114 POPULAR SCIENCE MONTHLY. 



by Mr. Marconi and others in the construction of such multiple wire 

 aerials. If, for instance, we put four insulated stranded 7/32 wires, 

 each 100 feet long, about six feet apart, all being held in a vertical 

 position, the capacity of the four together is not much more than twice 

 that of a single wire. In the same manner, if we arrange 150 similar 

 wires, each 100 feet long, in the form of a conical aerial, the wires 

 being distributed at the top round a circle 100 feet in diameter, the 

 whole group will not have much more than twelve times the capacity 

 of one single wire, although it weighs 150 times as much. 



The author has designed an aerial in which the wires, all of equal 

 length, are arranged sufficiently far apart not to reduce each other's 

 capacity. 



As a rough guide in practice, it may be borne in mind that a wire 

 about one 'tenth of an inch in diameter and one hundred feet long, 

 held vertical and insulated, with its bottom end about six feet from 

 the ground, has a capacity of 0.0003 of a microfarad, if no other 

 earthed vertical conductors are very near it. The moral of all this is 

 that the amount of electric energy which can be stored up in a simple 

 Marconi aerial is very limited, and is not much more than one tenth 

 of a joule or one fourteenth of a foot-pound, per hundred feet of 

 7/23 wire. The astonishing thing is that with so little storage of 

 energy it should be possible to transmit intelligence to a distance of 

 a hundred miles without connecting wires. 



One consequence, however, of the small amount of energy which 

 can be accumulated in a simple Marconi aerial is that this energy is 

 almost entirely radiated in one oscillation or wave. Hence, strictly 

 speaking, a simple aerial of this type does not create a train of waves- 

 in the ether, but probably at most a single impulse or two. 



We shall later on consider some consequences which follow from 

 this fact. Meanwhile, it may be explained that there are methods by 

 which not only a much larger amount of energy can be accumulated in 

 connection with an aerial, but more sustained oscillations created than 

 by the original Marconi method. One of these methods originated 

 with Professor Braun, of Strasburg, and a modification was first de- 

 scribed by Mr. Marconi in a lecture before the Society of Arts of 

 London.* In this method the charge in the aerial is not created by 

 the direct application to it of the secondary electromotive force of an 

 induction coil, but by means of an induced electromotive force created 

 in the aerial by an oscillation transformer. The method due to Pro- 

 fessor Braun is as follows: A condenser or Leyden jar has one ter- 

 minal, say its inside, connected to one spark ball of an induction coil. 

 The other spark ball is connected to the outside of the Leyden jar or 



* G. Marconi, * Syntonic Wireless Telegraphy,' Journal of the Society of 

 Arts, Vol. XLIX., p. 501, 1901. 



