36 



HABIT AND INTELLIGENCE. 



[chap. 



Explana- 

 tion. 



Electro- 

 static 

 nduction, 



and con- 

 tinuous 

 currents, 

 probably 

 both due 

 to mole- 

 cular 

 tension. 



Electro- ' 

 magnetic 

 induction. 



Experi- 

 ment V. 



Ex2)eri- 

 ment VI. 



Explana- 

 tion. 



According to my explanation, the momentary slackening of 

 the current at its commencement is caused by a portion of its 

 energy being taken up, and becoming static, in throwing the 

 wire into a state of molecular tension ; and the momentary 

 increase of the force of the current at the moment when it is 

 cut off is caused by the energy that was static in the -wire 

 returning to the state of kinetic, or current, electricity. 



I have now stated the most important facts of electro-dynamic 

 indtiction, and, as I believe, given a satisfactory interpretation 

 of them. And I believe, though the evidence is not nearly so 

 strong, that electro-static induction, as in a Leyden jar, wiU be 

 explained in some similar way ; namely, that the electric charge 

 consists in some kind of molecular tension. 



The explanation I have given above applies only to those 

 momentary currents which are produced by induction ; it does 

 not apply to continuous currents. But I believe that the hypo- 

 thesis of molecular tension will be found to explain the facts of 

 continuous currents also. The molecules of a conductor along 

 which a current is {lowing are, I think, shown to be in a state 

 of molecular tension (of a different kind, however, from that of 

 wire b) by the fact that all the successive portions of the same 

 current repd each other : as is easily proved by suitable experi- 

 mental arrangements.^ 



With respect to the theory of electro-magnetic induction, 

 however, there is no difficulty whatever : it admits of an ex- 

 planation exactly parallel to that which I have given of electro- 

 dynamic induction. In other words, the induction of momentary 

 magnetism in a soft iron bar is an exactly parallel fact to the 

 induction of a momentary current in a conducting wire. 



Let an electric current be flowing along a wire coiled into a 

 hollow spiral, and let a soft iron bar be put into the spiral ; the 

 iron will be instantaneously magnetised, and the current will for 

 the moment become less forcible. On removing the bar it at 

 once loses its magnetism, while the current, for the moment, 

 becomes more forcible. 



These facts admit of exactly the same interpretation as those 

 of induced currents. When the iron is put into the spiral, and 

 the current for a moment loses force, the slackening of the 

 current is caused by a portion of its energy being taken up and 

 becoming static in the iron, which it throws into a peculiar state 



^ De la Rive on Electricity, English translation, vol. i. p. 231. 



