

by Nicholas Bodley


One of the most delightful small mechanisms ever sold is the Curta brand of handheld mechanical calculators. These are a radical rethinking of the layout of a calculator, and were first put on the market roughly about 1953. They were very popular with sports-car rallyists because of their architecture; the number entered could be retained very easily (it had to be cleared out manually, one or a few digits at once).

I know of two models, the larger one being able to work with more digits. I used to own one of the smaller ones; it would accept an eight-digit number, and had an eleven-place accumulator with six positions. In one calculation, you couldn't see all 6 + 8 = 14 digits of a product.

Apparently, the inventor was Curt Herzstark, if memory serves me right. They were originally made in the Principality of Liechtenstein, which is generally east of Switzerland, on the border.

The Curta is basically a cylinder with a handcrank on the end. It has an amusing resemblance to a pepper mill. The mechanism that does the job of the carriage in desktop machines is like a large knob, on the crank end of the cylinder. The dials are tiny cylinders (thick disks) on shafts with their axes extending radially from the center. You shift the "knob carriage" (let's call it a "display") by pulling it against spring tension away from the main body of the machine and rotating it to its new position, then gently letting go.

You hold the calculator in your left hand, with thumb and forefinger on the display. Your right hand operates the handcrank. You always turn the crank clockwise, the "easy" and natural direction. (Sorry, as so often happens, this machine is distinctly right-handed!) You should NEVER turn the crank backwards! On my Curta, the anti-reversal ratchet is quite delicate, and easily overridden.

One turn enters the number into the accumulator once; multiplication requires several turns, typically, and shifts of the "knob". To subtract, you pull out on the handcrank, which snaps happily into place; a shiny collar shows that the handcrank is out. You turn the crank the same way, and the number is subtracted from the accumulator, but by adding the complement.

The display, in addition to the accumulator, contains a cycle counter that shows the number of machine cycles in each of its positions. There is a complete carry, and the sense of the count can be reversed for division and square root (use the "fives method") to show a correct quotient/root.

There is a finger ring that swivels from a stowed position and locks into place; this ring is attached to a rotating piece that clears either the accumulator, the cycle counter, or both. It normally remains in either of two positions. You need to lift the "knob" away from the main body to clear. Incidentally, everything is interlocked; you can't lift the "knob" while the machine is in mid-cycle, you can't clear the dials if the "knob" is in its normal position, and you can't turn the crank if the "knob" is pulled away from the body.

You enter numbers by positioning tiny sliding knobs that stick out through slots in the side of the housing. They detent nicely as you change settings; tiny dials show the number you've entered. These knobs position tiny 5-tooth gears to engage with the proper level of a big, central "stepped reckoner" which has interspersed layers of complement-count teeth for subtraction; if you carefully misposition a digit-entry slider, you can engage these teeth.

Pulling out the handcrank moves the stepped reckoner by (I think) one and one-half "digit spaces", referring to the digit-entry sliders.

Subtracting all zeros rotates the least-significant accumulator dial (that is engaged with the body, that is) by exactly one full turn; this is guaranteed to cause a carry. All other columns advance their dials by 9 places, and the carries propagate to return all to their original positions.

This is a really beautiful piece of machinery, the equal of the innards of a fine camera. Some idea of its relative sophistication can be gained by considering what is on the shaft for each display dial. First, of course, is the dial itself. Then, there's a 10-lobe detent cam that ensures that a dial is always positioned correctly, never between digits. There's a tiny spring-loaded ball that bears against this cam.

There's also a little pin or such that sets the carry slide when the dial moves between 9 and 0. There is, quite likely, also a Geneva-drive locking disk that ensures that a dial will rotate only when the main mechanism unlocks it; a dial is probably locked at all other times. This mechanism ensures that the dial doesn't overshoot if someone develops a really-fast handcrank technique. (All calculators except the newer Marchants have such a lock.)

There is still more; there's a 10-tooth gear to advance the dial when the shaft in the main frame turns a tiny 5-tooth crown gear. This gear, most likely, serves to clear the dial when you clear the display. To keep the dial from going past zero when clearing, a part of one tooth is machined away so that the dial will stop when it reaches zero. All of these various mechanical elements are mounted on a tiny shaft that is probably shorter than 1/2 inch.

If you are a careful person, you can see part of the mechanism by removing the screws on the end cover opposite the crank. You won't see much relating directly to the display, but you can watch the carry slides set and reset. The stepped reckoner in the middle is dramatically apparent, and it's quite easy to see how the setting slides and the stepped reckoner work.

In my calculator, I removed the setting shafts and knobs; I was astonished to find that the tiny porous-bronze bearings at the ends of the shafts were individually hand-fitted; I'd assumed they were interchangeable. They weren't; I did my best, having a big, long, trial-and-error session of swapping!

If you know what you're doing, this is a good opportunity to "clean house" and relubricate judiciously (use clock oil?) inside. It's a sacrilege to use a cheap oil! Reinstalling the cover is rather easy; there is a small "key" that positions it precisely. Rotate it back and forth a wee bit to make this "key" drop into its slot.

I would not recommend trying to unpin the handcrank unless you're an experienced machinist or mechanical technician; to begin with, on my Curta, at least, it's probably a taper pin, and it isn't obvious which end is bigger. The crank absolutely MUST be "backed up" with a large mass, such as a kilogram or so of lead. (I have not seriously tried, on mine.) Don't even think of laying the Curta on its side and banging away on a pin punch.

To a person who appreciates small, precision mechanisms, a Curta is a very special device, a real treasure.

(Since I wrote this, a really superb set of Web pages on disassembling a Curta has been prepared. Sorry not to have the URL. Skip Godfrey did, and might still have, copies of Curta service and parts manuals. -nb 97.10)

(In its present form, this set of articles could stand rewriting, because some introductory matter is redundant. Someday, I hope to have the time and energy to do so! My keyboard technique is only fair, although I am an excellent speller. I'd appreciate being notified about any needs for change, whether minor typos that neither I nor the spelling checker didn't catch, or something more significant. -nb)

Written 96.03 Revised 96.11 by Nicholas Bodley, Waltham, Mass., USA.

Copyright @1997 Nicholas Bodley

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