1 52 A nalyses.of Boohs. [Feb . 



sure which may be measured. The law of their attraction and repul- 

 sion is inversely as the square of the distance. His theory of the 

 magnetic phenomena of rotation has been shaken by the discoveries 

 of Faraday. 



III. Electro-dynamics. 1. To Oersted we are indebted for the 

 foundation of this branch of electricity. He demonstrated the action 

 of electric currents upon the magnet, while Biot and Savart prose- 

 cuting the subject discovered the law which regulates this action at 

 a distance, viz. : that the electro-dynamic force increases inversely as 

 the simple distance. M. Savary applying the formulae of Ampere 

 to the experiments of Biot and Savart, has found that the total 

 action of the wire was reciprocally proportional to the simple distance. 

 M. Colladon first observed the action of the Leyden phial upon the 

 needle. With a phial two feet square charged as strongly as possible, 

 the deviation of the needle was 32 °. Faraday has demonstrated fur- 

 ther, that a continuous deviation of the magnetic needle in the mul- 

 tiplier may be obtained with the common electrical machine, pro- 

 vided time be allowed to enable the action to be produced, by causing 

 the electricity to pass along imperfect conductors. 



2. But one of the most important consequences from the discovery 

 of the action of the electric fluid upon the magnet was the observa- 

 tion of M. Arago, that the same current developed the magnetic pro- 

 perty in plates of iron or steel which did not previously possess it, and 

 that to communicate magnetism to needles deprived of it, it is ne- 

 cessary to place them in a direction perpendicular to the forming wire, 

 or if a strong degree of magnetic influence is required, it is necessary 

 to introduce them into a helix, and make the current pass across the 

 wire. Savary has observed, that the magnetic influence is produced 

 inversely as the distance of the needle from the wire, and that a given 

 discharge produces always magnetic power, so much the more intense 

 as the length of the wire is greater in relation to its diameter ; and 

 that equal fragments of the same needle, in the interior of a helix, 

 were always equally magnetized, Arago having previously shewn, 

 that similar needles are equally magnetized. He considers that all 

 the phenomena which he has observed, may be deduced from the 

 hypothesis, which ascribes the dependence not only of the intensity, 

 but the magnetic influence to laws, according to which, the minute 

 motions are extinguished in the wire, in the medium which surrounds 

 it, and in the substance which receives and preserves the magnetic 

 power. Moll has shewn that the force of the communication of the 

 magnetic power, depends on the rapidity of the current. Lipkins 

 and Quetelet have proved that very powerful magnetic effects may 

 be produced with voltaic elements on a confined surface, if the che- 

 mical action is energetic, and that on varying the dimensions of the 

 voltaic elements and the portions of iron, the energy of the effects 

 depends less on the size of the first than on that of the second. 



In reference to the power of retention of magnetism on soft iron, 

 Mr. Watkins found that in applying a current across a helix to a piece 

 of iron, a weight of 120 pounds was sustained, but on interrupting 

 the current, 56 pounds only could be supported. He has concluded 

 that the power of suspension of electric magnets depends on a com- 



