Page:Elementary Text-book of Physics (Anthony, 1897).djvu/390

376 plates of the condenser, on the resistance of the circuit, and on its self-induction. In case the resistance of the conductor is greater than a certain value, determined by the self-induction of the circuit and the capacity of the condenser, the current will decrease steadily from its value at the beginning of the discharge to zero. But in most cases this condition is not fulfilled, and in these cases the current assumes another character. It goes through a series of alternations in opposite senses, which are periodic in the sense that the successive maximum values of the current follow each other at equal intervals of time, though 'the absolute values of these maxima diminish very rapidly. The discharge in this case is called an oscillatory discharge. That the discharge of an ordinary condenser is of this nature was discovered by Joseph Henry, from the manner in which needles were magnetized by the discharge passed through small coils of wire. The theory was afterwards indicated by William Thomson, and his conclusions were fully confirmed by the investigations of Feddersen. Feddersen observed the spark produced by the discharge in a rotating mirror, and found that instead of giving a single line of light in the mirror, it gave a series of lines at equal distances apart. He showed that the period of the oscillation could be changed by changing the conditions of the circuit, and that by sufficiently increasing the resistance without correspondingly increasing the self-induction, the period of the oscillations was increased till finally the discharge ceased to exhibit any oscillations whatever.

313. Electromagnetic Waves.— According to Maxwell's theory of electricity, an oscillatory discharge of the sort just described ought to set up a series of disturbances in the medium surrounding the circuit, which proceed outward from the circuit in the manner of waves set up in any medium by a disturbance at a point in it; such disturbances may be called electromagnetic waves. The existence of such waves was demonstrated by Hertz, and the examination of their properties by him and by others has shown that they conform practically in all respects to the predictions of the theory. The arrangement used by Hertz to set up electromagnetic waves, called