The Model F Comptometer followed on from the Models A to E in 1915. It was the first model to be mass-produced in quantity, with around 40,000 units built over a 5-year period.

Comptometers were available with 8, 10, or 12 columns, in all-decimal or Sterling currency models. (The model numbers refer to the design of the internal mechanism, and not to the number or denomination of the columns). This 10-column machine was built for Sterling currency, with the black and white keys colour-coded for pounds, shillings, pence, and farthings (quarter-pence). There is only a single "1" key in the ten-shillings column, 11 in the pence, and 3 in the farthings. There are no "zero" keys. The clearing handle is mounted at the right-hand rear, and is moved back and forward to reset the registers to zero.

Addition is performed directly, just by pressing the keys. Subtraction is performed by the addition of complements, using a second set of (smaller) numbers marked on the keys. The "subtraction cut-off levers" (located just above the register windows) avoid the need to left-fill the complement with 9s on subtraction. Multiplication can be performed by a simple procedure involving repeated additions. Division is a more complex procedure, and is often converted into a simpler multiplication using a table of reciprocals.

The small red button behind the farthings column is the "controlled-key" lock release. If the operator fails to fully depress a key, the keys in all the other columns will immediately lock. The operator can then complete the incorrect keystroke, clear the lock by pressing the red button, and continue with the calculation as if nothing had happened.

Errors due to incomplete upstrokes (ie, repeating a keystroke before the key has fully risen) are eliminated by means of a ratchet locking mechanism which engages automatically as the key rises. The lock is not reset until the key reaches its topmost position.

A 10-column Model H Comptometer for Sterling currency, with the classic "Comptometer" name embossed into the front of the case.

The Model H made significant changes to the clearing mechanism to further reduce the chance of error. Operators making perhaps 200,000 keystrokes and 3000 clearings per day would sometimes fail to clear the machine before starting a new calculation, and would be unaware of their mistake. The Model H Comptometer introduced a "start-from-clear" signal which provided threee separate indications - visual, audible, and tactile. The clearing handle was relocated to a more convenient position near the front of the machine, and needed only to be drawn lightly forward and released. After clearing, the zeros in the register were offset slightly to give a visual confirmation of the machine's state. The first keystroke of the next calculation required a firmer pressure, corrected the zeros, and rang a small bell, thus providing three separate signals to verify that the calculation was starting from clear.

Felt & Tarrant made much of the term "Enforced Accuracy" in their advertising, as it was impossible to be unaware of an operating error.

The mechanical changes needed to provide the clearing signals are described in US Patent 1357748, filed by engineer J.A.V.Turck in 1919. The escapement mechanism was redesigned and mounted in a hinged sub-frame, which was controlled by two over-centre toggle mechanisms triggered alternately by the keys and the clearing handle. The sub-frame could be removed relatively easily, which greatly simplified cleaning and adjustment. The front of the case was lengthened by half an inch to accommodate the new mechanism.

A 12-column Model J Comptometer for Sterling currency, originally used in the accounts department of a firm in Adelaide, South Australia.

The "copper-cased" Comptometers were never intended to have a bright copper finish. Rather, the copper plating was simply used as an undercoat, as zinc plating or galvanising is used today. The copper protected the steel, and provided a base for a light-brown translucent lacquer finish. Both layers are generally worn through to the bare steel on the corners and around the clearing handle. Cleaning must be done carefully to avoid producing patches of bright copper where the lacquer has deteriorated. The machine illustrated has had all of the old lacquer removed, and the copper has been cleaned to a dull finish. While it may look distinctive, it is certainly not authentic.

Some of the moulded keytops on this machine can be seen to be in poor condition. The black and white keys on the older models are usually still in good order. The white keys on the later models are often severely yellowed, shrunken, or brittle, while the new green keys on the Model J have sometimes decomposed into an oily mess. These problems with new synthetic materials are not unique to Comptometers, nor are they limited to the polymer science of the 1920s. Many relatively modern materials (such as the polyurethanes of the 1960s and 70s) have been introduced with great promise, but have proven to be total failures in long-term service.

