The first practical system of wireless telegraphy was invented and carried out by a young Italian engineer of the name of Marconi. Strictly speaking, it may be said that Marconi discovered nothing. What he did was to apply the scientific discoveries of his predecessors and make them practically useful. Marconi made the electric discharge pass between two round knobs, and to these two knobs he attached two wires. One of the wires went high into the air, and the other went down into the ground. The consequence was that when the discharge passed between the two knobs, electricity was in turn thrown up into the air and thrown down into the ground. The effect was very much like that produced by throwing a stone into water. A wave of electric force started from the wire and spread out all around it. As soon as any electricity was thrown into the earth, it spread out along the surface in great waves, which travelled over the ground at an incredible speed. As soon as electricity was thrust up along the wire, a want of electricity was produced in the earth, and this electric vacuum, as we might call it, spread out along the ground in the same way as the first pulse. It was indeed a process resembling the crest and the valley of a wave of water. This was, therefore, the state of things likely to produce the best results. Any other vertical wire at a distance along the surface of the earth would experience the effect of such waves. The electricity would surge up into the wire when the crest of the wave readied it, and would rush down again as the wave subsided. [Illustration: Fig. 71.--Guglielmo Marconi.] Now, every such motion of electricity constitutes an electric current. It remained to discover this current, and to devise a delicate instrument for discovering it even when it is very weak. The instrument which Marconi used for this purpose was called a “coherer.” It was a little glass tube filled with some metallic powder. When an electric wave passes through such powder, the particles cohere or stick together, and then the powder conducts the current easily. That being the case it was only necessary to have a battery in readiness to take advantage of the new state of affairs. That battery was connected to both the end wires of the coherer, and as soon as the wave reached the wire and the coherer began to conduct it, a current from the battery passed through and worked an ordinary telegraph apparatus. When the coherer was shaken or tapped with a little hammer, it lost its conductivity and was ready to receive the next wave. Marconi had a little hammer tapping the coherer at very short intervals, so that it was always ready to receive any wave that might be sent out from the sending station as a signal.