1821.] Prof Oersted on Elect ro-mrgnetism. 326 



by + s upon the north pole, and by — s upon the south pole, 

 must increase the motion towards the east. 



If the conductor be placed opposite a point in the northern 

 half of the needle, — s in a directed towards the east will not 

 act upon the north pole of the needle ; while the contrary will 

 be the case with — £ in c directed towards the west. It is true 

 that the attraction of + e in a will draw the needle towards the 

 east; but it will act upon a very weak magnetic point, and, 

 consequently, with but little power ; the north pole w ill then go 

 to the west. I do not reckon upon the advantage which the 

 lever — s in c possesses over + £ in a ; for it may be readily 

 shown that this advantage is not necessary to produce the phe- 

 nomenon in question ; it requires only that a magnetic needle 

 should be fixed perpendicularly to one of the ends of an arm of 

 a torsion balance, and that it should be arranged with the gal- 

 vanic connecting wire. When the conductor is so placed that 

 E represents a horizontal section of it, — £ in c will exert some 

 attraction upon the south pole of the needle, 'out + £ in a 

 repulses it with greater force ; this end will, therefore, turn 

 towards the east. I have already shown by the effect of A upon 

 C what will happen when the conductor is opposite the south 

 pole. It may also be readily foreseen, what will happen when 

 the conductor is placed on the western side of the needle p E 

 with respect to the needle F. When the conductor is at D, — e 

 of a will repel the north pole, but + s of c will attract it with 

 more force, the north pole will then turn towards the west, 

 whether the perpendicular conductor be placed on the eastern 

 or the western side. In the same manner, the south pole will 

 in both cases turn towards the west, which will be readily 

 explained by referring to the figure. 



If any one be desirous of seeing the contrary effect of the two 

 sides of the connecting wire in a more direct manner, it is only 

 requisite to give armoveable needle the same magnetism at both 

 ends. It is not that I consider this experiment as necessary 

 after so many similar ones, but it may, perhaps, be very use- 

 ful to represent the thing in the most simple manner in an 

 elementary lecture. With the same needle the experiment may 

 be made with the connecting wire placed horizontally opposite 

 the ends of the needle, and observe the movements from the top 

 to the bottom, or the bottom to the top. In this way it is pos- 

 sible to show in a manner very easy to be understood, the direc- 

 tion of the electro-magnetic forces in the connecting wire ; it is 

 also very easy to perform electro-magnetic experiments by the 

 following arrangement : Let A B, fig. 3, represent a small hollow 

 cylinder, which may be made of paper, and in this a very small 

 cylinder of wood must be made to turn with but little friction. 

 The end D carries the needle S N, and the whole is fastened with 

 a hair or a very fine metallic wire. When the effect of the con- 

 necting wire has been tried upon N, it is only necessary to lurn 

 C D in the cylinder, so that N may be uppermost ; and then the 



