THE AGE OF ELECTRICITY. 167 



helix; not that this be at first sight a natural consequence of the facts 

 - - - yet I thought it would not be an idle experiment to attempt." 

 The result was disappointing, and Fresnel concludes that curious 

 paper with the following words: 



"I must add, in behalf of M. Ampere, that the slight movements 

 that he had noticed in a magnetic needle when he brought near to it a 

 circuit of brass wire, a part of which was spirally wound, around a 

 magnet, were not repeated in a constant manner, and besides that 

 tbey were so slight that he would not have made that experiment 

 public had he not felt certain from the success of my own experiment, 

 of which he entertained no doubt, that these slight motions were also 

 caused by an electric current produced by the magnet acting upon tbe 

 helix wound around it.'' 



We may rely on the notorious awkwardness of Ampere for an assur- 

 ance that his helix was not attached to the magnet, that it was thrown 

 out of shape while a portion of the wire was brought near the needle, 

 and that he had actually discovered the induction currents; it wall 

 sufiice to repeat his exj)eriment without any modification. 



Shortly thereafter Daniel Colladon, in whom the last figure of an 

 illustrious generation has just disappeared, also investigated, by a 

 more delicate process, whether a very powerful magnet will produce 

 an electric current in a helix by being brought near it. The helix was 

 connected with a galvanometer and the latter was kept in a separate 

 room so as to preclude any direct action of the magnet. After making 

 all preparations for the exj)eriment and j)utting the magnet in posi- 

 tion, Colladon would go and inspect his galvanometer. He probably 

 did so without great haste, for the index was motionless and at the 

 same point where it stood before. 



"I had not suspected," said he, "that induction could only be 

 instantaneous. If I had had an assistant he would have seen for me 

 the displacement of the magnetic needle occurring at the instant when 

 I brought the magnet near the helix." 



It has sometimes been said that the induction currents are the sen- 

 tries charged with the protection of the principle of the conservation 

 of energy. Nothing is produced without an expense of labor, and the 

 failure of the preceding experiments was due to a singular oversight 

 of that i)rinciple. 



The discovery by Arago of the effect of copper plates in abating the 

 oscillations of magnetic bars, and of the attraction of these bars by 

 conductive mediums in motion was giving there and then the clew 

 to inductive currents and the energy necessary for their maintenance. 

 Again they were at hand when Ampere and Colladon went over the 

 same experiments by substituting helices conducting a current for the 

 bars, and when Fresnel, in an experiment that remained unpublished 

 for a long while, demonstrated that by increasing the weight of the 

 magnetic bar with a copper bar, the oscillations were modified with 

 greater rapidity than the increased mass of the whole would lead to 

 suppose. 



