66 POPULAR SCIENCE MONTHLY. 



The action of the telephone transmitter, whieli also consists of 

 minute conducting particles in which a battery terminals are im- 

 mersed, and the analogous coherer is microscopic, and there are 

 many theories to account for their changes of resistance to electri- 

 cal currents. We can not, I believe, be far wrong in thinking 

 that the electric force breaks down the insulating effect of the 

 infinitely thin layers of air between the particles, and thus allows 

 an electric current to flow. This action is doubtless of the nature 

 of an electric spark. An electric spark, in the case of wireless 

 telegraphy, produces magnetic and electric lines of force in space, 

 these reach out and embrace the circuit containing the coherer, 

 and produce in turn minute sparks. Similia sitniUhus — one action 

 perfectly corresponds to the other. 



The Marconi system, therefore, of wdiat is called wireless te- 

 legraphy is not new in principle, but only new in practical appli- 

 cation. It had been used to show the phenomena of electric waves 

 in lecture rooms. Marconi extended it from distances of sixty to 

 one hundred feet to fifty or sixty miles. lie did this by lifting the 

 sending-wire spark on a lofty pole and improving the sensitiveness 

 of the metallic filings in the glass tube at the receiving station. 



Fia. 6. — The coherer employed to receive the electric waves. (One aud a third actual size.) 



He adopted a mechanical arrangement for continually tapping the 

 coherer in order to break up the minute bridges formed by the 

 cohering action, and thus to prepare the filings for the next mag- 

 netic i^ulse. The system of wireless telegraphy is emphatically a 

 spark system strangely analogous to flash-light signaling, a system 

 in which the human eye with its rods and cones in the retina acts 

 as the coherer, and the nerve system, the local battery, making 

 a signal or sensation in the brain. 



Let us examine the sending spark a little further. An elec- 

 tric spark is perhaps the most interesting ])hcnomenon in elec-- 

 tricity. What causes it — how does the air behave toward it — 

 what is it that apparently flows through the air, sending out light 

 and heat waves as well as magnetic and electric waves? If we 

 could answer all these questions, we sh<nil<l know what elec- 

 tricity is. A critical study of the electric spark has not only its 

 scientific but its practical side. We see the latter side evidenced 

 by its employment in wireless telegraphy ;ni(l in the X rays; for 

 in the latter case we have an electric discharge in a tube from 

 which the air is removed — a special case of an electric spark. In 