A ten-column dual-register Comptometer for Sterling currency, without farthings.

The "Super Totalizer" is essentially a standard Comptometer with a second accumulator register added at the front. The front register can be engaged and cleared independently, so as to accumulate the grand total of a series of calculations. The front of the case has been extended by 3 inches to accommodate the additional mechanism.

US Patent 1391220 for an early dual-register Comptometer was issued to J.A.V.Turck in 1921. The machine illustrated was introduced in 1934, based on a later patent (2130364) to Turck and Niemann. The machine carries separate serial numbers for the main machine (J321777) and the front register mechanism (S1625).

The Model K was the first electrically-operated Comptometer, also introduced in 1934. An internal electric motor provided the power to drive the mechanism (in a similar manner to an electric typrwriter), resulting in a very light touch for the operator. (Models F to J required more than 1 pound force to operate the keys. Models A to E required up to 5 pounds).

The Model K was housed in a pressed-metal casing with a dark green painted finish. The case is raised at the rear to give a sloping keyboard with a uniform key height. The 10 and 11 pence keys have been moved sideways into the top of the farthings column.

The Model M Comptometer, which followed the Model J in 1938, had a more rounded pressed-metal casing with a mid-green painted finish. The machine is an inch wider than the corresponding Model J, with the clearing handle located within the limits of the body rather than externally.

The mechanism of the Model M was similar to the Model J, but included a number of changes which provided a lighter touch than on the earlier machines. It also introduced a new mechanism which aimed to reduce reading errors by supressing the display of leading zeros in the accumulator. The mechanism used modified numeral wheels with cascaded shutters to hide the unwanted zeros.

Model 3D11 was the last Felt & Tarrant Comptometer. It followed on from the very successful Model M in about 1950, and continued until the end of US production in Chicago in 1961.

A new feature of this model is the single key on the left-hand side which is marked with multiplication and division symbols. When latched down, this key modifies the operation of the "controlled key" mechanism so that it resets automatically when the errant keystroke is completed. This was especially useful in multiplication and division calculations, as it meant that the operator did not have to re-position her hands to correct the error. The red "controlled key" release button was also relocated to become a small green button amongst the subtraction cut-offs at the front of the machine.

This 12-column machine was originally built for Sterling currency. It was converted to 11-column decimal by the Australian agents (Peacock Bros) during the decimal currency change-over in 1966. This was not a trivial process, as it required removing the keys and key-plate, punching new key slots in the ten-shillings column, filling the empty slots in the pence and farthing columns, and disassembling the frame to replace the special side plates in the ten-shillings column. (Follow the links under the Model J above for a detailed description of the internal mechanism).

There was a significant trade in refurbished Comptometers during Australia's conversion to decimal currency in 1966. This 12-column Model J has been converted to decimal and re-finished in the new Comptometer green. The machine has been fitted with a new keyplate and a new set of late-model keytops, suggesting that the problems with the original keys were well evident at that time.

A 12-column electrically-driven Comptometer built by Felt & Tarrant's British subsidiary. The case is similar in style to the 3D11, with the clearing handle inboard and the key release button at the front.

The machine is built for Sterling currency, but has no farthings keys, and no 10 or 11 keys in the pence columns. The 10 and 11 keys were rarely used for addition, and their absence presented only a minor complication when subtracting by complements.
Felt & Tarrant Ltd patent label (29kb)

A late-1950s "Comptometer", made in England by the Bell Punch Company (BPC) for export to the Comptometer Corporation in America. These machines were sold in Australia through the Comptometer distributors. The machine is identical to the corresponding "Sumlock" model, except for the tan colour scheme and the bolt-on name badges. This machine was originally built for Sterling currency (with no farthings and no 10 or 11 keys), but has been converted to decimal.

