Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/458

 charging, similar to that which would be produced if the mass of a body at the surface of the earth were suddenly to become greater. Moreover, it was conjectured that the condenser, when freely suspended, would tend to move so as to assume the longitudinal orientation, which is that of maximum kinetic energy : the transverse position would therefore be one of unstable equilibrium.

For both effects a search was made by FitzGerald's pupil Trouton: in the experiments designed to observe the turning couple, a condenser was suspended in a vertical plane by a fine wire, and charged. If the plane of the condenser were that of the meridian, about noon there should be no couple tending to alter the orientation, because the drift of aether due to the earth's motion would be at right angles to this plane; at any other hour, a couple should act. The effect to be detected was extremely small; for the magnetic force due to the motion of the charges would be of order, where denotes the velocity of the earth; so the magnetic energy of the system, which depends on the square of the force, would be of order ; and the couple, which depends on the derivate of this with respect to the azimuth, would therefore be likewise of the second order in.

No couple could be detected. As the energy of the magnetic field must be derived from some source, there seems to be no escape from the conclusion that the electrostatic energy of a charged condenser is diminished by the fraction of its amount when the condenser is moving with velocity  at right angles to its lines of electrostatic force. To explain this diminution, it is necessary to admit FitzGerald's hypothesis of contraction. The negative result of the experiment may be taken to indicate that the kinetic potential of the system, when the FitzGerald contraction is taken into account as a