These pages give a detailed technical description of the internal mechanism of the Felt & Tarrant Model J Comptometer. The description can also be used (with suitable modifications) with any of the manually-operated "rock frame" Comptometers (Models H, J, M, WM, and 3D11) that were manufactured from 1920 to the 1960s. In the earlier Models A to F the keyboard and actuating mechanisms are generally similar to the Model J, but the register and clearing mechanisms are different. The electric machines (Models K, 992, 99C) use similar rock frame and register mechanisms to the Model J, but the keyboard and actuating mechanisms are different. Descriptions of other models may be added if there is sufficient interest.
The notes and photographs following were first prepared during the overhaul of a 12-column Sterling machine (Model J, S/N 254299) in 2002-3. They have been refined and expanded during the rebuilding of many Comptometers since. While I believe my descriptions to be substantially correct, I do not guarantee that they are entirely accurate or complete. No responsibility will be taken for any consequences arising from the use of these notes by others. Readers seeking to rebuild or restore a Comptometer will find detailed step-by-step instructions on the Rebuilding pages.
I am indebted to the late Mr Ray Mackay of Melbourne (formerly Service Manager for Peacock Bros, the Comptometer agents in Australia) for his assistance and encouragement with this project.
I would be happy to receive feedback, comments, or suggestions via the enquiry form.
Comptometer model names refer only to the construction of the
internal mechanism, and not to the number of columns or the
arrangement of the keyboard. The machines were generally made with
8, 10, or 12 keyboard columns, but larger machines were available on
special order. Non-decimal versions included machines for British
currency (with or without farthings), Indian currency, hours and
minutes, and Imperial lengths and weights.
The graph opposite shows the (approximate) progress of serial numbers by model and year for the "copper-cased " or "shoebox" Comptometers Models A to J. (Click the graph to enlarge).
Felt & Tarant's first machines (1886-1903) were housed in rectangular wooden cases, and were known simply as "Comptometers" without a model designation. The early mechanism required the keys to be pressed one-at-a-time, and had no operating safeguards.
The metal-cased "Model A Duplex Comptometer" from 1904 added a new automatic mechanism which allowed keys in adjacent columns to be pressed simultaneously, so that multi-digit multipliers and divisors could be entered at a single stroke.
Model B (1906) added "cut-off levers" to remove the need for filling the keyboard with nines on subtraction.
Model C (two versions, 1909 and 1911) reduced the force required to operate the keys and improved the visibility of the register. Many of the mechanical parts were redesigned for greater strength and durability.
The Model E (1913) introduced the "Controlled-key" mechanism. This involves an automatic "trigger" which detects incomplete keystrokes and allows the operator to correct them without needing to re-start the calculation. Model E was also the first (and only) Comptometer to have "interference guards" to eliminate errors from accidental movement of adjacent keys.
Model F (1915) re-calibrated the trigger mechanism and removed the interference guards. Production of Model F Comptometers (over 40,000 machines in 5 years) exceeded the total of models A to E combined.
Models A to F all have a a permanently-engaged register drive. To clear the machine, the forward stroke of the clearing handle advances all the register dials 9 (or their maximum capacity). The mechanism then injects a 1 into the first column, initiating a ripple carry and leaving the register at zero.
The Model H (1920) separated much of the tens-carry mechanism into a removable "rock frame" at the front of the machine. A light touch of the clearing handle would rock the frame forward, disconnecting the register drive and allowing the carry springs to return the dials to zero. A triple "start from clear" signal was added, and the case was further decorated with a large Comptometer name front and rear.
The Model J (1926) was the last of the "shoebox" Comptometers. It was essentially the same as the Model H, except for some modifications to the carry gear and escapement levers, and a re-calibration of the springs to provide a lighter touch. The keytops changed to green and white, using a new synthetic polymer material which has not lasted well. There was considerable overlap between models H and J at the time of transition, and there were many changes and improvements made during the 13 years of production.
Model K (1934) was the first electric Comptometer. It uses a similar rock-frame and register mechanism to the Model J, but has a completely different keyboard and a power-driven actuator. The Controlled-key mechanism still works in exactly the same manner as on the previous models. Model K has a deep brown painted case with short legs at the rear.
The manually-operated Model M (1939) has a green-painted case in a more rounded "Art-deco" style. The mechanism has leading-zero shutters on the numeral wheels, latches on the subtraction levers, and further refinements to the touch. Some of the parts were made from aluminium. The "war model" WM was the same as the M but with many parts redesigned to conserve materials.
The final changes occurred in 1950. The electric Model K was updated to become the Model 992, with the case and register display from the Model M and an automatic start-stop mechanism for the motor. Model M became Model 3D11, with changes to the Controlled-key mechanism to provide automatic re-setting during multiplication and division.
