Milestones:Anderson Bridge, 1891

Title

Anderson Bridge, 1891

Citation

Developed by Alexander Anderson here, formerly Queen's College Galway, the Anderson Bridge is a modified Maxwell Bridge specialised for measuring electrical inductance by comparing an unknown inductance value with the capacitance of a fixed reference capacitor. While operating in a manner similar to how the Wheatstone Bridge measures resistance, it was the first invention to enable precise measurements of inductance ranging from a few microhenrys to several henrys.

Street address(es) and GPS coordinates of the Milestone Plaque Sites

University of Galway, University Road H91 TK33. 53.2792N, -9.0617W, University of Galway, University Road H91 TK33. 53.2792N, -9.0617W

Details of the physical location of the plaque

In the Archway of the Quadrangle, near the Physics Laboratory, next to the Reception fully accessible to the public

How the plaque site is protected/secured

Fully visible and accessible to the public

Historical significance of the work

In the mid 19th century, as the field of electrical engineering was evolving, there was great interest in the measurement of electrical components viz. resistance, inductance and capacitance. Michael Faraday and Joseph Henry independently discovered inductance in 1831/32. James Clerk Maxwell introduced a ballistic deflection method for measuring inductance in 1865. This was a variation on the Wheatstone Bridge (1843) for measuring resistance. The Maxwell Bridge balanced the L/R time constant against a known RC time constant. Several modifications followed and the most famous one was the Anderson Bridge (1891) named after Alexander Anderson, Professor of Natural History at the Queen’s College, Galway, now known as the University of Galway [1]. The Anderson Bridge became the de facto bridge for measuring inductance, described in the Bulletin on the Bureau of Standards by E.B. Rosa and F.W. Grover, the leading exponents of all things inductance at the time [2]. The Anderson Bridge is still described in text books for Electrical Engineering [3]. It is also described in “A History of Impedance Measurements” [4].

At the August 1893 International Congress of Electricians, the unit of inductance was standardised as “henry” in honor of Joseph Henry.[8] As such, the unit "microhenry" is also proper. While these two plural forms are both used widely ("henrys" and "henries"), the former is used in the citation to best honor the name of Joseph Henry.

Justification for inclusion of the name of Alexander Anderson in the Citation

Alexander Anderson is indisputably the originator of what came to be known at the Anderson Bridge. His 1891 paper is Reference 1, and its first page can be seen at https://www.tandfonline.com/doi/abs/10.1080/14786449108620116. This came to be known at "Anderson's Method" in the paper at Reference 2, fully available at https://nvlpubs.nist.gov/nistpubs/bulletin/01/nbsbulletinv1n3p291_A2b.pdf. Anderson's 1891 paper cites this as "Anderson's Bridge" in Reference 4 on p. 8, fully available at https://www.ietlabs.com/pdf/GenRad_History/A_History_of_Z_Measurement.pdf. There appears to be no doubt about its originator being Alexander Anderson, and indeed the IEEE Dictionary calls it the "Anderson Bridge." Thus, it certainly is necessary to include this gentleman's full name in the citation.

Obstacles that needed to be overcome

The Maxwell Bridge, was based on the deflection of a ballistic galvanometer, which was difficult to calibrate due to its dependance on a moving coil to detect small movements near the balance points. The Anderson Bridge removed this obstacle by balancing the L/R time constant on an unknown inductor with the known RC time constant of a capacitor. Capacitors were more stable and easier to measure.

Features that set this work apart from similar achievements

The Anderson Bridge remained the standard bridge for measuring inductance until the advent of digital methods in the 1970’s. Most undergraduate textbooks up until that time referenced the Anderson Bridge for the measurement of inductance. It formed the trio of bridges, Wheatstone, Anderson and Schering Bridges for measuring resistance, inductance and capacitance respectively.

Significant references

On Coefficients of Induction, A. Anderson, Philosophical Magazine, vol. 31, pages 329-337
Measurement of Inductance by Anderson’s Method, E.B. Rosa and F.W. Grover, Bulletin of the Bureau of Standards, vol. 1, no. 3, pages 291-293
Optimization of the Anderson-Bridge Experiment, P.Arun, K. Kumar and Mamta, Resonance (Springer), Ch 15, pages 244-256, 2010
A History of Impedance Measurements, H.P. Hall, General Radio Company, page 8, 1992
Irish Innovators in Science and Technology, C. Mollan, W. Davis, B. Finucane, Royal Irish Academy, , page 184-185, 2002
Anderson’s Bridge, Wikipedia
Alexander Anderson (physicist), Wikipedia
https://siarchives.si.edu/collections/siris_sic_12595

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