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Henry Parsons Hall


A History of Impedance Measurements


This history was originally written as a chapter of a book on the histories of various types of electrical measurements that was to be published by the IEEE. This project was the idea of Joseph F. Keithley, founder of Keithley Instruments, who initiated the project in 1988. When he died in 1999, only a few of the chapters had been written, some never started, and the project was abandoned.

There were to be some nineteen sections on various subjects. The first, to be written by Joe himself, was to be on very early electrical measurements. Fortunately, he did publish it separately as “The Story of Electrical and Magnetic Measurements from 500 BC to the 1940s”, and it is available from the IEEE press. This history was put aside when the “Keithley Project” was dropped, but recently I decided that the effort I had put into it shouldn’t be wasted and the information I had gathered should not be lost.

Readers might note that many more GR (General Radio or GenRad) instruments are mentioned and referenced than are those of other companies. There are three reasons for this. First, GR made many more bridges than any other company, second, GR instruments were well documented in their little journal, The GR Experimenter, and finally, your author worked at GR for 42 years and may be a somewhat biased.


This booklet covers the history of impedance measurements dc resistance to low radio frequencies. By this we mean the range of circuits with lumped impedances, a range of frequencies from dc to about 100 MHz. However most of the discussion is about lower-frequency measurements, those at 1 MHz or lower. It deals mainly with instruments for production test and general-purpose measurements in science and industry. Some discussion is made of the techniques used in precision impedance measurements but it does not deal with the fascinating story of the history of determining the value of the Ohm that was to be in another chapter of the proposed book.

The early history in the first part repeats to some degree the material that was to be in the introductory chapter of the proposed book and now is in Joe Keithley’s book. This is necessary because many of the first measurements were dc resistance and ac impedance measurements. Scientists like Ohm, Wheatstone, Kelvin and Maxwell were important to the story of impedance measurements as well as to the early history of electricity.


The author wishes to thank those that helped him with this booklet, particularly the several reviewers who took the time, trouble and effort to read the drafts and comment on them. They were Erasmus Barlow, Norman Belecki, David Braudaway, Conrad Hoffman, William Lyons, Arnold Lynch, Gordon McCouch, Jack Riley and Robert Soderman. He also wants to thank others he contacted with specific questions, especially Doug Strain. And, of course, I thank my wife for her patience and understanding.