Page:The New International Encyclopædia 1st ed. v. 20.djvu/684

* WIRELESS TELEGRAPHY. 586 WIRELESS TELEGRAPHY. Fig. 7. CURVE of oscillatory CURRENT. amplitude decreases from its maximum value to zero potential in geometric proj;ression, as in- dicated by the curve in Fig. 7. This is due to the damping factor in- volved in the con- version of the en- ergy of the oscilla- tions into electric •waves. The fre- quency of the oscil- lations depends on the electrical dimen- sions of the circuit ; these are termed its coefficients, and consist of its electrostatic capacity, its inductance, and its resistance. The capacity of an oscillatory system is meas- ured by the quantity of electricity required to charge it to a certain critical potential where it will disrupt the air-gap, and the capacity of the system is directly proportional to the charge and inversely proportional to the potential. Induct- ance is the effect of a current flowing in the oscillator system on itself, and in dealing with high frequencies this effect is very marked, so that a finite current requires time to reach its maximum value and time for the current to fall to zero. The resistance of the oscillator sy.stem finds its analogue in the frictional resistance a pipe offers to flowing water; when the cur- rent is of high potential and frequency the re- sistance of the sj-stem may be regarded as negligible, since the high resistance offered by the spark-gap is broken down by the spark during the period the high-frequency current is oscil- lating through the system. In the resonator system the same conditions prevail, but instead of emitting electric waves as in the case of the oscillator the electric wavea impinging upon the aerial wire set up electric oscillations, and although very greatly reduced in potential compared with those originally emitted, the frequency is identical. The mode of electric wave propagation through the ether has been deduced on two different theories. The first is based on the rectilinear propagation of electric waves. An experiment to demonstrate that electric waves traverse space in straight lines may be made by em- ploying an ordinary Hertz oscillator, shown d iagrammat icallv in Fig. 8. Wlien high- frequency currents os- cillate tlirough the sys- tem splierical electric waves in the form of transverse vibrations in the ether are emitted as depicted in Fig. 9, each one Fia. 8. HKIITZ OPES-CIBCriT OSCILLATOR. Fig. 10. ELECTRIC WAVES REFLECT- ED BY STRATA OF RAREFIED AIR. waves, i.e. 180,500 miles per second. Where one terminal is grounded and the opposite terminal of the oscillator system is elevated, as shown in Fig. 6, the theory of rectilinear propagation as- sumes that the waves are still spherical and travel in a straight line until they come in contact with the upper stratification 'Of rarefied air, which is a con- ductor of elec- tricity and there- fore a non-con- ductor of electric waves, in which case the waves are reflected back to the surface of the earth. Electric ^^•aves are radiated in every direction at right angles to the oscillator system; now if a resonator system is placed in a direct visual line to the oscillator it will respond, but if the curvature of the earth intervenes as shown in the diagram Fig. 10, then the waves will be re- flected from the strata of rarefied air and di- rected again to the surface of the earth. This theory is in accordance with Hertz's physi- cal demonstration of the electro-magnetic 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 JMeans 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 sho-wn in Fig. 11. Fortunately, however, the theory of Mi ?J M " i ' Su ! 1 ! 1 ^ 1 I i .! ' :.-„ : i .,: ' i ' I WWr%. FlO. 9. HERTZ DETACHED ELECTRICAL WAVES. being detached from the oscillator and propagated through space at the velocity attained by light rface of Earth Fig. 11. FESSENDEN SLIDING HALF-WAVES. wave propagation has little to do with the prac- tical application of the art. Syntonic Wirei.es.s Telegr.m'hy. Since elec- tric waves are radiated in every direction, it is evident that any receiver in the fi<'ld of force will respond to any transmitter emit- ting the waves. To enable any njiera- tor or operators of different instru- ments in the same field to send and receive messages indci)endently of all others, apparatus constructed in ac- cordance with the principles of elec- tric resimance have been attempted. The phenomena of electric resonance may bp illustrated by Lodge's syntonic jars. Fig. 12. A and 1! represent the sending and receiv- ing circuits: the jar for emitting the waves, 1, A,