Comptometer-style mechanical calculators were once among the most common and popular forms of adding machine available. Though Felt & Tarrant originally invented and marketed the actual Comptometer brand calculating device, the word 'comptometer' has since become the vernacular. Comptometers are characterized by their massive array of keys. Unlike a modern calculator which only has a single set of numeric keys and forces the user to enter numbers one digit at a time, comptometers have a column of numeric keys for each digit. This allows a skilled operator to add numbers on the machine at a very fast pace, far faster than on a conventional calculator, by striking every column of keys at once. Comptometers are designed primarily for repetitive addition. Though capable of other mathematical operations, execution is not so straightforward. In fact, most of the entities that manufactured Comptometer-style calculators ran academies which companies could send their employees to, in order to learn how to operate these machines.

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The Lightning Portable Adding Machine is a device commonly referred to as an 'addometer', a popular type of calculator in the 1950s and 1960s due to their low cost and ease of use. Unlike complex comptometers and expensive 10-key calculators, addometer-style machines could be used by practically anyone, with little to no training. The controls of the machine consist of seven perforated wheels which resemble small rotary phone dials. Four of the dials are copper-colored; these dials are the cents and thousands indicators when working with currency. The perforations in the dials are marked with numbers.

To perform addition, the user inserts a metal stylus into the appropriate hole and rotates the dials until the stylus catches the hook on the right side of the dial aperture. A small window above each dial displays the sum of the numbers entered into the unit, and a slide on the left side of the machine resets the display to zero. The unit is mounted in an angled bakelite stand for easy use, but is designed to be removed from the stand and placed directly on the document being added when performing long calculations.

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The Addometer is a small, handheld, stylus operated adding machine that was produced in large quantities during the first half of the 20th century. The machine consists of a set of 8 digit wheels encased in a long rectangular metal enclosure with carry levers located above each wheel. Using the machine for addition is very simple; the stylus is inserted in the hole for the desired number on each dial and turned clockwise to add it to the total, which appears in the small round windows at the bottom of each dial. The machine can also be used for subtraction, in this mode the dials are turned counterclockwise and the smaller numbers are used for input. A sliding lever on the side of the device can be pulled out to reset the machine to zero. The Addometer was manufactured for nearly 40 years starting in 1927, and as such is quite common in the collectors market.

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The Curta, manufactured by Contina AG Mauren in Liechtenstein, is widely considered to be the smallest 4-function mechanical calculator in the world. Produced in both eight and eleven digit versions, the Curta is held upon an almost mythical pedestal by calculator and computer collectors, due to its war-torn origin story and intricate mechanical construction. It's inventor, Curt Herzstark, began design work on the Curta in the 1930s, but was captured by Nazi Germany and imprisoned in the Buchenwald concentration camp before the calculator could be manufactured. Curt Herzstark was forced to trade the plans for the Curta to the to the Buchenwald camp administrators in exchange for his own life. Nazi Germany never manufactured the device however, and after Buchenwald was liberated by the Allies in 1945, Curt Herzstark partnered with the Prince of Liechtenstein and was able to put the Curta into production. The heart of the Curta is a central stepped drum, which contains teeth representing each decimal digit. In operation input sliders on the outside of the device enable the output register to be interfaced with the stepped drum at various points, allowing addition to be preformed by rotating the crank attached to the top of the stepped drum. Controls on the Curta's shell allow the drum and output register to be manipulated in various ways to facilitate four function math. The unique combination of size and function made Curta a commercial success, and approximately 140,000 units were produced over the product's 20 year lifespan.

Despite it's ample production run, the Curta has been a highly-sought collectors item for decades. At a time in the past when calculator collecting was still in it's infancy, Curtas were already commanding 4 digit prices on Ebay. High-value Curta collecting was even a major plot point in the 2003 novel Pattern Recognition, in which the protagonist was forced to trade a low-serial number Curta for access to a global spy network. So many collectors have been priced out of the market for a Curta that in 2016, a maker named Marcus Wu designed a version of the Curta that can be printed on a 3D printer. Limitations of 3D printer technology force the Marcus Wu Curta to be printed a 3 times the size of an original, but the print is otherwise fully mechanically functional, and for most people represents the only realistic option to obtain one of these unusual calculators for themselves.

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This colorful calculator was produced by Eldorado Electrodata, one of the many companies that jumped into the market during the calculator gold rush of the 1970s. The Mathmagic B is attractively packaged in a striking blue-green enclosure, and also attractive for its 8 digit neon Panaplex display. The processor for the Mathmagic is a single chip CZ550, made by General Instruments, a chip best known for it's use in a number of British Sinclair kit calculators. The display is driven by discrete transistor drive circuits, and the CZ550 IC is socketed for easy replacement, a welcome feature. The Mathmagic B also includes an internal battery, which should be removed as quickly as possible by the modern collector, lest its corrosive internal juices leak and destroy the rest of the calculator. The Mathmagic B appears to be a fairly uncommon device... Eldorado Electrodata filed for bankruptcy in 1972 and was fully liquidated by 1976, allegedly a result of the company financially starving their existing test equipment businesses to try to break into the calculator market. The Mathmagic B is believed to have been released sometime between 1972 and 1974.

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The Craig 4505 is a small, horizontal format four-function calculator that was first released in 1974. Numerous calculators were made in this horizontal layout in the 1970s, in which the keypad is located to the right instead of below the display. This calculator makes use of a 9 digit, tubular single envelope VFD, similar to the Futaba 9CT06 or Soviet-made IV-21 tube. The Craig 4505 is equipped with a Rockwell 15330 calculator-on-a-chip IC to handle most of its functions, this chip is located in a square cutout in the PCB directly below the display. In 1974, the list price for this calculator was $110 US dollars.

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The Todd Protectograph 'Star' adding machine is quite small when compared to a comptometer from the same era, such as the Felt & Tarrant Model J. The front of the machine is bristling with ribbed sliders by which numbers are entered into the device. Each rib is labeled with a number mounted to an adjacent plate. To enter a number into the device, the user places their finger onto the appropriate rib and then pulls the slider down until their finger had pushed down the small button at the base of each slider. This is quite analogous to the operation of the digits dial on a rotary telephone.

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The Business Analyst is a common finance-oriented derivative of the Texas Instruments TI-30 and was released in 1976. The calculator has a nine digit LED display, one digit of which is reserved for the displaying the sign. Each digit has a convex magnifier lens attached to increase the readability of the tiny die-based LED strip display. The calculator uses rechargeable batteries which can be recharged while installed inside the calculator, though any original batteries have likely erupted into a blob of green corrosion long ago. This calculator uses a TMC0982 chip, the same chip as used in the extremely popular TI-30 calculator. This particular example was manufactured in the 18th week of 1977, and would have originally sold for $49.95.

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The Victor 6834 uses a comptometer-style input system, with a row of sequential input keys for each digit. Although this machine is capable of displaying up to nine digits, only eight of then have input keys, as is normal for comptometers. The unit has no display; each number entered into the machine is printed on a standard roll of adding machine paper that is loaded into the back of the device.

There is a mechanical accumulator suspended from the underside of the machine, and the lever to the right of the display is used to print and erase the contents of the accumulator. Subtraction is also possible by using the lever on the left side. Unlike many other comptometers from this era, the device has no motor. All mechanical power comes from pulling the lever on the side of the machine.

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