HERTZIAN WAVE WIRELESS TELEGRAPHY. 367 



turns, and hence the magnetic field due to the primary circuit remains 

 the same. Where it is desired to put a number of turns upon a coil, 

 and yet at the same time keep the inductance down, the writer has 

 adopted the device of winding a silk or hemp rope well paraffined 

 between the turns of the circuit, so as to keep them further apart from 

 one another, and as the inductance depends on the turns per centimeter, 

 this has the effect of reducing the inductance. 



The next and most important element in any transmitting station 

 is the aerial or radiator, and it was the introduction of this element by 

 Mr. Marconi which laid the foundation for Hertzian wave telegraphy 

 as opposed to mere experiments with the Hertzian waves. We may 

 consider the different varieties of aerial which have been evolved from 

 the fundamental idea. The simple single Marconi aerial consists of a 

 bare or insulated wire, generally about 100 or 150 feet in length, sus- 

 pended from a sprit attached to a tall mast. As these masts have gen- 

 erally to be erected in exposed positions, considerable care has to be 

 taken in erecting them with a large margin of strength. To the end 

 of a sprit is attached an insulator of some kind which may be a simple 

 ebonite rod, or sometimes a more elaborate arrangement of oil insu- 

 lators, and to the lower end of this insulator is attached the aerial 

 wire. As at the top of the aerial we have to deal with potentials 

 capable sometimes of giving sparks several feet in length, the insulation 

 of the upper end of the aerial is an important matter. 



In the original Marconi system, the lower end of the aerial was 

 simply attached to one spark ball connected to one terminal of the 

 induction coil, and the other terminal and spark ball were connected 

 to the earth. In this arrangement, the aerial acted not only as radia- 

 tor, but as energy-storing capacity, and as already explained, its radi- 

 ating power was on that account limited. The earth connection is 

 an important matter. For long distance work, a good earth is essen- 

 tial. This earth must be made by embedding a metal plate in the 

 soil, and many persons are under the impression that the efficiency 

 of the earth plate depends upon its area, but this is not the fact. It 

 depends much more upon its shape, and principally upon the amount 

 of its 'edge.' It has been shown by Professor A. Tanakadate, of 

 Japan, that if a metal plate of negligible resistance is embedded in 

 an infinite medium having a resistivity r, the electrical conductance 

 of this plate is equal to 4;rA times the electrostatic capacity of the 

 same plate placed in a dielectric of infinite extent. Hence in design- 

 ing an earth plate, we have to consider not how to give it the utmost 

 amount of surface, but how to give it the greatest electrostatic capacity, 

 and for this purpose it is far better to divide a given amount of 

 metal into long strips radiating out in different directions, rather 

 than to employ it in the form of one big square or circular plate. 



