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POPULAR SCIENCE MONTHLY. 



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F I G. 12 N O N- 



EADiATivE Closed 

 Loop Aerial. 



A closed vertical loop A^Ao (see Fig. 12) is formed by erecting 

 two parallel insulated wires vertically a few feet apart and joining 

 them together at the top. At the bottom these wires are connected, 

 with the secondary terminals of an induction coil, a 

 condenser C or Ley den jar being bridged across the 

 terminals and a pair of spark balls 8 inserted in one 

 side of the loop. It will readily be seen that on 

 setting the coil in action, oscillations will take place 

 in these vertical wires, but that if the oscillations are 

 simply the fundamental note of the system, then at 

 any moment corresponding to a current going up one 

 side of the loop of wire, there must be a current 

 coming down the other. Accordingly, an arrangement 

 of this kind, forming what is called a closed circuit, 

 will not radiate or radiates but very feebly. Pro- 

 fessor Slaby found, however, that it might be con- 

 verted into a powerful radiator if we give the two sides of the loop un- 

 equal capacity or inductance, and at the same time earth one of the 

 lower ends of the loop, as shown in Fig. 13. By this means it is pos- 

 sible to set up in the loop electrical overtones or harmonics of the fun- 

 damental oscillation, and if we cause the system to vibrate so as to 

 produce its first odd harmonic, there is a potential node at the lower 

 end of both vertical sides of the loop, a potential node on both vertical 

 sides at two thirds of the way up, and a potential antinode at the 

 summit of the loop; then, under these circumstances, the closed loop 

 of wire is in the same electrical condition as if two simple Marconi 

 aerials, both emitting their first odd harmonic oscilla- 

 tion, were placed side by side and joined together at 

 the top. 



It is a little difficult without the employment of 

 mathematical analysis to explain precisely the manner 

 in which earthing one side of the loop or making the 

 loop unsymmetrical as regards inductance has the 

 effect of creating overtones in it. The following 

 rough illustration ma}^, however, be of some assist- 

 ance. Imagine a long spiral metallic spring sup- 

 ported horizontally by threads. Let this represent a 

 conductor, and let any movement to or fro of a part of 

 the spring represent a current in that conductor. Suppose we take hold 

 of the spring at one end, we can move it bodily to and fro as a whole. 

 In this case, every part of the spring is moving one way or the other in 

 the same manner at the same time. This corresponds with the case in 

 which the discharge of the condenser through the uniform loop con- 



FlG. 13. Slaby's 

 Loop Radiator. 



