440 POPULAR SCIENCE MONTHLY. 



As in all other cases of oscillatory motion, the principle of resonance 

 may here be brought into play to increase immensely the amplitude of 

 the current oscillations thereby set up in the receiving aerial. As 

 already explained, any vertical insulated wire placed with its lower end 

 near the earth has capacity with respect to the earth, and it has also 

 inductance, the value of these factors depending on its shape and 

 height. Accordingly, it has a natural electrical time-period of its own, 

 and if the periodic electromotive impulses which are set up in it by the 

 passage of the waves over it agree in period with its own natural 

 time-period, then the amplitude of the current vibrations in it may 

 become enormously greater than when there is a disagreement between 

 these two periods. Before concluding these articles we shall return 

 to this subject of electric resonance and syntony, and discuss it with 

 reference to what is called the tuning of Hertzian wave stations. 

 Meanwhile, it may be said that for the sake of obtaining, at any rate 

 in an approximate degree, this coincidence of time-period, it is gen- 

 erally usual to make the receiving aerial as far as possible identical 

 with the transmitting aerial. If the receiving aerial is not insulated, 

 but is connected to the earth at its lower end through the primary coil 

 of an oscillation transformer, we can still set up in it electrical oscilla- 

 tions by the impact on it of an electric wave of proper period; and if 

 the oscillation transformer is properly constructed we can draw from 

 its secondary circuit electric oscillations in a similar period. 



One problem in connection with the design of a receiving aerial is 

 that of increasing its effective length and capacity, so as to increase 

 correspondingly the electromotive force or current oscillations in it. 

 It is clear that if we put a number of receiving wires in parallel so that 

 each one of them is operated upon by the wave separately, although we 

 can increase in this way the magnitude of the alternating current 

 which can be drawn off from the aerial, we cannot increase the electro- 

 motive force in it except by increasing the actual height of the wires. 

 Unfortunately, there is a limit to the height of the receiving aerial. 

 It has to be suspended, like the transmitting aerial, from a mast or 

 tower, and the engineering problem of constructing such a permanent 

 supporting structure higher than, say, two hundred feet, is a diffi- 

 cult one. 



Since any one station has to send as well as receive, it is usual to 

 make one and the same aerial wire or wires do double duty. It is 

 switched over from the transmitting to the receiving apparatus, as 

 required. This, however, is a concession to convenience and cost. In 

 some respects it would be better to have two separate aerials at each 

 station, the one of the best form for sending, and the other of the 

 best form for receiving. 



In Mr. Marconi's early arrangements, the so-called coherer or 



