Page:Popular Science Monthly Volume 63.djvu/373

Rh therefore for any given voltage varies as the capacity, and therefore as the length of the transmitting aerial. If, therefore, the transmitting and receiving aerial have the same length, the minimum energy varies as the square of the electromotive force in the receiving aerial, and therefore as the fourth power of the length of either aerial, since the electromotive force varies as the product of the lengths of the aerials. Hence when the distance between the aerials is constant, the minimum working energy varies as the fourth power of the height of either aerial, but when the lengths of the aerials are constant, the energy caught up by the receiving aerial must vary inversely as the square of the distance D between the aerials. Hence if we call e this minimum working energy, e must vary as 1/D2 when L is constant, or as L4 when D is constant, and since e is a constant quantity for any given arrangements of receiver and transmitter, it follows that when the height of aerial and distance vary, the ratio L4/D2 is constant, or, in other words, D2 varies as L4 or D varies as L2, i. e., distance varies as the square of the height of the aerial, which is Marconi 's Law. The curve therefore connecting height of aerial with sending distance for given arrangements is a portion of a parabola.

Otherwise, the law may be stated in the form $$L = a \sqrt D,$$ where a is a numerical coefficient. If L and D are both measured in meters, then for recent Marconi apparatus as used on ships $$a = 0.15$$, roughly. (See a report on experiments made for the Italian navy 1900-1901, by Captain Quintino Bonomo—'Telegrafia senza fili,' Home, 1902.)

This law, however, must not be used without discretion. After Mr. Marconi had transmitted signals across the British Channel, some people, forgetting that a little knowledge is a dangerous thing, predicted that aerials a thousand feet in height would be required to signal across the Atlantic, but Mr. Marconi has made such improvements of late years in the receiving arrangements that he has been able to receive signals over three thousand miles in 1903, with aerials only thirty-three per cent, longer than those which, in 1899, he employed to cover twenty miles across the British Channel.

We turn, in the next place, to the consideration of those devices for putting more power into the aerial than can be achieved when the aerial itself is simply employed as the reservoir of energy. Professor Braun of Strasburg, in 1899, described a method for doing this by inducing oscillations in the aerial by means of an oscillation transformer, these oscillations being set up by the discharges from a Leyden jar or battery of Leyden jars, which formed the reservoir of energy. The induction coil is employed to produce a rapidly intermittent series of electrical oscillations in the primary coil of an oscillation transformer by the discharge through it of a Leyden jar. Mr. Marconi