John Wolff's Web Museum

The Felt & Tarrant Model J Comptometer - Technical Description

Contents Introduction The case and frame The addition mechanism The keyboard mechanism The tens-carry mechanism The "Controlled Key" mechanism The clearing mechanism and the "Zero Signal" The Sterling models Notes on overhaul
	Model J Comptometer, c.1930

Introduction

These pages give a detailed technical description of the internal mechanism of the Felt & Tarrant "Comptometer". They relate primarily to the "copper-cased" Models H and J, which were manufactured from 1919 to 1935. The notes and photographs were prepared during the overhaul of a 12-column Sterling machine (Model J, S/N 254299) during 2003-04.

These notes are intended to complement (and especially, to illustrate) some of the previous works on the Comptometer mechanism:

• the Description of Comptometer Operation, prepared by Mr Ray Mackay of Melbourne, Australia, in 1997, and available on James Redin's web site. Ray's notes are a first-hand description of the principles and operation of the mechanism, recalled from his personal experience as a Comptometer service engineer in the 1960s.
  
  
• the Repair Instructions for the Model H Comptometer, published by the Felt & Tarrant Manufacturing Company in 1919 and available in HTML format on Brooke Boering's Comptometer reference site. This official repair manual for the Model H Comptometer provides an authoritative description of the mechanism and its individual components, from the viewpoint of a factory-trained serviceman in the field. There are only minor differences between Model H (1919) and Model J (1926).
  
  
• the Repairman's Reference Book for the Model K Comptometer, published by Felt & Tarrant in 1934. The Model K is a motorised Comptometer in which about half of the components are carried over from the previous models. This book of 135 pages contains a detailed trouble-shooting guide and an extremely wordy description of the function of every component, some of which are relevant to the operation of Models H and J.

My approach to describing the Comptometer mechanism is to break the machine down into a number of basic functional areas and to describe the principles of each, and then to re-assemble each function in easy stages to show the individual components and the ways in which they work together.

While I believe my descriptions to be substantially correct, I do not guarantee that they are accurate, complete, or applicable to any other machine. I would welcome any corrections, or suggestions for improvement.

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

Resources for Further Information

• The comptometers page on this site gives general information about key-driven calculators from Felt & Tarrant, Burroughs, and other manufacturers.
  
  
• Brooke Boering's Comptometer reference site provides a wealth of detailed information about the history and products of the Felt & Tarrant company.
  
  
• The items referenced in the introduction (above) provide technical descriptions of the Comptometer mechanism from quite different viewpoints.
  
  
• The Technical Description of the Bell Punch "PLUS" (on this site) describes a rather different mechanism which addresses the same ends.