108 Mr. Watkins on Magneto-electric Induction, 



that I have thus rendered the magnetic toy somewhat more 

 interesting, and am therefore induced to think it worthy of 

 notice in your valuable Journal. 



The revolving magnets I have had in motion for eleven 

 hours without superintendence, and they were only stopped 

 when my workshop was closed for the day. The chemical 

 action on the copper and zinc elements of the voltaic battery 

 employed to induce polarity in the soft iron by means of the 

 copper wire surrounding it, is produced merely by salt and 

 water (not nearly so strong as sea water); and I have a solu- 

 tion of this kind constantly in use, which has been mixed 

 above a month, and when the metallic elements are now 

 placed in it, the magneto-electric machine in question acts 

 without sensible diminution of force. 



The pendulum and suspended magnetic needles of this toy 

 at times exhibit in a modified form a beautiful experiment of 

 M. Plateau recorded in Correspondence Mathematique et Phy- 

 sique, par M. Quetelet, torn. vi. p. 70 (1830.); and as some of 

 your readers may be curious on this matter, and not be able 

 to obtain a sight of the work, you will probably insert the 

 translation. I stop for a moment to state that my attention 

 was originally called to the article in the foreign Journal 

 through the kindness of Mr. Babbage. 



M. Plateau heads his paper thus : " On the action which 

 a bar magnet exerts on a magnetic needle moving in a parallel 

 plane above it" He then goes on to say: "Arrange a bar 

 magnet so that it can turn in a horizontal plane about an 

 axis passing through its centre, and place above this bar a 

 magnetic needle sustained on a pivot, or suspended by a thread 

 devoid of torsion. If you cause the bar to rotate slowly, the 

 needle (as one would expect) follows and turns in the same 

 direction with it; but if you augment the rotation of the bar 

 to a certain point, the needle ceases to go all round, but vi- 

 brates in large arcs: on increasing still more the velocity of 

 the rotations of the bar, the vibrations of the needle diminish 

 in amplitude; and at length, at another certain and greater 

 degree of velocity, it is found they cease altogether, and the 

 needle reposes in the magnetic meridian just as quietly as if 

 the bar were not present or in motion. This fact tends to 

 prove that the transmission of the magnetic action is not in- 

 stantaneous ; and M. Plateau asks whether it could not be 

 employed to measure the necessary time for the development 

 of magnetic influence by induction in its full effect." 



M. Quetelet, the editor, then adds a note to the following 

 effect: " It seems to us that M. Plateau has very well pointed 

 out the advantage we might derive from this very simple ex- 



