October 1, 1900.] 



KNOWLEDGE. 



233 



diameter, aiid separated by a distance of about two 

 yards, aud fouud that when the first jai- wa.s charged 

 "and discharged the waves set up in the second circuit 

 could be made to cause it to overflow across a short air- 

 gap, provided by pasting a slip of tinfoil over the lip 

 of the second jai-, by experimentally adjusting the slides 

 shown in the illustration. Lodge calls this syntonising 

 the pair of jars. A closed circuit such as this is a feeble 

 radiator, because it is not well adapted for the transfer 

 of its cuergv to the siurounding ether, some thirty or 

 fortv oscillations taking place before there is any serious 

 damping. Great precision of tuning is therefore neces- 

 sary. 



It will be instructive to compare this an-angenient of 

 Professor Lodge's with a standard llcrtz oscillator aud 

 resonator, as shown in Fig. 2. A powerful induction 



o^o. 



Fuj. 2.— Hertz Oscillator aud Resonator. 



S.B. — C, C, are spheres with wires shown through lenlres, aud 

 therefore represented by circles. D is scjuare of wwe. 



coil, A, having the terminals of its secondary circuit 

 connected with the oscillator, which consists of a pair 

 of brass rods terminating in small polished knobs, B, 

 the distance between which is adjustable, while two large 

 metal spheres, C, C, slide on the brass rods. By 

 altering the positions of these spheres the oscillator can 

 be tuned into syntony with the resonator, D, consisting 

 of a wire rectangle or circle, tenninating in a pair of 

 polished brass knobs, which should be vei-y close 

 together. 



If Lodge's exciting jar had its two coatings removed 

 to a considerable distance apart, and the dielectric 

 sepai'ating them were made to extend out into the room, 

 we should obtain the equivalent of the Hertz oscillator, 

 which is of the most suitable form to facilitate the trans- 

 ference of its electric wave energy to the surrounding 

 ether. 'When the coatings are close together, as in 

 Lodge's form, the magnetic energy largely predominates 

 over the electrostatic. When the distance between them 

 is increa-sei and the dielectric more exposed, the electro- 

 static energy becomes more nearly equal to the magnetic, 

 and therefore the arrangement gains in efficiency as a 

 radiator, since in true radiation the two energies must 



Fig. 3.— Oscillations of Dumb-bell Hertz Oscillator. 



be nearly equal. The spheres, C, C, may, if desired, be 

 replaced by large metal plates. 



By means of calculations from the readings of an 

 electrometer inserted in the air-gap, D, Fig. 2, Bjcrknes 

 succeeded in obtaining curves representing the il.imping 

 of the oscillations. Figure 3 shows the oscillations 

 obtained with a dumb-bell oscillator, such as that illus- 

 trated in Fig. 2, and it will be observed that they die 

 away with extreme rapidity. 



The persistent character of the oscillations excited in 

 a ring resonator by an oscillator tuned to syntonism 

 with it is shown in Fig. -l. 



I u 



Fio. 4.— Oscillation nt Rin^;. shaped Hertz Kesoualur excited by 

 Syntonic Oscillation. 



Ju.st as in the case of acoustic resonance, when the 

 resonator has its natiu'al oscillations strongly damped, 

 the tuning of the oscillator into syntony with it is of 

 comjiaratively small importance, but if its oscillations 

 are persistent then exact tuning is essential. Exact 

 syntony is also necessai-y whenever the exciter is a per- 

 sistent oscillator, as otherwise it will tend to destroy at 

 one moment the oscillations which it set up a moment 

 before. This is well shown in Fig. 5, which exhibits 



Fio. 5. — Oscillation of Rinfj-shnped Hertz Resonator cieited by 

 Oscillation not cjuito Syntonic with it. 



the oscillation of a ring resonator, excited by an oscil- 

 lation not quite in syntony with it. 



To understand hew an electrical oscillation, or its 

 equivalent, an oscillating charged body, ca.n excite electric 

 waves in the ether, I will ask my readers to refer to 

 Fig. 6 in my last article. Let the rack represent the 

 electrically charged body, and imagine it is oscillating 

 backwards aud forwards in the direction of its length. 

 This will set up a rotary oscillation in the wheelwork, 

 and the wheelwork being, as has been assumed through- 

 out, elastic, this rotai-y oscillation will bo propagated 

 with a velocity depending on the elasticity and the 

 density, as has already been explained. The axes of the 

 wheels represent the direction of tho magnetic rotary 

 oscillation, and this is perpendicular to the line of rack 

 which represents the direction of the electrical oscil- 

 lation. The direction in which the wave is advancing 

 is perpendicular to both of thcni. Hertz by exploring 

 with his resonator the space in the neighbourhood of an 

 oscillator succeeded, not only in demonstrating the 

 existence of electric waves, but in differentiating between 

 the electrostatic and magnetic oscillations. He also 

 succeeded in proving they liatl all the well-known pro- 

 perties of light and heat waves. 



