Miniature Machine Performs Engineering Calculations


Last Update: July 26, 2010 -- THE CURTA REFERENCE

By using a single step drum to actuate successively each digit setting of the machine instead of an individual drum for each setting, Contina, Ltd., Mauren Liechtenstein has made the Curta Calculator small enough to be handheld during operation.

On its circumference the step drum has 19 rows of gear teeth, each row with a number of teeth ranging from one to ten. Nine rows are for addition, and nine cows carry the complementary digits of the addition rows, since subtraction is performed by adding complements. One role has ten teeth for first digit subtraction. When subtracting, all digits from too to eight are complemented to the false complement nine, but digit No. 1 must be complemented to the true complement ten. So the axle of digit No. 1 has an additional Cog wheel which is struck by the top row of ten teeth when subtracting.

Calculations are performed by driving the drum past each of the setting wheels to transfer a number corresponding to the number of teeth encountered. An automatic locking device prevents rotation in the wrong direction so that there is minimum wear of rotating parts. The calculator has been tested with 40,000,000 revolutions without appreciable wear.

Major components of the machine are: the carriage that can be moved to Six different positions for multiplication and division; the number transmission mechanism; the ten's carry mechanism; and the result and counter dials.

This precision calculator that weighs eight oz performs addition, subtraction, multiplication, division, squaring, cubing, and square root operations. The number set is transferred to the result dial once for each turn of the operating handle while the revolution counter records the number of these turns. Automatic devices prevent errors from mishandling. For instance, if one turn too many is made, it can be eliminated by a turn of the handle with the axis in the other position (subtracting instead of adding).

Capacity of the machine is 8x6x11 places. Eight place digits can be set for addition or subtraction; six place multipliers can be used; and maximum results of eleven places can be obtained.

In Canada, the Curta is distributed by the General Adding Machine Company, Toronto; and in the United States the distributor is the Curta Calculator Company, Chicago, Ill.

Carriage can be rotated into any one of six positions for multipliers of more than one digit. If indicating arrow points to 1 of result dial place numeration the number set is transferred unaltered to the result dial, with arrow at 2, number is transferred 10 times; with arrow at 3, number is transferred 100 times, and so on. Turns are recorded in the counter dial accordingly, as 1, 10, or l00.

TRANSMISSION OF NUMBERS As setting knob is pulled down, it rotates the setting axle so that the proper number appears on the setting dial. At the same time it pulls down the setting wheel to a position on the toothed step drum where the number of teeth corresponds to the number set. Turning the operating handle revolves the main axle which rotates the drum so that it acts successively on each of the eight setting wheels. The rotation of each setting wheel is transmitted through the transmission axle to a pinion which drives the numeral wheel in the result dial. This cut shows one digit operation.

TEN'S TRANSFER MECHANISM. As the zero on the numeral wheel is moved so that it is visible in the result dial, the tens transfer pin in the wheel depresses the tens transfer slide of the next digit. The slide then depresses it's tens transfer pinion so that it contacts a single tooth in a plate above the step drum and on the same axle. This tooth moves the pinion one step forward adding one to the next digit. The pinion is then returned to its original position by a curved plate behind the tooth.

TOP VIEW of machine show six place counter dial, and 11 place result dial. Clearing level is locked during operation by release button. When released, one complete turn clears both dials. But each dial can be cleared independently, by rotating the clearing lever overall the openings of that dial. Stops at the boundries between the counter and result dials assist independent clearing.

corrected image (right) from Romano Manaresi (7/11/2010)

SCHEMATIC of toothed portion of step drum explains how addition and subtraction operations are performed. During normal operation, teeth on the addition lines mesh with the setting wheels, adding a number corresponding to the number of teeth encountered. For subtraction, the operating handle is extended, raising the drum so that subtraction lines engage the setting wheels, subtraction is performed by adding the complement of the number that is set. Here wheel is set to add 6.


Historical contribution from Dan Weinstock (

The Calculator Reference by Rick Furr (
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