252 



ANNUAL EEPOET SMITHSONIAN INSTITUTION, 1913. 



of the energy as it leaves the sending or transmitting antenna and 

 spreads out to fill a wider and wider space aroimd it. The higher 

 the sending antenna the greater the distance which can be reached 

 before the attenuation is too great for imparting signals. 



Let us consider for a moment by the aid of a figure the actions 

 which must occur in wireless transmission on the sending out of 

 energy from the transmitting antenna. Referring to figure 17, we 

 will represent by e — e the surface of the earth as if it were flat, and 

 for moderate distances this will be substantially the case. We will 

 erect on that surface a tall mast A of conducting wire or wires which, 

 at the top, shall have an extension to increase its capacity. This 

 might be a large ball of sheet metal. Usually, for construction to be 

 practicable, it is a set of wires — a sort of cage or a skeleton body. 

 Now, by any system, inductively, conductively, or otherwise, or by 

 what is known as close or loose inductive coupling or what not (figs. 18, 



19, 20) we cause elec- 

 Fl^.16. n^. 19. Fi^. 20. trie disturbances, such 



that at one instant the 

 top of the antenna be- 

 comes positive and at 

 the next instant nega- 

 tive, many thousands^ — 

 even hundreds of thou- 

 sands — of times per 

 second. In other words, 

 we impress a high-fre- 

 quency wave upon this 

 vertical mast. We will 

 try to present an in- 

 stantaneous picture or form an instantaneous image of what the 

 condition is at the beginning of the process. 



Let us suppose that the charge is positive at the top, and necessarily 

 the surface below and surrounding the mast will be negative. Elec- 

 trostatic lines will extend from the mast, and particularly from the 

 expansion at the top down to the earth's surface in all directions 

 around the antenna, as in the figure. The medium around the 

 antenna will be stressed electrostatically. This would be all, pro- 

 vided the charges were stationary, but the system we are considering 

 is dynamic. The plus charge is replaced by a minus charge at the 

 top, and a current of a high frequency runs up and down the antenna, 

 but so also does this current extend into the sea radially from the 

 foot of the antenna, replacing the negatively charged area by a posi- 

 tively charged zone, as it were, while the top of the antenna is now 

 negative where it was formerly positive. (Fig. 21 A (p. 251), one 

 side only shown, and fig. 21 B, in plan.) 



