io8 POPULAR SCIENCE MONTHLY. 



The wire has capacity with respect to the earth, and it acts like 

 the inner coating of a Ley den jar, of which the dielectric is the air 

 and ether around it, and the outer coating is the earth's surface. 

 Wlien the discharge takes place, we may consider that electrons rush 

 out of the wire and then rush back again into it. At the moment 

 when the electrons rush out of or into the aerial wire, we say there 

 ie an electric current flowing into or out of the wire, and this electron 

 movement, therefore, creates the magnetic flux which is distributed in 

 concentric circles round the wire. This current, and, therefore, motion 

 of electrons, can be proved to exist by its heating eSect upon a fine 

 wire inserted in series with the aerial, and in the case of large aerials 

 it may have a mean value of many amperes and a maximum value 

 of hundreds of amperes. Inside the aerial wire we have, therefore, 

 alternations of electric potential or charge and electric current, or 

 we may call it electron-pressure and electron-movement. 



There is, therefore, an oscillation of electrons in the aerial wire, 

 just as in the case of an organ pipe there is an oscillation of air 

 molecules in the pipe. Outside the aerial we have variations and dis- 

 tributions of electric strain and magnetic flux. The 

 "" i" resemblance between the closed organ-pipe and the 



../ simple Marconi aerial is, in fact, very complete. In 



■;' the case of the closed organ-pipe, we have a longitudinal 



•/ oscillation of air molecules in the pipe. At the open 



end or mouthpiece, where we have air moving in and 



out, the air movement is alternating and considerable, 



but there is little or no variation of air pressure. At 



7jj the upper or closed end of the pipe we have great vari- 



"^[p ation of air pressure, but little or no air movement 



FIG. 5. AMPLi- (see Fig. 5). 



TUDE OF pres- Compare this now with the electrical phenomena of 



IN A Closed Or- the aerial. At the spark ball or lower end we have 



GAN Pipe, indi- little or no variation of potential or electron pressure, 



CATED BY THE , . . , „ , 



ordinates op but we nave electrons rushing into and out of the aerial 

 THE Dotted Line ^^t each half oscillation, forming the electric discharge 

 or current. At the upper or insulated end we have 

 little or no current, but great variations of potential or electron pres- 

 sure. Supposing we could examine the wire inch by inch, all the way 

 up from the spark balls at the bottom to the top, we should find at each 

 stage of our journey that the range of variation and maximum value 

 of the current in the wire became less and those of the potential became 

 greater. At the bottom we have nearly zero potential or no electric 

 pressure, but large current, and at the top end, no current, but great 

 variation of potential. 



We can represent the amplitude of the current and potential values 



