Page:Elementary Text-book of Physics (Anthony, 1897).djvu/392

378 Hertz showed that the electromagnetic waves were reflected from metal surfaces according to the law for the reflection of light (§ 333), and that nodal points or points of interference between the waves advancing from the vibrator and those returning from the mirror could be detected, and thus the wave-length of the disturbance determined. If the wave-length and the period be known, the velocity of the wave may be calculated; an approximate calculation of the period was made from the dimensions of the vibrator, and the velocity of the waves determined to be of the same order of magnitude as the velocity of light. Subsequent experiments, under more favorable conditions and with vibrators which permit a more precise calculation of the period, have confirmed the conclusion of theory, that the velocity of very short electromagnetic waves is the same as the velocity of light.

Hertz also proved that the electromagnetic waves are refracted (§ 334) when they pass from one medium into another. By the use of a large prism of pitch he obtained a considerable deviation of the waves and was able to calculate the index of refraction of pitch for such waves; he obtained a number of the same order of magnitude as the index of refraction of ordinary refracting bodies for light.

Owing to the way in which these waves are generated by an oscillatory discharge in one line, the waves which proceed from them are polarized (§ 376), that is, the electromotive forces transmitted through the air have always the same direction. Hertz interposed in the path of the waves a screen made of a number of parallel wires; he found that when the wires were parallel with the line of the discharge or with the electromotive forces in the successive waves, the waves were almost entirely absorbed by the wires. If, on the other hand, the wires were set so as to be at right angles to the electromotive forces in the waves, the waves passed through the screen without modification. The screen therefore exhibits a property analogous to that of tourmaline in polarized light (§ 379). Righi and others have observed similar effects produced by the interposition of blocks of wood in the path of the waves, which