All the commercial wireless systems of to-day depend upon the production and transmission of electric waves. About the year 1837 it was discovered that the discharge of a Leyden jar did not consist of only one sudden rush of electricity, but of a series of electric oscillations, which surged backwards and forwards until electric equilibrium was restored. This discovery was verified by later experimenters, and it forms the foundation of our knowledge of electric waves. At this point many readers probably will ask, “What are electric waves?” It is impossible to answer this question fully, for we still have a great deal to learn about these waves, and we only can state the conclusions at which our greatest scientists have arrived after much thought and many experiments. It is believed that all space is filled with a medium to which the name “ether” has been given, and that this ether extends throughout the matter. We do not know what the ether is, but the important fact is that it can receive and transmit vibrations in the form of ether waves. There are different kinds of ether waves, and they produce entirely different effects. Some of them produce the effect which we call light, and these are called “light waves.” Others produce the effect known as heat, and they are called “heat waves”; and still others produce electricity, and these we call “electric waves.” These waves travel through the ether at the enormous speed of 186,000 miles per second, so that they would cross the Atlantic Ocean in about 1/80 second. The fact that light also travels at this speed suggested that there might be some connexion between the two sets of waves, and after much experiment it has been demonstrated that the waves of light and electricity are identical except in their length. Later on in this chapter we shall have occasion to refer frequently to wave-length, and we may take this opportunity of explaining what is understood by this term. Wave-length is the distance measured from the crest of one wave to the crest of the next, across the intervening trough or hollow. From this it will be seen that the greater the wave-length, the farther apart are the waves; and also that if we have two sets of waves of different wave-lengths but travelling at the same speed, then the number of waves arriving at any point in one second will be greater in the case of the shorter waves, because these are closer together.