June 29, 191 1] 



NATURE 



bo I 



INCLUDING, SP^R^ 



in the elevated conductor, and by the employment of the 

 right value of capacity or inductance required in the con- 

 denser circuit, the two circuits were brought into elec- 

 trical resonance, a condition which I first pointed out as 

 being essential in order to obtain efficient radiation and 

 good tuning. 



The receiver (as shown in Fig. 4) also consists of an 

 elevated conductor or aerial connected to earth or capacity 

 through an oscillating transformer. The latter also con- 

 tains the condenser and detector, the circuits being made 

 to have approximately the same electrical time period as 

 that of the transmitter circuits. 



At the long-distance station situated at Clifden in 

 Ireland, the arrangement which has given the best results 

 is based substantially upon my syntonic system of 1900, 

 to which have been added numerous improvements. 



An important innovation from a practical point of view 

 was the adoption at Clifden and Glace Bay of air con- 

 densers composed of insulated metallic plates suspended 

 in air at ordinary pressure. In this manner we greatly 

 reduce the loss of energy which would take 

 place in consequence of dielectric hysteresis 

 were a glass or solid dielectric employed. 

 A very considerable economy in working 

 also results from the absence of dielectric 

 breakages, for, sliould the potential be so 

 raised as even to produce a discharge from 

 plate to plate across the condenser, this does 

 not permanently affect the value of the 

 dielectric, as air is self-healing, and one of 

 the few commodities which can be replaced 

 at a minimum of cost. 



Various arrangements have been tried 

 and tested for obtaining continuous or very 

 prolonged trains of waves, but it has been 

 my experience that, when utilising the best 

 receivers at present available, it is neither 

 economical nor efficient to attempt to make 

 the waves too continuous. Much better 

 results are obtained when groups of waves 

 (Fig. 5) are emitted at regular intervals in 

 such a manner that their cumulative effect 

 produces a clear musical note in the 

 receiver, which is tuned not only to the 

 periodicity of the electric waves transmitted, 

 but also to their group frequency. 



In this manner the receiver may be 

 doubly tuned, with the result that a far 

 greater selectivity can be obtained than by 

 the employment of wave-tuning alone. 



In fact, it is quite easy to pick up simul- 

 taneously different messages transmitted on 

 the same wave-length, but syntonised to 

 different group frequencies. 



So far as wave tuning goes, very good 

 results — almost as good as are obtainable 

 by means of continuous oscillations — can be 

 achieved with groups of waves, the decre- 

 ment of which is in each group 0-03 or 

 '0-04, which means that about thirty or forty 

 useful oscillations are radiated before their 

 amplitude has become too small to affect 

 receiver. 



The condenser circuit at Clifden has a decrement of 

 from 0-015 to 0-03 for fairly long waves. 



This persistency of the o^c ill;uit)n> has been obtained by 

 the employment of the sy>tfni shown in Fig. 6, which I 

 first described in a patent tak^n out in September, 1007. 

 This method eliminates almost completely the spark gaj) 

 and its consequent resistance, which, as is well known, 

 is the principal cause of the damping or decay of the 

 waves in the usual transmitting circuit. 



Thr apparatus shown in Fig. Ti consists of a metal disc 

 a having copper studs firmly fi.Ked at regular intervals in 

 its J). riphriN and placed transversely to it- pl.ui.'. I lii< 

 disc i- caiisi'd to rotate very rapidly l).i\\'"ii, luo oih. i- 

 discs /) h\- means of a rapidlv r<'vnlvinL; .■b'ctric motor or 

 slcain luriiinc. '\'\m-.<- sidr cli-rs arr aKo made slowly to 

 turn roLind in a plane at right angles to that of the middl^^ 

 disc. The (onneitions are as illustrated in the tii;uie. 

 The studs an; of such length as just to touch the side 



discs in passing, and thereby bridge the gap between the 

 latter. 



With the frequency employed at Clifden, namely, 

 45,000, when a potential of 15,000 volts is used on the 

 condenser, the spark gap is practically closed during the 

 time in which one complete oscillation only is taking 

 place, when the peripherical speed of the disc is about 

 600 feet a second. The result is that the primary circuit 

 can continue oscillating without material loss by resist- 

 ance in the spark gap. Of course, the number of oscilla- 

 tions which can take place is governed by the breadth or 

 thickness of the side discs, the primary circuit being 

 abruptly opened as soon as the studs attached to the 

 middle disc leave the side discs. 



The sudden opening of the primary circuit tends to 

 quench immediately any oscillations which may still 

 persist in the condenser circuit ; and this fact carries with 

 it a further and not inconsiderable advantage, for, if the 

 coupling of the condenser circuit to the aerial is of suit- 

 able value, the energy of the primary will hav(? practicallv' 



RL5DNflNCL CURVL 

 Of CLIFDEN 

 PRIMARY CIRCUIT 



TOTM RESISTANCE 



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NO. 2174, VOL. 86] 



