HERTZIAN WAVE WIRELESS TELEGRAPHY. 119 



ductor is a flow of electricity, all in one direction one way or the other. 

 The current is in the same direction in all parts of the loop at the same 

 time, and, therefore, if the current is going up one side of the loop it is 

 at the same time coming down the other side. Hence the two sides of 

 the loop are always in exact opposition as regards the effect of the 

 current in them on the external space, and the loop does not radiate. 

 Eeturning again to the case of the spring. Supposing that we add a 

 weight to one end of the spring by attaching to it a metal ball, and 

 then move the other end to and fro with certain periodic motion, it 

 will be found quite easy to set up in the spring a pulsatory motion 

 resembling the movement of the air in an open organ-pipe. Under 

 these circumstances both ends of the spring will be moving inwards 

 or outwards at the same time, and the central portions of the spring, 

 although being pressed and expanded slightly, are moving to and fro 

 very little. This corresponds in the case of the looped aerial with a 

 current flowing up or down both sides at the same time ; in other words, 

 when this mode of electrical oscillation is established in the loop, its 

 electrical condition is just that of two simple Marconi aerials joined 

 together at the top and vibrating in their fundamental manner. Ac- 

 cordingly, if one side of the double loop is earthed, we then have an 

 arrangement which radiates waves. Professor Slaby found that by 

 giving one side of the loop less inductance than the other, and at the 

 same time earthing the side having greater inductance at the bottom, 

 he was able to make an arrangement which radiated, not in virtue of 

 the normal oscillations of the condenser, but in virtue of the harmonic 

 oscillations set up in the conductor itself. The mathematical theory 

 of this radiator has been very fully developed by Dr. Georg Seibt. 



It will be seen, therefore, that there are several ways in which we 

 may start into existence oscillations in an aerial. First, the aerial may 

 be insulated, and we may charge it to a high potential and allow this 

 charge suddenly to rush out. Although this process gives rise to a 

 disturbance in the ether, as already explained, it is analogous to a pop 

 or explosion in the air, rather than to a sustained musical note. The 

 exact acoustic analogue would be obtained if we imagine a long pipe 

 pumped full of air and then suddenly opened at one end. The air 

 would rush out, and, communicating a blow to the outer air, would 

 create an atmospheric disturbance appreciated as a noise or small ex- 

 plosion. This is what happens when we cut the string and let the cork 

 fly out from a bottle of champagne. At the same time, the inertia 

 of the air rushing out of the tube would cause it to overshoot the mark, 

 and a short time after opening the valve the tube, so far from contain- 

 ing compressed air, would contain air slightly rarefied near its mouth, 

 and this rarefication would travel back up the tube in the form of 



