SIGNALING THROUGH SPACE WITHOUT WIRES. 



255 



ularize the work of Hertz and his followers, has given the name. 

 " coherer" to this form of apparatus. Marconi "decoheres" by making 

 the local current very rapidly vibrate a small hammer head against 

 the glass tube, which it does effectually, and in doing so makes such a 

 sound that reading Morse characters is easy. The same curreut that 

 decoheres can also record Morse signals on paper by ink. The exhausted 

 tube has two wings, which, by their size, tune the receiver to the trans- 

 mitter by varying the capacity of the apparatus. ' Choking coils pre- 

 vent the energy escaping. The analogy to Prof. Silvanus Thompson's 

 wave apparatus is evident. Oscillations set up in the transmitter fall 



ft Marconi Experiments 



— .Eleotro.Ma&'notlo induotlon Experiments 



BREAN DOWrKs 



Fig. ;J. — Map of locality where the experiments were carried out. 



upon the receiver tuned in sympathy with it, coherence follows, cur- 

 rents are excited, and signals made. 



In open clear spaces within sight of each other nothing more is 

 wanted, but when obstacles intervene and great distances are in ques- 

 tion, height is needed — tall masts, kites, and balloons have been used. 

 Excellent signals have been transmitted between Penarth and Brean 

 Down, near Weston-super-Mare, across the Bristol Channel, a distance 

 of nearly 9 miles (fig. 3). (The system was here shown in operation.) 



Mirrors also assist and intensify the effects. They were used in the 

 earlier experiments, but they have been laid aside for the present, for 

 they are not only expensive to make, but they occupy much time in 

 manufacture. 



1 The period of vibration of a circuit is given by the equation T=27r t/KL, so that 

 we have simply to vary either the capacity K or the so-called " self induction" L to 

 tune the receiver to auy frequency. It is simpler to vary K. 



