HERTZIAN WAVE WIRELESS TELEGRAPHY. 



107 



B— 



Fig. 3. Simple Marconi 

 Radiator. B, battery ; /, 

 induction coil ; A", signal- 

 ling key; 6', spark gap ; A, 

 aerial wire; .B, earth plate. 



closed lines of strain in the ether which move off through space, form- 

 ing, when cyclically distributed, an electric wave. 



We may next proceed to apply these principles to the explanation 

 of the action of the simplest form of Hertzian wave telegraphic radia- 

 tor, viz., the Marconi aerial wire. In its original form this consists of 

 a long vertical insulated wire A, the lower end 

 of which is attached to one of the spark balls 

 S of an induction coil I, the other spark' ball 

 being connected to earth E, and the two spark 

 balls being placed a few millimeters apart (see 

 Fig. 3). When the coil is set in action, oscil- 

 latory or Hertzian sparks pass between the balls, 

 electric oscillations are set up in the wire and 

 electric waves are radiated from it. Deferring 

 for the moment a more detailed examination of 

 the operations of the coil and at the spark gap, 

 we may here say that the action which takes 

 place in the aerial wire is as follows: The wire 

 is first charged to a high potential, let us sup- 

 pose, with negative electricity. We may imagine this process to con- 

 sist in forcing additional electrons into it, the induction coil acting as 

 an electron pump. Up to a certain pressure the spark gap is a perfect 

 insulator, but at a critical pressure, which for spark gap lengths of four 

 or five millimeters and balls about one centimeter in diameter approxi- 

 mates to three thousand volts per millimeter, the insulation of the air 

 gives way, and the charge in the wire rushes into the earth. In con- 

 sequence, however, of the inertia of the medium or of the electrons, the 



charge, so to speak, overshoots the mark, and 

 the wire is then left with a charge of oppo- 

 site sign. This again in turn rebounds, and 

 so the wire is discharged by a series of elec- 

 trical oscillations, consisting of alternations 

 of* static charge and electric discharge. We 

 may fasten our attention either on the events 

 taking place in the vertical wire or in the 

 medium outside, but the two sets of phe- 

 nomena are inseparably connected and go on 

 together. When the aerial wire is statically 

 charged, we may describe it by saying that 

 there is an accumulation of electrons or co- 

 electrons in it. Outside the wire there is, 

 however, a distribution of electric strain, the strain lines proceeding 

 from the wire to the earth (see Fig. 4), 



. ' . 9 ' ' I 



I < 



I 



j__L 



[E 



Fig. 4. Lines of Electric 

 Strain (Dotted Links) ex- 

 tending BETWEEN A MaRCONI 

 Aerial A and the earth ee 



BEFORE DISCHARGE. 