The large nameplate found on the base of Bell Punch Company machines was greatly abbreviated for their US Comptometer range. The maker's name and the British patent details have been completely removed, leaving only a model number and a few US and foreign patents. The last US patent listed was issued in 1956.
BPC Comptometer nameplate (28kb)

The Model 12D is a dual-register machine with an electric motor drive. The case is similar to the manually-operated model from the same period (above), but is raised at the rear to accommodate the grand-total register behind the keyboard. The machine is built for Sterling currency, but has no farthings and no 10 or 11 keys in the pence column. There is an identical machine in the Sumlock range.

A late-model 9-column "Comptometer" for decimal currency, in a re-styled 1960s casing. The machine is identical to the Sumlock FigureFlow.

Early-model Comptometers were supplied with a substantial sheet-metal dust cover with the name printed on both ends.
The fine print reads:
        COMPTOMETER
 (pronounced like thermometer)
 
Detail view (29kb)

This attachment contains a look-up table to assist the operator in calculations involving pence and farthings. It clamps on to the flat section at the rear of the early Comptometers, but has no connection to the internal mechanism.

The printed table has 48 rows for pence and farthings from 0 to 12, and five columns for multipliers of 1, 10, 100, 1000, and 10,000. The table is wrapped around a drum of 4" diameter, which can be turned to any of 48 positions by the knob at the left-hand end. The multiplier is selected by a lever at the top left, which opens a shutter to reveal the desired column. The drum is turned to show the pence value (eg threepence-ha'penny) in the rightmost window, the lever is set to the number of items (eg, 100), and the result (£1-9-2) is read from the 100s window.

The British copyright for the table was entered at Stationers' Hall in 1909.
Separate view (25kb)
Scale detail (underneath view) (29kb)

The Addicalco was built by the firm ACCA (loosely translated as "Adding, calculating, accounting, etc.") in Milan from 1946. The first machines were built using a set of tooling from Felt & Tarrant's superseded Model J Comptometer, presumably supplied as part of the post-war European reconstruction program.

This Addicalco Model 53 is a 10-column machine for Sterling currency (with farthings), which is generally similar to the Model M Comptometer. It was imported and sold in Australia in competition with the Comptometer, but only in fairly small numbers. This machine has a leather carrying case that was custom-built by a firm in Melbourne.
Addicalco badge (24kb)
ACCA badge (24kb)

In the 1950s the Comptometer Corporation sought to broaden their product line with a ten-key adding and listing machine sourced from Walther in Germany. The name "Comptograph" was taken from Felt's original recording calculator of 1889.

When the Comptograph was discontinued in favour of the Victor machines following the merger in 1961, the Australian Comptometer agents continued to supply the Walther machines direct from Germany.

Part of the decorative scrollwork which is embossed into the sides of the early Comptometer cases.

The classic Comptometer script trade mark, embossed into the front panel of the Model H and J cases.

The rear panel of the Model J Comptometer shows a series of US patent dates, from the first production model in 1887 up to the introduction of the Model H in 1920.

Th engraved nameplate fastened to the top of the Models F to J Comptometers, just behind the keys.

Models K and M have the Comptometer name embossed into the front panel, with a small die-cast "Felt & Tarrant" name badge behind the keys. The engraved nameplate with the "back-handed" text still exists, but it has been banished to the underside of the case.

Model 3D11 has a plain front panel, with the "Comptometer" name appearing only on the small badge behind the keys.

A re-styled Comptometer badge in plastic rather than metal, from a British-made Felt & Tarrant machine.

The Model 101 Comptograph from the 1950s uses an inverted version of the same badge on the front of the machine. It refers to "Comptometer Ltd (Great Britain)", and carries the ribbon logo of the German Walther company.

Peacock Bros. were a long-established firm of office systems suppliers based in Melbourne, Australia. Over the years they handled the Swiss "Millionaire" and "MADAS" calculators, the American "Comptometer", the German "Walther" ten-key machines, and eventually the Japanese "Busicom" electronic calculators.