Some principles

All radio listeners know how to precisely perform, on their receiver, the operation that consists of "tuning" it to the broadcast they wish to hear. For many of them, this maneuver, which the improvements made to receiving devices allow to be performed by turning a single knob, retains a mysterious character. We will say a word about the phenomenon to which the adjustment or tuning of a receiver to a specific broadcast corresponds. This phenomenon is called resonance. It is not specific to radioelectricity and extends to all physical phenomena that involve a vibratory movement. If, for example, a tuning fork is struck very close to a piano, the strings of the piano corresponding to the note of the tuning fork are heard to vibrate. The other strings remained silent, or, in any case, their vibration was insignificant. To give a simple explanation of this phenomenon, we will take the example of the swing, which is set in motion by giving it successive impulses. At the first impulse, the swing starts, moves away, then returns; if we give our second impulse, as well as the following impulses, at the moment when the swing returns to its initial position, we observe that each time the amplitude of the movement increases and that our efforts add their effects to each other. If, on the contrary, after the first impulse, we had given the following ones without method, by pushing the swing while it is in any position, we would have seen the oscillations remain of small amplitude despite our efforts. In the first case, we have put our efforts "in resonance" with the oscillations of the swing. Electrical resonance is quite analogous. When the Hertzian wave encounters an antenna, the electrons begin to move and execute an oscillation between the two ends of the wire. If a second wave arrives at the moment when these electrons, having returned to their starting point, begin their second oscillation, this wave will add its effect to the first: the movement of the electrons will be amplified. It is thus conceived that Hertzian waves, even weak, can, under such conditions, produce an appreciable effect. There will have been resonance between the waves and the oscillations of the electrons of the antenna. But how to establish this resonance? By matching the rhythm of arrival of the waves with that of the electrical oscillations of the antenna; in other words, it is necessary that, in the interval between the arrival of two successive waves, the electrons have completely traversed the length of the antenna (back and forth) to return to their initial position. It can therefore be said that, for resonance to be achieved, the duration of a complete oscillation in the antenna must be equal to the time interval that separates two successive waves. But this oscillation duration depends on the path that the electrons have to take, that is to say, the length of the antenna. We will therefore shorten or lengthen it to obtain the desired tuning. For the convenience of the maneuver, we will insert at the base of the antenna a coil of wire of which we will use more or fewer turns. The tuning thus achieved, the transmitting antenna and the receiving antenna will vibrate in unison. Rhythm creates resonance; resonance amplifies the effect and makes it perceptible.