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Page:Popular Science Monthly Volume 56.djvu/72

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bell or sounder when the electric wire excites the iron filings. In Fig. 2 this receiving apparatus is shown diagrammatically. B is the battery which sends a current through the sounder M and the

PSM V56 D0072 Magnetic whirls around the sending wire.png
Fig. 4.—Magnetic whirls about sending wire.

coherer N when the magnetic whirls coming from the sending wire W embrace the receiving wire W’.

The term wireless telegraphy is a misnomer, for without wires the method would not be possible. The phenomenon is merely an enlargement of one that we are fully conscious of in the case of telegraph and telephone circuits, which is termed electro-magnetic induction. Whenever an electric current suddenly flows or suddenly ceases to flow along a wire, electrical currents are caused by induction in neighboring wires. The receiver employed by Marconi is a delicate spark caused by this induction, which forms a bridge so that an electric current from the relay battery can pass and influence magnetic instruments.

Many investigators had succeeded before Marconi in sending telegraphic messages several miles through the air or ether between two points not directly connected by wires. Marconi has extended the distance by employing a much higher electro-motive force at the sending station and using the feeble inductive effect at a distance to set in action a local battery.