HERTZIAN WAVE WIRELESS TELEGRAPHY. m 



lating any complete theory of the aerial. The first is the operation 

 taking place in the vertical wire, which is described by saying that 

 electrical oscillations or vibratory movements of electrons are taking 

 place in it, and, on our adopted theory, it may be said to consist in a 

 longitudinal vibration of electrons of such a nature that we may 

 appropriately call the aerial an ether organ pipe. Then in the next 

 place, we have the distribution and movement of the lines of electric 

 strain and magnetic flux in the space outside the wire, constituting the 

 electric wave; and lastly, there are the electrical changes in the con- 

 ductor over which the wave travels, which is the earth or water sur- 

 rounding the aerial. In subsequently dealing with the details of trans- 

 mitting arrangements, attention will be directed to the necessity for 

 what telegraphists call 'a good earth' in connection with Hertzian 

 wave telegraphy. This only means that there must be a perfectly free 

 egress and ingress for the electrons leaving or entering the aerial, so 

 that nothing hinders their access to the conducting surface over which 

 the wave travels. There must be nothing to stop or throttle the rush of 

 electrons into or out of the aerial wire, or else the lines of strain can 

 not be detached and travel away. 



We may next consider more particularly the energy which is avail- 

 able for radiation and which is radiated. In the original form of 

 simple Marconi aerial, the aerial itself when insulated forms one coat- 

 ing or surface of a condenser, the dielectric being the air and ether 

 around it, and the other conductor being the earth. The electric energy 

 stored up in it just before discharge takes place is numerically equal 

 to the product of the capacity of the aerial and half the square of the 

 potential to which it is charged. 



If we call C the capacity of the aerial in microfarads, and V the 

 potential in volts to which it is raised before discharge, then the energy 

 storage in joules E is given by the equation. 



2.106 



Since one joule is nearly equal to three quarters of a foot-pound, the 

 energy storage in foot-pounds F is roughly given by the rule 

 2^ = 1 CV^/10^. For spark lengths of the order of five to fifteen 

 millimeters, the disruptive voltage in air of ordinary pressure is at the 

 rate of 3,000 volts per millimeter. Hence if S stands for the spark 

 length in millimeters, and C for the aerial capacity in microfarads, it 

 is easy to see that the energy storage in foot-pound is 



F^= — 7^ — 



