ELECTRO-MAGNETISM. 



of the negative electricity is from the 

 west side of the wire. 



(17.) When the uniting wire, instead 

 of being horizontal, is placed vertically, 

 as shown in fig. 5, either to the north or 

 south of the needle, and then brought 

 near to the adjacent pole, if the upper 

 extremity of the wire receives the nega- 

 tive electricity, that pole moves towards 

 the east ; but when the wire is brought 

 opposite to a point between the pole and 

 the middle of the needle, as in Jig. 6, the 



Fig. 6. 



n 



same pole deviates to the west. When 

 the upper end of the wire receives posi- 

 tive electricity, the phenomena are re- 

 versed. 



(18.) Oersted found that these expe- 

 riments succeeded equally well if t'he 

 uniting conductor consisted of one or of 

 several wires, or metallic ribbons, con- 

 nected together. Neither is the effect 

 altered in its kind, though it may vary 

 somewhat in degree, when different 

 metals are used : thus, platinum, gold, 

 silver, brass, iron, lead and tin, and even 

 mercury contained in a tube, when em- 

 ployed as the conductors of the electricity, 

 have a similar influence on the magnetic 

 needle. The conductor still exerts this 

 power, although it be interrupted by 

 water, provided the interval between the 

 metals does not extend to several inches 

 in length. The magnetic influence of 

 the wire on the needle is not prevented 

 by the interposition of glass, metals, 

 wood, water, resin, stones, or any other 

 substance that was tried. The effect 

 produced, nevertheless, is referable 

 purely to magnetism, for it is exerted on 

 magnetic bodies only, and has no in- 

 fluence on needles of brass, glass, or 

 gum lac. It appears to depend, not upon 

 the intensity of the circulating electri- 

 city, but solely on its quantity ; and ac- 

 cordingly Oersted found that he could, 

 with a single galvanic arc, repeat all the 

 experiments which he had at first made 

 with a compound Voltaic battery. In 

 nis way, also, he was enabled to detect 



the reciprocal action which the poles of 

 a magnet exert on the conducting wire ; 

 for, by placing a plate of zinc, six inches 

 square, between two plates of copper 

 formed into a trough, in order to hold 

 the acid which is to act upon the for- 

 mer, but kept from touching them by 

 small pieces of cork interposed on each 

 side, on forming a communication be- 

 tween the two plates by an extended 

 wire, and then suspending the whole 

 apparatus by a thread, the effect of a 

 magnet in moving the wire could be 

 readily ascertained. 



(19.) The announcement of the im- 

 portant discovery of Oersted excited the 

 greatest interest among all the philoso- 

 phers of Europe, and they immediately 

 occupied themselves in repeating and 

 extending his experiments. Among 

 those who were early distinguished by 

 their zeal and activity in this research 

 were Ampere and Arago, in France, and 

 Sir H. Davy and Faraday, in England. 

 So many were the cultivators in this 

 new field of inquiry, and so eagerly did 

 they pursue the path thus unexpectedly 

 opened, that a great number of interest- 

 ing facts were speedily brought to light ; 

 and where all were pressing forward in 

 the same career, it is scarcely possible 

 to adjust the claims to priority of dis- 

 covery, with respect even to the most 

 important facts. Instead, therefore, of 

 attempting to give a chronological view 

 of the progress of knowledge in this de- 

 partment of science, we shall adopt the 

 following more didactic, and, we trust, 

 more instructive plan. We shall first 

 state those general principles to which 

 philosophers have arrived by gradual 

 and successive inductions; secondly, 

 we shall trace the various combinations 

 of those principles in different ways, and 

 under different circumstances, and the 

 effects resulting from them ; and lastly, 

 point out the explanations which they 

 afford of particular phenomena, in the 

 order which appears most conducive to 

 clear and comprehensive views of the 

 whole subject of electro-magnetism. 



CHAPTER III. 

 Fundamental Law of Electro-Magnetic 



Action. 



(20.) An attentive examination of 

 the facts described in the preceding 

 chapter will soon convince us that the 

 magnetic force which emanates from the 

 electrical conducting wire is entirely dif- 

 ferent in its mode of operation from all 

 the other forces in nature with which 



