124 Mr. Willoughby Smith [June 6, 



this pin is balanced a steel magnet. I will place this on the glass 

 cover over the centre of the copper plate. Now, if the copper plate 

 be made to revolve, you will see that the magnet will revolve also in 

 the direction of the copper disc. There it goes! No doubt you 

 observed how sluggish its movements were at first, and that it was 

 some time before it followed the movements of the disc ; this was 

 owing to the attraction of the earth's magnetism on the magnet, which 

 held it in bondage until the speed of the disc was sufficient to over- 

 come its attraction, then, once released, how merrily it appeared to 

 obey the influence of a superior power. The disc now being at rest, 

 the needle has returned into bondage ; but if I judiciously use the 

 influence of this magnet to partially neutralise the influence of the 

 earth's attraction, you will observe how much more quickly it obeys 

 the influence of the mysterious power of the revolving disc. There 

 it goes ! Apparently more readily than before. Were I lecturing on 

 moral philosophy, I certainly should make use of the similes which 

 might be drawn with advantage from experiments with this simple 

 instrument ; but my subject being of a different nature, I will resume 

 without further digression. If the order were reversed and the magnet 

 revolved, the copper disc would act in the same way as the magnet 

 has just done. This is the phenomenon discovered by Arago, who 

 also asserted that the effect takes place, not only with all metals, but 

 with all substances. On this latter point there has always been a 

 difference of opinion between experimentalists who have endeavoured 

 to verify Arago's statement. As far as my experiments have gone, I 

 have only obtained reliable results from good conductors of electricity ; 

 but I believe that, theoretically, Arago is right ; for as all substances 

 are, in a certain degree, conductors of electricity, it, I think, neces- 

 sarily follows that we only want sufficiently sensitive instruments to 

 develop the phenomenon, as Arago asserts, in every substance. It 

 has been stated that all substances when subjected to a sufficiently 

 strong magnetic force are found to give indications of polarity, and 

 also, when magnetic force acts on any medium, whether magnetic, 

 diamagnetic, or neutral, it produces within it a phenomenon called 

 magneto-induction. If this be the case it materially strengthens the 

 correctness of Arago's assertion. Arago's discovery that copper, a 

 non-magnetic metal, was influenced by a rotating magnet, or that 

 a magnet was, in the same way, affected by a rotating disc of copper, 

 was looked upon at the time as a remarkable and new phenomenon in 

 induced magnetism by philosophers both in England and other 

 countries ; but it was Faraday who, by the assistance of his know- 

 ledge of the evolution of electricity from magnetism, gave the true 

 solution, by proving it to be the effect of electrical currents induced 

 in the disc on account of its motion in a magnetic field. He not only 

 proved by simple experiments the correct interpretation of the phe- 

 nomenon, but he saw the way to making the discovery of Arago a 

 new source of electricity, not despairing, by the aid of his knowledge 

 of teirestrial-magneto-induction, of being able to construct a new 



