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Philip L. Marston
Postal:
Physics and Astronomy Department Washington State University Pullman, Washington 99164-2814 USA
Email:
marston@wsu.edu
James Clerk Maxwell and the Physics of Sound
The 19th century innovator of electromagnetic theory and gas kinetic theory was more involved in acoustics than is often assumed.
Introduction
The International Year of Light in 2015 served in part to commemorate James Clerk Maxwell’s mathematical formulation of the electromagnetic wave theory of light published in 1865 (Marston, 2016). Maxwell, however, is also remem- bered for a wide range of other contributions to physics and engineering includ- ing, though not limited to, areas such as the kinetic theory of gases, the theory of color perception, thermodynamic relations, Maxwell’s “demon” (associated with the mathematical theory of information), photoelasticity, elastic stress functions and reciprocity theorems, and electrical standards and measurement methods (Flood et al., 2014). Consequently, any involvement of Maxwell in acoustics may appear to be unworthy of consideration. This survey is offered to help overturn that perspective.
For the present author, the idea of examining Maxwell’s involvement in acous- tics arose when reading a review concerned with the propagation of sound waves in gases at low pressures (Greenspan, 1965). Writing at a time when he served as an Acoustical Society of America (ASA) officer, Greenspan was well aware of the importance of the fully developed kinetic theory of gases for understanding sound propagation in low-pressure gases; the average time between the collision of gas molecules introduces a timescale relevant to high-frequency propagation. However, Greenspan went out of his way to mention an addendum at the end of an obscure paper communicated by Maxwell (Preston, 1877). There, Maxwell ex- amined the relationship between kinetic theory and recent measurements of the speed of sound in mercury vapor (Kundt and Warburg, 1876). The context and importance of these developments are clarified in the present article.
Also relevant to Maxwell’s involvement with, and influence in, acoustics was his stature in mathematical physics during his short lifetime (1831-1879). Maxwell’s stature resulted in a secondary involvement in acoustics, his services as a peer reviewer of several important acoustics manuscripts, examples of which are dis- cussed here. Another secondary influence was through his teaching and writing. For example, Horace Lamb (1849-1934) studied Maxwell’s highly mathematical Treatise on Electricity and Magnetism (Maxwell, 1873a) and was taught by Max- well at Cambridge in the 1870s. Of Maxwell’s professional interactions, however, those with Lord Rayleigh (1842-1919) are emphasized here.
Resources and Chronological Summary
Any endeavor to examine Maxwell’s life and thought is aided by the availability of four resources: (1) the early biography (Campbell and Garnett, 1882) commis- sioned by Maxwell’s widow and designated here as the Life; (2) a compilation of many of Maxwell’s scientific papers (Niven, 1890); (3) compilations of Maxwell’s
20 | Acoustics Today | Winter 2016 | volume 12, issue 4 ©2016 Acoustical Society of America. All rights reserved.