WIRELESS TELEGRAPHY. 



63 



of a remarkable phenomenon which has made Marconi's system 

 of wireless telegraphy possible. In order that an electrical current 

 may flow through a mass of particles of a metal, a mass, for in- 

 stance, of iron filings, it is necessary either to compress them or to 

 cause a minute spark or electrical discharge between the particles. 

 ]^ow, it is supposed that the magnetic whirls, in embracing the dis- 

 tant receiving circuit, cause these minute sparks, and thus enable 

 the electric current from the battery B to work a telegraphic 

 pounder or boll ]\f. The metallic filings are inclosed in a fflass tube 



Fio. 3. — I'hotograph ot the electric lines which einauale frcnu the eud of the wire at the send- 

 ing station, and which are probably reproduced among the metallic filings of the coherer 

 at the receiving station. 



between wires which lead to the battery, and the arrangement is 

 called a coherer. It can be made small and light. Fig. 6 is a 

 representation in full size of one that has been found to be very 

 sensitive. It consists of two silver wires with a few iron filings 

 contained in a glass tube between the ends of the wires. It 

 is necessary that this little tube should be constantly shaken up 

 .in order that after the electrical circuit is made the iron filings 

 should return to their non-conducting condition, or should cease 

 to cohere together, and should thus be ready to respond to the 

 following signal. My colleague. Professor Sabine, has employed 

 a very small electric motor to cause the glass tube to revolve, and 

 thus to keep the filings in motion Avliile signals are being received. 

 Fig. 7 shoW'S the arrangement of the receiving apparatus. 



The coherer and the motor are shown betw^een two batteries, 

 one of which drives the motor while the other serves to work the 



