Page:Popular Science Monthly Volume 63.djvu/372

368 The importance of the 'good earth' will have been seen from our discussion on the mode of formation of electric waves. There must be a perfectly free access for the electrons to pass into and out of the aerial. Hence if the soil is dry, or badly conductive in the neighborhood, we have to go down to a level at which we get a good moist earth. In fact, the precautions which have to be taken in making a good earth for Hertzian wave telegraphy are exactly those which should be taken in making a good earth for a lightning conductor.

Whilst on the subject of aerials, a word may be said on the localization of wireless telegraph stations on the Marconi system. For reasons which were explained previously, the transmission of signals is effected more easily over water than over dry land, and it is hindered if the soil in the neighborhood of the sending station is a poor conductor. Hence all active Hertzian wave telegraph stations, like all active volcanoes, are generally found near the sea. In those cases in which a multiple aerial has to be put up consisting of many wires, one mast may be insufficient to support the structure, and several masts arranged in the form of a square or a circle have to be employed. The illustrated papers have reproduced numerous pictures of the Marconi power stations at Poldhu in Cornwall, Glace Bay in Nova Scotia, and Cape Cod in the United States. In these stations, after preliminary failures to obtain the necessary structural strength with ordinary masts, tall lattice girder wooden towers have been built, about 215 feet in height, well stayed against wind pressure, and which so far have proved themselves capable of withstanding any storm of wind which has come against them.

An important question in connection with the sending power of an aerial is that of the relation of its height to the distance covered. Some time ago Mr. Marconi enunciated a law as the result of his experiments, connecting these two quantities, which may be called Marconi's Law. He stated that the height of the aerial to cover a given distance, other things remaining the same, varies as the square root of the distance. Let D be the distance and let L be the length of the aerial, then if both the transmitting and receiving aerial are the same height, we may say that D varies as L2. This relation may be theoretically deduced as follows: Any given receiving apparatus for Hertzian wave telegraphy requires a certain minimum energy to be imparted to it to make it yield a signal. If the resistance and the capacity of the receiver is taken as constant, this minimum working energy is proportional to the square of the electromotive force set up in the receiving aerial by the impact on it of the electric waves. This electromotive force varies as the length of the receiving aerial, and as the magnetic force due to the wave cutting across it, and the magnetic force varies as the current in the transmitting aerial, and