324 Professor Oliver Lodge [June 1, 



surge to and fro several times first; and these surgings or electric 

 oscillations must, according to Maxwell, start waves in the ether, 

 because at the end of each half-swing they cause electrostatic, and at the 

 middle of each half-swing they cause electromagnetic effects, and the 

 rapid alternation from one of these modes of energy to the other con- 

 stitutes ethereal waves.* If a wire is handy they will run along it, 

 and may be felt a long way off. If no wire exists they will spread 

 out like sound from a bell, or light from a spark, and their intensity 

 will decrease according to the inverse square of the distance. 



Maxwell and his followers well knew that there would be such 

 waves ; they knew the rate at which they would go, they knew that 

 they would go slower in glass and water than in air, they knew that 

 they would curl round sharp edges, that they would be partly absorbed 

 but mainly reflected by conductors, that if turned back upon them- 

 selves they would produce the phenomena of stationary waves, or 

 interference, or nodes and loops ; it was known how to calculate the 

 length of such waves, and even how to produce them of any required 

 or predetermined wave-length from 1000 miles to a foot. Other 

 things were known about them which would take too long to enume- 

 rate ; any homogeneous insulator would transmit them, would refract 

 or concentrate them if it were of suitable shape, would reflect none 

 of a particular mode of vibration at a certain angle, and so on, and 

 so on. 



All this was Imoivn, I say, known with varying degrees of confi- 

 dence ; but by some known with as great confidence as, perhaps even 

 more confidence than, is legitimate before the actuality of experimental 

 verification. 



Hertz supplied the verification. He inserted suitable conductors 

 in the path of such waves, conductors adapted for the occurrence in 

 them of induced electric oscillations, and to the surprise of every one, 

 himself, doubtless, included, he found that the secondary electric 

 surgings thus excited were strong enough to display themselves by 

 minute electric sparks. 



I shall show this in a form which requires great precision of 

 tuning, or syntony, both emitter and receiver being persistently vi- 

 brating things giving some 30 or 40 swings before damping has a 

 serious effect. 1 take two Leyden jars with circuits about a yard in 

 diameter, and situated about two yards apart, Fig. 4. I charge and 

 discharge one jar, and observe that the surgings set up in the other 

 can cause it to overflow if it is syntonised with the first.f 



A closed circuit such as this is a feeble radiator and a feeble 



* Strictly speaking, in the waves themselves there is no lag or difference of 

 phase between the electric and the magnetic vibrations ; the difference exists in 

 emitter or absorber, but not in the transmitting medium. True radiation of 

 energy does not begin till about a quarter wave-length from the source, and 

 within that distance the initial quarter period difference of phase is obliterated. 



t See ' Nature,' vol. xli. p. 368 ; or J. J. Thomson, * Kecent Eesearches,' 

 p. 395. 



