WIRELESS TELEGRAPHY. 
117 
other sources. Marconi signals, on the other hand, are to a certain 
extent liable to confusion from unforeseen disturbances such as a flash 
of lightning. I do not know whether the Marconi system has ever 
been tried in a thunderstorm, but there appears to be some element of 
danger from this cause. If a satisfactory form of delicate relay could 
be devised which would bring in action a local battery, the first and 
most serious disadvantage of Preece’s system, which I spoke of, would 
be remedied. It is a difficult problem, but I believe not impossible. 
I am not quite certain whether a certain degree of success has not 
been attained already, and I think apparatus is now being experimented 
with which promises success. 
I shall now leave Preece’s system, which serves fairly well as a 
introduction to a more recent invention-—which I shall call Marconi’s sys¬ 
tem. To sketch the development of what is called Marconi’s system is 
to show how truly cosmopolitan science is. The system really may be 
said to be the result of the study of one of the oldest phenomena 
known in electricity, namely, the electric spark. 
Forty or fifty years ago it was discovered that a spark from a 
Leyden jar is under certain conditions oscillatory in character; it con¬ 
sists not of a single discharge from one point to another but of a series 
of surgings to and fro, the oscillations continuing until the energy of 
the discharge is exhausted, the whole discharge occupying, however, 
only a very small fraction of a second of time. This was probably- 
known to Faraday in England and Henry in America. In 1855 Lord 
Kelvin gave the mathematical conditions for a spark to be oscillatory. 
Perhaps the best anology to give is the release of a stretched spring. 
When a spring under tension is suddenly released it does not contract 
to its original dimensions, but vibrates backwards and forwards with 
a series of oscillations. The instant before the spark leaps across the 
gaps separating the spheres of the transmitting instrument (shewn) on 
the table, the space bewteen the spheres is in a state of strain. The 
fact of a spark passing, is the outward and visible , sign of the release 
of this state of strain, and in the same manner as the spring, the elec¬ 
tricity surges backwards and forwards with vibrations of ever-dimin¬ 
ishing amplitude, until the discharge is complete. Every sudden dis¬ 
charge then consists of a series of surgings, but with so great 
a rapidity do they take place, that the whole discharge appears prac¬ 
tically instantaneous to the eye. 
Shortly after Lord Kelvin’s mathematical theory was enunciated. 
Clerk Maxwell developed his famous theory of the identity of 
electric waves and light waves, and showed that while an electric 
oscillation is in progress there must be thrown out at right angles to 
the line of oscillation—in this instrument (see Plate II) the line of 
oscillation in the line joining the centres of the spheres—an electro¬ 
magnetic wave which moves with the same velocity as light but is of 
different wave length and frequency. Seeing that the time occupied 
by a single one of those oscillations may be less than one hundred 
millionth part of a second, it may be gathered what difficulties lay in 
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