hertz's experiments. 225 



tliea'ther is inucli stiniiu'd; w hoic wide, the a-ther is l)ut little stiaiii('<l. 

 Each tube must be looked upon as a tube of unit strain. 



The nature of the strain of the a'ther is uot known; it is, most i)iob- 

 ably, some increased motion in a perfect Ii<]uid. \Vc n)ust not be 

 snri)rise<l at the nature «f the strain being" unknown. We do uot 

 know the nature of the change in a piece of India, rubber when it 

 is strained, nor indeed in any solid, and though the a^tlier is mucli 

 sim])ler in structure than india rubber, it can hardly be wondered 

 at that we have not yet discovered its structure, for it is only within 

 the present century that the existence of the aether was demonstrated, 

 while men have known solids and studied their proi)erties and struc- 

 tiue for thousands of years. Any way, there is no doubt that the 

 a'ther is strained in these tubes of force when ^l and /> are oppositely 

 charged, and that the energy per cubic centimeter of unstrained icther 

 is less than that of strained aether, and that the work done in what is 

 called charging A and B is really done in straining the a'ther all rcmnd 

 them. When the air-ga]) breaks down, and an eh'ctric si)ark takes 

 its place, there is (piite a new series of phenomena produced. Sud- 

 denly, the strained a'ther relieves itself, and in doing so, sets up new 

 motions in itself. The strained state was probably a peculiar state of 

 motion, and in changing back to ordinary .Bther a new and quite dis 

 tinct state of motion is set uj). This new state of motion all round tlie 

 (•(mdnctors is most intense near the spark, and is usually described as 

 an electric current in the conductors and across the spark, or as a rush- 

 ing of the electric charge from one conductor to the other. The elec- 

 tric current is accompanied by magnetic force in circles round it, and 

 the tubes of magnetic force detine the nature of the new movement in 

 the a'ther as far as we know it. 



Hitherto, for the sake of simplicity, the existence of this magnetic 

 force has been unnoticed. It is due to a i)eculiar motion in the a'ther 

 all ronnd what aie called electric currents. The currtuit in fact con- 

 sists of little else than a line, all round which this moxement is going 

 on; like the movement surrounding an electrified body, but also un- 

 like it. Whenever electric forces are changing, or electrified bodies 

 moving, or electric currents running, there this other peculiai- mo- 

 tion exists. We have every reason for thinking that this, whicli may 

 be called the magnetic strain in the a'ther, as the nio\M'ment all lound 

 electrified bodies was callcMl the electric strain — that this magnetic 

 strain only exists in these three cases: (1) When the electric strain is 

 changing; (li) when electrified bodies are moving, and {'.j) when electric 

 currents are running. Thes(^ three may be all cases of one action; 

 certainly the magnetic strain that accompanies each is the same, and 

 it seems most likely that the electric change is only another as])ect 

 of the magnetic- strain. There are analogies to this in the motion of 

 matter that partly helj) and i)artly annoy, because tlie\ partly agree 

 and i)aitly will not agree- Avith the a'thcrial phenomena. Take the case 

 H. Mis. 114. -15 



