264 



WIRELESS TELEGRAPHY. 



by Lord Kelvin in 1853. Hertz set up electric oscillations by means 

 of an electric oscillator, shown in lig. 1. This consists of an ordinar}^ 

 large induction coil, A, the terminals of the secondary coil being- con- 

 nected to brass balls, or knol)s, and to which short metal rods, or 

 wings, ?/;, are attached. The knobs are separated by a small air space, 

 across which sparks jump when the coil is in operation. At such 

 times electric oscillations are set up, the rate of which varies with the 

 electrical dimensions of the circuit. 



Hertz assumed that if the electric oscillations thus produced set up 

 corresponding waves in the ether of free space, these waves should, 

 in turn, set up electric oscillations of corresponding frequency in 

 a suitable receiver, or "eye,'' within the range of their influence. 



He therefore adopted as a detector 

 of these waves a copper wire, D 

 (tig. 1), of nearly circular shape, 

 about IG inches in diameter, but 

 broken at one point. On the ends 

 of this wire he placed small metal 

 knobs, the distance between which 

 could be easily regulated. This wire 

 was held by an insulated handle,' a 

 few feet from the oscillator. With 

 the room darkened, minute sparks 

 were observed passing between the 

 discharge knobs of the receiver; and 

 the results of this simple experi- 

 ment have ])een generally accepted 

 as proof of the existence of electric 

 waves in free space. 



Hertz, however, was not satisfied 

 with this demonstration of the accu- 

 racy of Maxwell's theory, but also, 

 in the course of his su])sequent mas- 

 terl}' experiments, showed that, like sound, heat, and light waves, 

 the Hertzian waves could also be reflected, refracted, concentrated in 

 paralhd rays, and to a focus, etc. 



By the Hertz receiver the distance at which electric waves could 

 ))e detected was very limited, perhaps 10 or 12 feet at most, and 

 hence it is not likely that much would have been done in the utiliza- 

 tion of Hertzian waves for telegraphic purposes had progress rested 

 there. Fortunately, it did not. Shortly after the experiments of 

 Hertz, Dr. Branly discovered that loose metal tilings, which in a 

 normal state have a very high electrical resistance, lose this i-esistance 

 in the presence of electric oscillations and become practically conduc- 

 tors of electricity. This he showed by placing metal filings in a glass 



Fig. I.— An oscillator and a Hertz tlete'Ctor. 



