Lord Kelvin on Magnetism and Molecular Rotation. 2^)7 



to be embedded in ether, viewed as an incompressible solid, 

 and attached to the ether in contact with it firmly enough to 

 prevent slipping. The circuital impulse on the ring by the 

 generation of the magnetic field will give rise to a rapidly 

 subsiding train of waves of transverse vibration, of the kind 

 which, in communications to Section A of the British Associa- 

 tion* at its meeting in Bristol last September, I described 

 as a solitary wave of the simplest possible kind in an elastic 

 solid, and again, for periodic motion, as a very simple and 

 symmetrical case of a train of periodic waves of transverse 

 vibration. The work done by the circuital force on the ring 

 is spent on waves of this class travelling outwards through 

 ether, and in a very short time the ring comes practically to 

 rest. It does not come to perfect rest suddenly by the 

 departure from it of waves carrying away all its energy ; it 

 subsides to absolute rest in an infinite time according to the 

 law e~ pt smqt. The resinously electrified fluid within the 

 ring continues revolving with unaltered energy as long as 

 the force of the magnetic field is maintained constant. 



§ 4. The simple molecular arrangement thus imagined 

 supplies the rotatory or revolutional motion, and the " moment 

 of momentum, v which, forty-three years ago, I pointed oiitf 

 as wanted to explain, " simply by inertia and pressure, '* the 

 rotation of the plane of polarization, then recently discovered 

 by Faraday, for light transmitted through heavy glass in a 

 powerful magnetic field along the lines of force. In my 

 Baltimore Lectures I showed that embedded gyrostats would 

 in fact produce exactly the rotation of the plane of polariza- 

 tion in a magnetic field discovered by Faraday. The idea 

 which forms the subject of the present communication shows 

 how the fly-wheels of the gyrostats may be started into rotation 

 in virtue of the generation of the magnetic field and stopped 

 when the magnetic field is annulled. 



§ 5. The simply embedded gyrostat has not, however, the 

 vibrational quality which is the essential of the Stokes- 

 Maxwell l-Sellmeier vibratory molecule. For this a gyrostatic 

 vibrator, capable of originating from a single blow on itself 

 a subsidential train of at least 200,000 waves of light, must 



* " On the Simplest Possible &c. ; " " On Continuity in Undulatory 

 Theory, &c," B. A. Report, 1898. 



t " Dynamical Illustrations of the Magnetic and the Helicoidal Rota- 

 tory Effects of Transparent Bodies on Polarized Light." — Proc. R. 8. L., 

 vol. viii. June 1856 ; Phil. Mag. March 1857. 



% See Rayleigh, Phil. Mag. July 1899, quoting from Camb. Univ. 

 Calendar, 18t>9. 



Phil. Mag. S. 5. Vol. 48. No. 291. Aug. .1899. S 



