1897. 



071 Signalling through Space without Win 



473 



the cure more troublesome tlian the disease, and its use had to be 

 abandoned. The same action is very common in granulated carbon 

 microphones like Huuning's, and shaking has to be resorted to to 

 decohere the carbon particles to their normal state. Mens. E. Branly 

 (1890) showed the effect with copper, aluminium and iron filings. 

 Jr'rof. Oliver Lodge, who has done more than anyone else in England 

 to illustrate and popularise 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 



6 



^ 



STEEPHOLM 



<» Marconi Experiments 

 •"— Eiectro-Magnetio Induction CxporlmontS 



eREAN OOWMn^ 



Fig. 3. — Map of locality where the experiments were carried out. 



doing so makes such a sound that reading Morse characters is easy. 

 The same current 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 transmitter by varying the capacity of 

 the apparatus.* Choking coils prevent the energy escaping. The 

 analogy to Prof. Silvanus Thompson's wave apparatus is evident. 

 Oscillations set up in the transmitter fall upon the receiver tuned in 



• The period of vibration of a circuit is given by the equation T = 2 ir VK L, 

 80 that we have simply to vary either the capacity K or the so-called " self- 

 induction " L to tune the receiver to any frequency. It is simpler to vary K. 



