This deory is in accordance with Hertz's physi cal demonstration of the electromagnetic theory of light, as elucidated in his paper on "Electric Radiation." in which he shows how long, invisible free electric waves are propagated the same as the microscopic visible free waves of light.
The second theory is also based on Hertz's ex periments as deduced and reviewed in his paper "On the Propagation of Electric Waves by Means of Wires," in which he shows how electric waves may slide over the surface of conducting wires with the wave front perpendicular to the path of the wire.
In wireless telegraphy this theory offers the earth as the conducting path, and according to Professor Reginald A. Fessenden the wave is vir tually cut in half, and is therefore a half-wave skimming along the surface of the earth as shown in Fig. 11. Fortunately, however, the theory of wave propagation has little to do with the prac tical application of the art.
WiftELEsss TELEnitArnr. Since elec tric waves are radiated in every direction. it is evident that any rceeiver in the field of force will respond to any transmitter emit ting the waves. To enable any opera tor or operators of different instru ments in the same field to send and receive messages independently of all others. apparatus constructed in ac cordance with the principles of elec tric resonance have been attempted. The phenomena of electric resonance may be illustrated by Lodge's syntonic jars. Fig.
12. A and 11 represent 11le sending and receiv ing circuits: the jar for emitting the waves, 1, A, is charged to a high potential ; it is connected in series with the spark-gap, 2, and the loop of wire, 3; when the jar is discharged through the gap electric oscillations of high frequency are set up in the circuit ; in virtue of the circuit being closed—in contradistinction to the llertz open circuit oscillator—the oscillations are very per sistent, that is, it requires a much longer time for their conversion into electric waves than in the I 'pen-circuit type, but the emitted waves are much more feeble than where the oscillations are damped out rapidly. The receiving jar 1 11 is
connected through a spark-gap, 2, and a closed circuit, 3, 3. the induction of which is made variable by the sliding metal bar 4. When the feeble electric waves impinge upon the circuit they are transformed into electric oscillations, and by their cumulative effect the jar is charged to its sparking capacity when it breaks down the gap, thus indicating the presence of the waves.
This action takes place if the jars, A, B, are in tune, i.e. if their inductance capacity and re sistance are identical; but if B is not in tune or syntonized to A it will not respond. Syntonie or selective wireless telegraphy is based on these factors, hut inasmuch as the open-eircuit oscil lations are damped out too quickly to obtain good resonance effects and the closed circuit oscil lators are too feeble to cover long distances, these inherent difficulties have not been overcome with any degree of success in a practical way.