John Wolff's Web Museum

The Felt & Tarrant Model J Comptometer - Technical Description



Model J Comptometer
Model J Comptometer, c.1930


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 only minor modifications) with any of the manually-operated "rock frame" Comptometers (Models H, J, M, WM, and 3D11) that were manufactured from 1919 to the 1960s.

The removable "rock-frame" containing the tens-carry mechanism was first introduced in 1919 on the Model H. The Model J from 1926 is 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. Model J also introduced two additional components - the accumulator latch and the latch lifter - which were sometimes retro-fitted to Model H machines during major overhauls.

The Model M from 1938 added leading-zero shutters on the numeral wheels and latches on the subtraction levers, and further refinements to the touch. The rectangular "copper" casing was replaced with a more rounded "Art-deco" design. The final changes in the 3D11 (c.1950) were in the trigger touch-off mechanism and the column locks, which were modified to provide automatic resetting during multiplication and division. Apart from these gradual additions, the basic Comptometer mechanism continued more or less unchanged from 1920 until the end of production.

The earlier Model F (1915-1920) has the same keyboard and actuator mechanisms, but has a permanently-engaged register drive and a different clearing mechanism. The electric models (K, 992, 99C) have the same rock-frame and register mechanisms as the Model J, but have completely different keyboards and power-driven actuators. Descriptions of these less-common mechanisms may be added later 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 2003. 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 restore or rebuild a Comptometer will find detailed step-by-step instructions on the Rebuilding page.

I am indebted to the late Mr Ray Mackay of Melbourne for his assistance and encouragement with this project.

I would be happy to receive feedback, comments, or suggestions via the enquiry form.

Resources for Further Information

Original text and images Copyright © John Wolff 2004-12.
Use at own risk; beware of errors; suggestions for improvement welcome.
Last Updated: 8 January 2012

Next:   The case and frame
Back to:   Home    Calculating Machines    Key-driven calculators    Comptometer    Model J    Tech Index