We have to consider in the next place the arrangements of the receiving station and the various forms of receivers that have been devised for effecting telegraphy by Hertzian waves. Just as the transmitting station consists essentially of two parts, viz., a part for creating electrical oscillations and a part for throwing out or radiating electric waves, so the receiving-station appliances may be divided into two portions; the function of one being to catch up a portion of the energy of the passing wave, and that of the other to make a record or intelligible signal in some manner in the form of an audible or visible sign. Accordingly, there must be at the receiving station an arrangement called a receiving aerial, which in general takes the form of a long vertical wire or wires, similar in form to the transmitting aerial, There is, however, a distinct difference in the function of the transmitting aerial and the receiving aerial. The function of the first is effective radiation, and for this purpose the aerial must have associated with it a store of energy to be released as wave energy; but the function of the receiving aerial is to be the seat of an electromotive force which is created by the electric force and the magnetic force of the incident electric wave. In tracing out the mode of operation of the transmitting aerial, it was pointed out that the electric waves emitted consisted of alternations of electric force in a direction which is perpendicular to the surface of the earth, and magnetic force parallel to the surface of the earth. These two quantities, the electric force and the magnetic force, are called the _wave vectors_, and they both lie in a plane perpendicular to the direction in which the wave is travelling and at right angles to one another, the electric force being perpendicular to the surface of the earth. In optical language, the wave sent out by the aerial would be called a plane polarised wave, the plane of polarisation being parallel to the magnetic force. Hence, if at any point in the path of the wave we erect a vertical conductor, as the wave passes over it, it is cut transversely by the magnetic force of the wave and longitudinally by the electric force. Both of these operations result in the creation of an alternating electromotive force in the receiving aerial wire.