Page:Elementary Text-book of Physics (Anthony, 1897).djvu/391

§ 313] by him the vibrator, consisted, in a typical form, of two metal plates set up in the same plane, each carrying a rod on the end of which was a small sphere; the plates were so placed that the spheres were near together, with a short air-gap left between them. When the plates were joined to the two terminals of a Holtz machine or to the two terminals of an induction-coil, a series of sparks passed across the air-gap between the spheres. It may be shown that the electrical oscillations in the sparks are practically independent of the peculiarities of the Holtz machine or the induction-coil, and depend only on the capacity of the plates and the resistance and self-induction of the plates and rods carrying the spheres. The electromagnetic waves originate at the spark-gap. The instrument used by Hertz to detect the waves, called by him the resonator, consisted simply of a plane circuit broken at one point by a very small gap; the presence of an electrical disturbance in this circuit could be detected by the appearance of sparks in the gap. The dimensions of the resonator were so adjusted that the period of the electrical oscillations which would originate in it if a momentary discharge were sent through it was the same as the period of the discharge of the vibrator. The electromagnetic waves coming from the vibrator set up electrical disturbances in the resonator, which were detected by the passage of sparks across the gap.

By the aid of these instruments. Hertz first proved the existence of electromagnetic waves; he then impressed upon a wire an electrical oscillation of the same period as that sent through the air around the wire, and compared the rate of propagation of the two disturbances. Hertz's own experiments were misleading, for reasons which perhaps cannot now be given, but Sarasin and de la Eive, working under more favorable circumstances, reached the conclusion, which was accepted by Hertz, that the velocity of propagation of the wave in the wire was the same as that in air, when the periods of vibrations were very small. This result is in accordance with theory. It follows immediately from the view we have taken that the current is due to the movement of tubes of force through the dielectric surrounding the circuit.