232 



KNOWLEDGE. 



[October 1, 1900. 



WIRELESS TELEGRAPHY. -IV. 



By G. W. DE TUNZELMANX, B.SC. 



ELECTRIC WAVES. 

 As far back as 1842, the American professor, Joseph 

 Heni-y, pointed out that the phenomena accompany- 

 ing the discharge of a Leyden jar suggested that it was 

 of an oscillatory character; and Helmholtz, in 1847, 

 in his celebrated essay on the Conservation of Energy, 

 made the same assumption, and pointed out that the 

 oscillations would become continually smaller until the 

 entire energy was dissipated by the opposing resistances. 

 The time of a complete oscillation, as mentioned in my 

 first art.icle, is 



T = 2 ir \^L S, 

 where L and S are the self induction and ca,pacity of 

 the circuit respectively. 



The meaning of these electrical constants will be 

 more clearly understood by a comparison of the elec- 

 trical oscillations with mechanical oscillations of a 

 simple character such as those of a straight spring fixed 

 at one end and having a weight attached to the other. 



The flexibilitv of the spring is the analogue of the 

 capacity, and the inertia of the loaded spring that of 

 self induction. An increase in either of them will 

 diminish the rate of oscillation. In the electincal case 

 the capacity of the circuit may be increased by making 

 the jar larger, and as the self induction is due to the 

 magnetisation of the medium surrounding the current 

 it may be augmented by increasing the length of the 

 circuit. Owing to the fact that there is very little 

 magnetising effect, except close to the conductor, the 

 area included in the circuit makes very little difference, 

 so that the circuit may be wound into a coil, making 

 the arrangement more compact. If the oscillations 

 were slower the self induction might be still further 

 increased by filling the space inside the coil with iron, 

 but with these extremely rapid oscillations the iron 

 is protected from magnetisation by the currents, opposed 

 to those in the coil, which ai'e induced by the latt«r 

 in the outer skin of the iron, and the result is that 

 the introduction of iron does not increase the self in- 

 duction but actually diminishes it. 



\Vhen the spring is set in motion the vibrations 

 rapidly die away. This damping action is caused by 

 the friction of the difiEerent portions of the spring, the 

 energy of vibration being thereby dissipated into mole- 

 cular \abrations or heat. It may be increased still 

 further by immei-sion in a viscous medium, and if 

 suificiently viscous the motion may become dead-heat, 

 that is to say, simply a single excursion and return 

 to the position of equilibrium. Another cause of 

 damping is the transference of energy to the medium 

 by the production of waves in it, and if the spring is 

 so shaped as to increase this effect the damping will 

 also be increased. The electric oscillations which occur 

 when a Leyden jai' is discharged are damped in a very 

 simDar manner, the resistance of the circuit con-espond- 

 ing to friction, in the case of the spring, but in order 

 to destroy its oscillpvtory chai-acter, except in the case 

 of very large condensers, such as are used in submarine 

 telegraphy, it is necessary to include in the circuit some 

 very bad conductor, such as a wet string or a block of 

 wood. The rapid damping of the oscillations of a 

 Leyden jar discharge when the circuit is so designed 

 as to be an efficient exciter of electric waves follows 

 necessarily from the principle of the conservation of 

 energy, just as in the case of the spring. 



The Hertz oscillator, or exciter of electric waves, is 

 simply a Leyden jar of such design as to facilitate the 

 transference of the energy of the electric oscUlations 

 of its discharge to the surrounding ether, and therefore 

 a compai'atively large amount of energy is required to 

 maintain it in action. Several years before Hertz's 

 experiments were made. Professor Fitzgerald, of Dublin, 

 had suggested, from theoretical considerations, that it 

 should be possible to excite such electric waves in the 

 ether by means of the discharge of Leyden jars of suitable 

 design, and about the same time that Hertz began these 

 investigations Professor Oliver Lodge was, in connection 

 with the theoi-y of the lightning conductor, making a 

 series of experiments on tlie discharge of small con- 

 densers, which led him on to the observation of ether 

 waves within the wires, and not waves transmitted by 

 the material of the wires themselves. 



As Hertz himself suggests. Professor Lodge would in 

 all probability have succeeded in discovering the ether 

 waves in air had he not anticipated him. 



Hertz tells us that in 1886 he was experimenting 

 with a pair of what he calls Riess or Knochenhauer 

 spirals, but which should be more properlv called Henry 

 spirals, spirals of silk-covered copper tape first used by 

 Professor Joseph Henry about 1838 in his researches 

 on mutual and self induction. Hertz noticed that in 

 order to obtain sparks in one of these spirals the large 

 batteries which had hitherto been employed might be 

 replaced by even a small Leyden jar, provided — and this 

 was the important point — that the discharge was made 

 to spring across a spark-gap. This observation led to 

 the splendid series of researches which experimentally 

 demonstrated the truth of Maxwell's theory of electro- 

 magnetic waves, and laid the foundation for the method 

 of telegraphy which Signer Mai-coni and others have 

 so successfully developed into a practical system. 



It is well known to musicians as well as to students 

 of acoustics that when a certain musical note is sounded, 

 a string or pipe which would give out this note will 

 respond to it, and in a similar manner an electric con- 

 ductor may be adjusted or tuned to respond to the 



S'-der for Tunifif. 



Fic. 1. — Lodge's Experiments witli Syntonic Leyden Jars. 



¥ro>a Lodge*s " SitjnalUug throuKb Space without Wires." 



oscillations set up by the discharge of a Leyden jar. 

 This is well shown in an experiment maJe by Professor 

 Lodge after reading Hertz's papers. He took a pair of 

 Leyden jars (Fig. 1) with circuits about a yard .n 



