120 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 



An important innovation from a practical point of view was the 

 adoption at Clifden and Glace Bay of air condensers, 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, should the 

 potential be so raised as to even 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 expe- 

 rience that, when utilizing the best re- 

 ceivers 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 manner that 

 their cumulative effect produces a 

 clear musical note in the receiver, 

 which is tuned not only to the period- 

 icity 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 tun- 

 ing alone. 



In fact, it is quite easy to pick up simultaneously different messages 

 transmitted on the same wave length, but syntonized to different 

 group frequencies. 



As 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 decrement of which is in each group 0.03 

 or 0.04, which means that about 30 or 40 useful oscillations are 

 radiated before their amplitude has become too small to perceptibly 

 affect the receiver. 



The condenser circuit at Clifden has a decrement of from 0.015 

 to 0.03 for fairly long waves. 



This persistency of the oscillations has been obtained by the 

 employment of the system shown in figure 6, which I first described 

 in a patent taken out in September, 1907. This method eliminates 



Fig. 4. 



