"So," my friend Mike Sebastian asked, "how are you going to present your new CURTA as a programmable calculator?"

A good question. But whether you can call it programmable or not, the amazing CURTA deserves mention in any respectable collection of vintage calculators.

From its introduction in 1948 until it gave way to electronic calculators in the early 1970s, the CURTA was the instrument of choice for many nerds. A mechanical marvel in a compact package, the CURTA is a remarkably versatile handheld calculating tool.

The CURTA, invented by Curt Herzstark (who worked out the basics of the design while he was a concentration camp inmate!) is a mechanical calculator utilizing the so-called stepped drum mechanism. A stepped drum mechanism uses a gear with a variable number of cogs along its length. In the CURTA, sliding levers are used to set the corresponding drums to the desired digits; when the crank is then turned, the digits are transferred to the accumulator by addition. This, along with an ingenious carry mechanism is what turns the CURTA into an efficient adding machine.

The ability to shift the entry mechanism relative to the accumulator is what makes it possible to use the CURTA for multiplication; you can shift the machine to the desired decimal position, perform the necessary number of rotations, then shift to the next position as you multiply with each successive digit.

The crank of the CURTA can be lifted to reverse its operation: subtraction instead of addition. A sliding switch enables the counter to count either upwards or downwards. Together, these features make it possible not only to easily divide multidigit numbers, but also to compute square roots, a method for which is described in the manual.

The CURTA came in two basic models: type I and type II CURTAs are distinguished by the number of digits that the machine can handle. The picture on this page is that of a type II CURTA calculator, shown after a successful calculation of the square root of 5.

So what makes the CURTA a programmable calculator? Well... fortunately, I don't have to figure out the answer to that one. CURTA enthusiasts have already done it for me, as a description of the CURTA "programming model" at CURTA.ORG demonstrates. From that article we learn that the CURTA has three registers, an instruction set with four instructions, and a clock frequency of ±1 per revolution.

This 1957 (according to the serial number) machine in my hands still operates absolutely flawlessly nearly half a century after it was made. A symbol of a bygone era, but lest we forget, it was CURTAs and slide rules, not Pentium-III computers, that put a man on the Moon.