368 POPULAR SCIENCE MONTHLY. 



The importance of the 'good earth' will have been seen from our dis- 

 cussion 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 neighbor- 

 hood, 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 locali- 

 zation of wireless telegraph stations on the Marconi system. For rea- 

 sons 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 con- 

 ductor. 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 Mar- 

 coni power stations at Poldhu in Cornwall, Glace Bay in Nova Scotia, 

 and Cape Cod in the United States. In these stations, after prelim- 

 inary 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 L^. 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 work- 

 ing 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 



