1832.] Progress of European Science. 373 



The induction of electricity by the momentary passage of a magnetic pole, so 

 analogous to that of the induction of magnetism by a current of electric fluid, 

 leads at once to the consideration of another branch of magnetic phenomena which 

 originated with Mr. Arago, and has since been studied assiduously by Messrs. 

 Herschel, Christie, Barlow, and Sturgeon, and others, namely, the magnetic 

 effects of metallic plates in rotation, with which we think should be classed, the 

 practical methods of magnetising steel bars, by the motion of magnets over them. 



That magnetism existed in brass had been pointed out by Mr. Barlow in 1823, 

 but the magnetic force in most metals except iron, nickel, and cobalt was far too 

 feeble to be detected by the simple application of the most delicate needle. Mr. 

 Arago judiciously applied the principle of the magnetic momentum generated by the 

 rapid motion of discs of these metals, and demonstrated that all possessed magnetic 

 energy in various degrees*. The novel phenomenon of a magnetic needle rotating 

 on its pivot by simply placing it above a revolving plate of copper, had something 

 in it so fascinating, and presented so striking a similitude to the electro-magnetic 

 rotations then familiar to philosophers, that for a while it was doubted whether 

 or not the revolving plate possessed electric properties. Another mode of solv- 

 ing the problem was, that the copper plate, like all ferruginous bodies, actuated 

 the needle through what is called induced magnetism by the influence of the earth, 

 and that the needle was put into motion by a rapid succession of transient magnetic 

 poles induced in the plate. When however it was found that light copper discs would 

 of themselves rotate when suspended over a revolving horse-shoe, or other powerful 

 magnet, it occurred that all the phenomena emanated from the action of the 

 magnet employed, exciting transient polarity in the metals under examination. 



Mr. Sturgeon, from whose paper the foregoing facts are drawn, was from the 

 want of an adequate hypothesis led to examine the distribution of magnetic pola- 

 rity in metallic discs, by making them revolve or oscillate between the poles of a 

 horse-shoe magnet, and trying the conditions of various parts of the plate by a 

 small dipping needle. The number of vibrations performed by a thin copper disc 

 under the influence of a magnet, were reduced even five-fold when compared 

 with its free vibrations in air. 



This subject is still under investigation, and will doubtless receive fresh eluci- 

 dation from the able men engaged upon it : we have already found that magnetism 

 is but another name for electricity, and shall soon know how to elicit it from every 

 substance, and perhaps to draw benefits as important in mechanical as the pola- 

 rity of the needle is in nautical science. What can be more wonderful than the pro- 

 digious mechanical force set in action by what appears a very inadequate cause, — 

 the elector-magnet, an instrument now familiarly known, in which a coil of wire 

 is wound round a horse-shoe of soft iron, and connected by its two extremities 

 with two small plates of zinc and copper. One of these instruments, constructed 

 by Mr. Walsh of Woolwich, was exhibited at the evening meetings of the Insti- 

 tution, which when the plates were dipped in weak acid, sustained a weight of be- 

 tween 300 and 400 pounds, though before the application of the voltaic power it 

 did not support an ounce. 



The last fact to be noted in the progress of electrical knowledge, is the produc- 

 tion of sparks from the magnetic needle in Italy : Signor Nobili's announcement 

 has been confirmed by Mr. James D. Forbes' experiments, before the Edinburgh 

 Royal Society, in April, 1832. And the power of drawing sparks from the natural 

 magnet is now established. All we know as yet of the procedure is that, it " rests 

 upon the recent discoveries of our distinguished countryman Mr. Faraday." 



* W. Sturgeon, Phil. Mag. April, 1832. 



