The antenna plays the part of one of the metal plates in the Hertz oscillator. Early experiments were made with Hertz apparatus, but the range of such a contrivance is very limited. For one thing, it neglects to take advantage of the earth. It is little realised what an important part the earth plays in the carrying of wireless messages. A very great step was taken when Marconi dispensed with one of the plates of Hertz, and used the earth instead; while the other plate gave place to the elevated wires, the most familiar part of the apparatus to most people. The condenser is thus formed by the earth as one plate, the elevated wires as the other, and the intervening air as the insulator. The "capacity" must be exceedingly small in such an apparatus, but it is sufficient; while the long lines of electrical force stretching from the high antenna to the earth produce waves of great carrying power. Lastly, when the earth forms a part of the condenser the waves cling to it, so that instead of being largely dissipated into space, they move along the surface of the earth. The advantage of this is obvious. At first it was customary to place the spark-gap in the wire leading from the antenna to the earth, as in the accompanying sketch. Later, however, it was found better to place the coil and spark-gap in a local circuit in which the oscillations are first produced. These oscillations pass through a coil which is interwound with another one connected to the antenna and to earth, and thus the local oscillations, as we might call them, induce similar oscillations in the antenna, just as the fluctuations in one part of an induction coil induce fluctuations in the other. Indeed the coil in the local circuit and the one in the antenna circuit actually constitute an induction coil. The advantage of this is that by introducing condensers the capacity of which can be varied, and coils the inductance of which can be varied, into the oscillation circuit it becomes possible to "tune" the circuits effectively. Thus resonance comes into play and the power expended can be made to produce the maximum effect.