Compare now the motion of the tuning-fork, after the pressure of the fingers has been removed, to the electrical motion when the air-gap has broken down and the action of the electrical generator suspended. The prongs are driven back to their normal position by the elastic reaction due to the bending; but when they reach that position they are moving with a certain velocity, and their momentum carries them beyond that position; they move on until the energy of the moving mass has been expended in the work of bending the prongs in the direction opposite to that of the original bending. The prongs begin then to move back in the opposite direction, starting the second cycle of motion. The same line of reasoning will carry us into the third and fourth and every succeeding cycle of motion. It is obvious that these cycles will follow each other during equal intervals of time, which gives a definite pitch to the tuning-fork. A periodic motion of this type is called an oscillation or vibration; and it is clear that it is a periodic transformation of the energy of elastic bending into energy of motion of the mass of the prongs including the surrounding air, and vice versa. The motion is finally reduced to rest when the energy of bending, produced at the start by the work of the fingers, has been used up. The question, what has become of that energy? is very important in this connection. The answer is: It is used up partly in overcoming internal friction and _partly in overcoming the reactions of the surrounding air, which result in sound-waves_. A sound-wave is a short name describing the physical fact that in the air here are compressions and dilatations alternating at periodically recurring intervals. The production of sound-waves in the air is a proof that the air in the space surrounding the tuning-fork participates in the motions of the tuning-fork. A perfectly analogous experiment was performed by Hertz with his electrical oscillator, and his principal object was to find whether the electrical field, that is, the electrified space surrounding the oscillator, reacted as did the air driven by the vibrating tuning-fork; if it did it would develop electrical waves. If these electrical waves actually existed, what did Hertz expect them to be? In the description of the oscillator and of its action, given above, two things only were mentioned: the action of the electrical machine which charges the oscillator and the reaction of the lines of force against the tensions and pressures which crowd them into the surrounding space. The electrical waves can, therefore, be nothing else than periodic variations of the tensions and pressures in the lines of force, that is to say, periodic variations in the destiny of the lines of force in the space surrounding the oscillator. This was what Hertz had found.