More than 15 years ago, while restoring two antique telephones, I tried to find out how they worked and learn something about their technical development. Getting that information was surprisingly difficult.
General histories are readily available, but most are on the broader subject of telephony -- with considerable attention to transmission cables, switching, corporate chronologies, and business personalities. Some books, such as those by Robert Bruce (1973) and John Brooks (1975), do treat early developments of the telephone itself, but in a limited and qualitative manner. Perhaps the best historical account yet written is that of Frederick Rhodes (1929). However, even with his meticulous attention to detail, Rhodes' account is incomplete with regard to the induction coil and the magneto, and it does not describe any of the electrical circuits. Others, such as the excellent books of Kempster Miller (several from 1903 to 1933), fill in some of the gaps, but most of these sources are out of date, out of print, and difficult to locate.
Some information on early telephone instruments is available from collectors, and a few of their picture books have been published (Ron Knappen 1978, John Dommers 1983, Kate Dooner 1992, 1993, and Richard Mountjoy 1995). These, however, contain little descriptive information, and they are not generally known outside of museum and collector circles.
A major addition to the literature on the development of telephone technology was the AT&T technical history, which was edited by M. D. Fagen (1975). That account, however, has only one chapter (albeit a lengthy one) on the telephone. It covers only Bell System developments, and addresses only the first fifty years of the telephone, ending its coverage around 1925. Later books, such as that of Frank Lee (1985), provide training in the fundamentals for telephone technicians, but discussions of circuit theory often contain arcane information that obscures the true operating principles..
After locating some of this material, I began to bring together a more complete body of information. The premise of my work was that the telephone is the simplest of all electronic devices, and that it could be easily understood. Because telephone history is a patently technical subject, it was my purpose to trace that history to reveal the technical evolution of commonly available telephones and their operating principles. In the process, the developments described by Rhodes, Miller, and Fagen (AT&T), were updated by some fifty years to cover several decades rich in technical developments.
In tracing the technical developments, a large number of other references were consulted. These are listed in the Bibliography and in footnotes. Copies of all relevant patents were obtained, and each was read for technical insights. Frequent use was made of facilities at the Smithsonian's National Museum of American History, the Library of Congress, and the National Institute of Standards and Technology (the old National Bureau of Standards). Several journals that were published by the Bell System provided a wealth of technical articles. And significant information was also gleaned from a collection of loose-leaf notes called Bell System Practices (now AT&T Practice Standards).
At times, however, it was easier to get technical information directly from the telephones themselves by using the methods of an archeologist. I was able to purchase or borrow specimens of almost every telephone needed for a comprehensive study. These telephones have been examined, restored when necessary, and photographed for the purpose of illustrating the discussions in the text. With a state-of-the-art digital multimeter and some other instruments, every component and circuit described in this book has been tested, using artifacts in operating condition.
As presumed at the outset, telephone circuits have proven to be quite simple. There are, in fact, only five conceptually different circuits used from the very beginning to the time of Bell System divestiture.1 These, and several variations of historical interest, are described with the aid of diagrams drawn specifically to facilitate the reader's understanding. Typical measured voltages are used to permit quantitative discussions, while avoiding complex mathematical analysis.
Although it was not my intention to do original research on the early history, I discovered several interesting facts. First, Gray's early receivers, as described in his patent of 1875, utilized the Page effect and probably could not have reproduced intelligible speech; this concept was corrected in Gray's patent caveat of 1876. Second, the induction coil, long in existence when the telephone was invented, played a role in telephone circuits that was not fully understood for years; no mention is made in early patents or texts of the concept of impedance matching accomplished by the coil. Third, Werner Siemens developed the magneto in its useful form in Germany 20 years before the telephone was invented; this development is almost never mentioned in American texts, and it appears to have been overlooked by Bell and Watson. These new insights are presented within the context of a technically oriented account of the development of the telephone.
The resulting book is divided into four parts, the first three of which cover approximately the same period of time, from the beginning of the telephone in 1875 to divestiture of the Bell System in 1984. Part One covers development of the concept of the telephone and of the electrical components (transmitter, receiver, induction coil, etc.) that comprise a telephone. These developments are carried through to recent times and are illustrated with patent drawings, a few beautiful line drawings from earlier sources, and some equally beautiful new drawings made with computer graphics.
Part Two describes telephone instruments that have been in commercial service in the U.S. Each chapter in Part Two describes a group or generation of telephones that are similar in their mechanical design and vintage. Within each group, the Bell System's Western Electric product line is covered rather completely. In the first edition, Kellogg and Stromberg-Carlson telephones were also covered to illustrate similarities across the industry. In the second edition, Automatic Electric telephones have been added to complete the coverage of the top four telephone manufacturers in the U.S. during the historical period. Photographs of all of these telephones are shown, and, except for several of the earliest telephones, all of the photographs are original.
Part Three describes electrical circuits used throughout the historical period and still in service today. This is accomplished with unique circuit diagrams that reveal operating principles in a simple way while demonstrating how the circuits evolved historically. These diagrams were also drawn by computer and were intentionally produced to have the appearance of ink tracings, like those from the developing period. Even the terminology that is used adopts the language of the developing trade (e.g., condenser instead of capacitor, and cycles per second instead of Hertz).
Part Four contains three chapters on the restoration and repair of these rugged electromechanical instruments, which are, by design, compatible from generation to generation and have virtually unlimited service lifetimes.
An Appendix is also included to provide background information. It resembles a brief text book on physics for just those topics needed to understand the telephone. Selected numerical examples were worked out in the Appendix so the reader will not have to perform any calculations to follow the text.
The first edition of this book became a standard reference volume for telephone collectors and for the small industry that deals in restored equipment. Correspondence from these communities has identified areas for expansion, improvement, and correction. The book's popularity and these comments have thus led to the second edition. The major expansion occurred with the coverage of Automatic Electric telephones, which are not only interesting for their aesthetic design, but also for their novel circuit features. Significant improvements were made in the description of transmitter designs, in examples in the Appendix, and in several other areas (e.g., discussion of the patent for the booster circuit). And a correction in the definition of ringer equivalence numbers (RENs) led to complete re-measurement of ringer properties and the addition of a useful table of REN values for pre-registration telephones.
It has now taken more than 15 years to develop (and improve) the information for the following chapters and to write the book I had hoped to find in the beginning.
1 (1) series circuit, (2) local-battery circuit, (3) booster circuit, (4) booster circuit with anti-sidetone feature, and (5) anti-sidetone bridge circuit. Other circuits are variations of these.