ELECTRO-MAGNETISM. 



be more clearly understood from an in- 

 spection of Figs. 1 and 2, where N, S 

 present the two opposite poles of the mag- 

 netic needle, halanced upon its pivot ; and 

 p n the uniting wire ; the end p being 

 connected with the positive or copper 

 end of the simple galvanic battery, and 

 the other end, n, being connected with 

 the negative or zinc end of the same 

 battery. (See the Treatise on GALVA- 

 NISM, $ 15.) So that the direction of the 

 positive electric current is from p to n t as 

 described by the arrows in the figure. 



Fig. 2. 



When the uniting wire is above the 

 needle, as \njig. 1, the pole S, which is 

 next to the negative side of the battery, 

 or towards which the current of positive 

 electricity is flowing, will move towards 

 W, the western side of the horizontal 

 dotted circle; and the needle will as- 

 sume the position N' S'. When the 

 wire is below the needle, as mfig. 2, the 

 same pole S will move towards E, the 

 east point of the horizon ; and its new 

 position will be N' S', inclined in a di- 

 rection the reverse of that which it as- 

 sumed in the former case. 



(14.) For the more easy retention of 

 these facts in the memory Oersted used 

 the following formula : namely, ' the 

 pole above which the negative electricity 

 enters, is turned to the west; under 

 which, to the east: Another, and more 

 convenient formula, however, will pre- 

 sently be given, comprehending not only 

 these but many other facts, which are 

 derived from a more universal principle 

 applicable to all of them. 



(15.) When the uniting wire is si- 

 tuated in the same horizontal plane as 

 that in which the needle moves, and is 

 at the same time parallel to it, no decli- 

 nation takes place either to the east or 

 west ; but the needle is inclined, so that 

 the pole next to the end of the wire at 

 which the negative electricity enters is 

 depressed, when the wire is situated on 

 the west side, and elevated when situ- 

 ated on the east side. 



Thus, if the uniting wire p n, fig. 3, 



be placed on a level with the needle 

 NS and parallel to it, on its eastern side, 

 the pole S, next to the negative end of 

 the wire n, will be elevated, and the pole 



Fig. 3. 



N depressed, so as to assume the posi- 

 tion represented by the dotted needle 

 N'S'. If the uniting wire had been 

 placed on the western side of the needle 

 the pole N would have been elevated, 

 and S depressed ; and the axis of the 

 needle would have been in the position 

 N"S". 



(16.) If the uniting wire, instead of 

 being parallel to the needle, be placed at 

 right angles to it, that is, extending from 

 east to west, whether above or below it, 

 the needle remains at rest, unless it be 

 brought very near to one of the poles ; 

 in which case the pole is elevated when 

 the entrance of the negative electricity 



Fig. 4. 



is from the west side of the wire, and 

 depressed when from the east side. Thus 

 the pole S, fig. 4, is elevated when the 

 current of positive electricity proceeds 

 from p to n ; that is, when the entrance 

 Fig. 5. 



