1821 .] Historical Sketch of Electro-magnetism. 199 



point which before attracted both poles will ncrw repel them both. 

 If when the wire is near the extremity of the needle where the 

 at'traction is strongest, it be moved round the end so as to go 

 from one side to the other, keeping the same point constancy 

 towards the needle, its attractive power over the needle will be 

 found to increase as it approaches the end but remains on one 

 side of it will diminish as it turns the end, will become null 

 when exactly opposed to the pole, and as it passes on the other 

 side will assume repulsive powers which will be strongest at the 

 extremity of the pole on the opposite side to where the wire was 

 situated at first : fig. 4. ■■ 



In all these cases, the positions assumed by the wire and 

 needle whether the result of attraction or repulsion, are the 

 same as those before described, except that the wire is now near 

 the end of the needle instead of the middle as at fig. 5 where 

 there are two positions of the wire, either of which will attract 

 the pole opposite to it, and it will be found that all the attrac- 

 tions and repulsions may be reduced to four positions of the 

 needle to the wire, in which it forms tangents with it: fags, o, 7, 

 show the positions in which the two poles are attracted 1 ; in tig. b, 

 the north pole; in fig. 7, the south pole; if in either of them he 

 poles of the needle be reversed, the tangents remaining in the 

 same direction, repulsion will take place. Hence it is easy to 

 see how any individual part of the wire may be made attractive 

 or repulsive of either pole of the magnetic needle by mere change 



of position. , . , • 



1 have been more earnest in my endeavours to explain tins 

 simple but important point of position, because I have met 

 with a great number of persons who have found it difficult to 

 comprehend; and it constitutes a very important part ol M. 

 Oersted's discovery. Having, however, given the best view ot 

 it I am at present able to do, I will hasten to enumerate some 

 other facts of the discovery. 



The magnetic property does not depend upon the metal 

 employed or its form, but is exerted by any of them which forms 

 the circuit between the poles : even a tube filled with mercury is 

 effectual: the only difference is in the quantity of effect pro- 

 duced. It continues also, though the conductor be interrupted 

 bv water, unless the interruption be of great extent. 



The magnetic influence of the wire extends through all sorts 

 of substances, and acts on the needle beyond, just as with com- 

 mon magnetism. It does not act on needles of brass, glass, or 



gU fn a second paper on this subject, M. Oersted shows that not 

 intensity, but quantity, is wanting in the voltaic apparatus to 

 produce this effect most eminently. A single galvanic arc is 

 sufficient for the purpose. A plate of zinc six inches square 

 placed in a trough of copper, filled with diluted acid enabled 

 the wire which connected the two metals to act powerfully; and 

 with a similar arrangement, the zinc plate havmg a surface ot 



