Professor Hertz proved in 1888 that a spark, or disruptive discharge of electricity, caused electro-magnetic waves to radiate away in all directions through the ether. The waves acted exactly like ripples that radiate from a stone when it strikes the water. These Hertzian waves were found to travel with the same velocity as light, and would circle the world eight times in a second. As soon as the existence of these waves was known many scientists began to consider whether they could not be used for telegraphy. But the problem was a very difficult one. The questions were how to transmit the energy to a distance, and how to make a receiver that should be sensitive enough to be affected by it. Let us picture a body of still water with a twig floating upon its surface. If a stone is thrown into the water ripples radiate in all directions, these waves becoming weaker as the circles they form become larger, or in other words as they grow more distant from the point where the stone struck the water. When the waves reach the floating twig they will move it, and when they cease the twig will be motionless again. Should there be grasses or rocks protruding up from the water the motion given to the twig by the waves would be lessened, or distorted, or changed in many ways, depending on the intervening object. Whether the waves will actually impart motion to the twig will depend on the force by which these waves were started and upon the lightness of the twig, or its sensitiveness to the ripples as they radiate. If the water were disturbed by some other force than the stone the twig would be moved by that other force, and the observer could not tell from what direction the motion had come, or how it had been caused. Applying this to wireless telegraphy one may say that a device must be used that will send out waves of a certain length, and that the receiver must be constructed so that it will respond only to waves of the length sent by that transmitter. There must therefore be accurate tuning of the two instruments. Let a weight be fastened at the end of a spiral spring and then be struck. The weight will oscillate at a uniform rate, or so many times a minute. If this be held so that it strikes the water the movement of the spring will create a certain number of waves a minute. If now a second weight, attached to a second spring, be hung down into the water, the waves caused by the first will reach the second, and if the springs be alike the movements or oscillations will correspond. But if the springs were not alike, or if, in other words, the two instruments were not in tune, the wave motions would not be received and copied accurately. Therefore in wireless telegraphy the instrument that is to impart the motion to the electro-magnetic waves that fill the ether must be tuned in accord with the instrument that is to receive the motion of those waves.