PROGRESS IN WIRELESS TELEGRAPHY. 277 



effect of the film of celluloid, which does not })enetrate into the inte- 

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

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

 oscillations 2:)robably accounts for the variations in the resistance of 

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

 icall}^; but these devices were not extensively used in practice, if 

 at all. 



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 aiitocoherer was used by Marconi in his fii-st ex])eri- 

 ments in trans-Atlantic wireless telegraphy. Tt consists of a tube 

 similar to the filings coherer, l)ut instead of metal filings l)etween the 

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

 employed. In the circuit with the coherer there is a small l)attery 

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

 mercury apj^ears to cohere to the carbon or iron, with the result that 

 the resistance decreases, but innnediately rises when the oscillation 

 ceases, these variations in the resistance of the circuit setting up noises 

 in the telephone, 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 ojjeration, to prevent which 

 fre(|uent renewals of the mercury and cleansing of the tube are neces- 

 sary. 



Subsequently JNIarconi 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 })rimary and secondary 

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

 of which is connected to the vertical wire in a manner practically 

 similar to that in which the filings coherer is coiuiected. The outer 

 coil contains in its circuit a teleplione 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 abour rapid 

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

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

 receiver. 



The slow changes of magnetization referred to are In'ought about as 



follows: An endless rope or core of fine iron 



wires is j-epresented in figure 1 by c <■ ; p p are 



pidleys alxMit an inch in diameter, operated by 



Fig. 1.— Marconi magnetic clockwork. The iron ropc passes over the pul- 



detector. ]^,^,^ .^j^^l tiu-ong]^ .^ small glass tubc upon 



which the coils refei-red to, ir ic, are coiled. The iron core is magnet- 

 ized by two horseshoe magnets (not showMi in figure), (he ends of 



