Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/380

this way shown to be finite and of the same order as the velocity of light.^[1]

Later in 1888 Hertz^[2] showed that electric waves in air are reflected at the surface of a wall; stationary waves may thus be produced, and interference may be obtained between direct and reflected beams travelling in the same direction.

The theoretical analysis of the disturbance emitted by a Hertzian radiator according to Maxwell's theory was given by Hertz in the following year.^[3]

The effects of the radiator are chiefly determined by the free electric charges which, alternately appearing at the two sides, generate an electric field by their presence and a etic field by their motion. In each oscillation, as the charges on the poles of the radiator increase from zero, lines of electric force, having their ends on these poles, move outwards into the surrounding space. When the charges on the poles attain their greatest values, the lines cease to issue outwards, and the existing lines begin to retreat inwards towards the poles; but the outer lines of force contract in such a way that their upper and lower parts touch each other at some distance from the radiator, and the remoter portion of each of these lines thus takes the form of a loop; and when the rest of the line of force retreats inwards towards the radiator, this loop becomes detached and is propagated outwards as radiation. In this way the radiator emits a series of whirl-rings, which as they move grow thinner and wider; at a distance, the disturbance

1. Hertz's experiments gave the value 45/28 for the ratio of the velocity of electric waves in air to the velocity of electric waves conducted by the wires, und 2 x 10¹⁰ ems. per sec. for the latter velocity. These numbers were afterwards found to be open to objection: Poincaré (Comptes Rendus, cxi (1890). p. 322) showed that the period calculated by Hertz was ${\sqrt {2}}\times$ the true period, which would make the velocity of propagation in air equal to that of light $\times {\sqrt {2}}$ . Ernst Lecher (Wiener Berichte, May 8, 1890; Phil. Mag. xxx (1890), p. 128), experimenting on the velocity of propagation of electric vibrations in wires, found instead of Hertz's 2 x 10¹⁰ ems. per sec., a value within two per cent. of the velocity of light. E. Sarasin and L. De La Rive at Geneva (Archives des Sc. Phys. xxix (1893)) finally proved that the velocities of propagation in air and along wires are equal.
2. Ann. d. Phys. xxxix (1898), p. 610. Electric Waves (English edition), p. 124.
3. Ibid., xxxvi (1889), p. 1. Electric Waves (English edition), p. 137.