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Page:Popular Science Monthly Volume 56.djvu/74

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The action of the telephone transmitter, which also consists of minute conducting particles in which a battery terminals are immersed, and the analogous coherer is microscopic, and there are many theories to account for their changes of resistance to electrical 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 similibus—one action perfectly corresponds to the other.

The Marconi system, therefore, of what is called wireless telegraphy is not new in principle, but only new in practical application. 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.

PSM V56 D0074 Coherer employed to receive the electrical waves.png
Fig. 6.—The coherer employed to receive the electric waves. (One and 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 magnetic pulse. 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 electric spark is perhaps the most interesting phenomenon in electricity. 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 should know what electricity 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 and 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