PROGRESS IN WIRELESS TELEGRAPHY. 277 



('H'cct of llu> Uliii of celluloid, which docs iiol |)ciicti'iitc into (he inte- 

 rior of the slits, is to prevent the tlissipatioii of the particles of sil- 

 ver in the slits, and whose motion under the influence of electric 

 oscillations prohahly accounts for the vai'iations in the resistance of 

 the circuit. Carbon tilings were also found to decohere automat- 

 ically; but these devices were not extensively used in practice, if 

 at ail. 



The next most important autocoherer was that due to Castelli, 

 known for a time as the Solari coherer, also as the Italian navy 

 coherer. This autocoherer was used by Marconi in his first e\])ei-i- 

 nients in trans-Atlantic wireless telegraphy. It consists of a tube 

 similar to the filings coherer, but instead of metal filings between the 

 ends of the iron or carbon rods within the tube, a drop of mercury is 

 emi)loyed. In the circuit with the coherer there is a small, battery 

 and a telephone receiver. On the arrival of electric oscillations the 

 mercurv appears to cohere to the carbon or iron, with the result that 

 the resistance decreases, but immediately rises when the oscillation 

 ceases, these variations in the resistance of the circuit setting u]) noises 

 in the telejihone, which can be read as dots and dashes when messages 

 are transmitted. This autodetector has the disadvantage that after 

 some use it becomes rather unreliable in operation, to prevent Avhich 

 frequent renewals of the mercury and cleansing of the tube are neces- 

 sary. 



Subsequently ISIarconi devised an autocoherer, known as a magnetic 

 detector, which has been used in his trans- Atlantic and other long-dis- 

 tance experiments. This coherer consists of a primar}^ and secondary 

 coil of wire (wound over a coil of fine iron wires), the inner coil 

 of which is connected to the vertical wire in a manner i^ractically 

 similar to that in wdiich the fllings coherer is connected. The outer 

 coil contains in its circuit a telephone receiver, but no battery. This 

 detector of electric waves is based on the observed fact that when 

 a magnet, such as the iron core, is caused to undergo slow changes 

 of magnetism, electric oscillations in the outer coil bring about rapid 

 changes in the magnetism of the core, Avhich, in turn, set up currents 

 HI the inner coil, and these are heard as clicks in the telephone 

 receiver. 



The slow changes of magnetization referred to are brought about as 

 follows : An endless rope or core of fine iron 

 wires is represented in figure 1 by c c; p p are 

 pulleys about an inch in diameter, operated by 

 Pio. i.-Miirconi magnetic clockwork. The iroii ropc passes over the pul- 

 detector. j^^.^.j. ^^^^ tlirough a small glass tube upon 



which the coils referred to, w w, are coiled. The iron core is magnet- 

 ized by two horseshoe magnets {not shown in figure), the ends of 



