PHYSICAL SCIENCE. 



xlix 



When these disks were cut with a pair of 

 scissors in various radii, stretching from the cir- 

 cumference nearly to the centre, the force was 

 greatly weakened; but it was nearly restored 

 again, by soldering these intervals with any 

 metal whatever. 



Silver was found to hold a high rank among 

 the metals, and gold a very low one. Mercury 

 lies between antimony and bismuth. The only 

 other substance, besides the metals, in which any 

 traces of magnetism have been perceived, is char- 

 coal, in that state in which it is deposited at a 

 red heat from coal gas, lining the retorts, and 

 gradually filling them. 



Copper revolving produces no effect upon 

 copper. It was always necessary, in order that 

 motion should be produced, that one of the bodies 

 should be a magnet It is obvious that magnetism 

 is introduced into the copper and the other 

 metals by induction. 



The intensity of the directive force of a 

 magnetic needle may be measured, by causing 

 the needle to vibrate in small arches, and count- 

 ing the number of vibrations performed in a 

 given time; the intensity of the directing force 

 being inversely, as the square of the number of 

 vibrations. 



The effect of an electric current upoa a mag- 

 netic needle was accidentally discovered by pro- 

 fessor (Ersted of Copenhagen, in the year 1819. 

 While a galvanic battery was in action, he placed 

 a needle under the wire which united the two 

 poles of the battery, and observed that it placed 

 itself at right angles to the conducting wire. 

 When placed above the conducting wire, it also 

 placed itself at right angles to it ; but in that 

 situation the needle was reversed, the north pole 

 pointing the contrary way from what it did 

 when the needle was under the wire. If the 

 needle be above the conducting wire, the north 

 pole turns to the left of the current of positive 

 electricity. If it be below the conducting wire, 

 the same pole turns to the right. When the 

 needle is placed on the west side of the conduct- 

 ing wire, (supposed to be nearly in the magnetic 

 meridian) and parallel to it the north pole will 

 be elevated ; but when placed on the east side of 

 the conducting wire, the same pole is depressed. 



Arago and Davy observed, about the same 

 time, that the conducting wire, while the galvanic 

 battery is in activity, attracts iron filings ; but 

 the moment the contact is interrupted, the filings 

 drop off; but bodies not. magnetic are not at- 

 tracted by it. 



Arago observed, that if a copper wire be wound 

 spirally round a glass tube, and then made to 

 unite the opposite poles of a galvanic battery, if 

 a steel needle be introduced into the inside of 

 the tube it is instantly converted into a magnet. 



If the copper wire be twisted round a mass of 

 soft iron, shaped like a horse shoe magnet, and 

 made to unite the opposite poles of a galvanic 

 battery in action, the soft iron becomes a power- 

 ful magnet, and retains its virtue while the 

 battery continues in activity ; but if we interrupt 

 the connexion of the copper wire with either of 

 the poles of the battery, the magnetic virtue 

 disappears. 



Next to the discoveries of (Ersted, those of 

 Mr Faraday are the most important. lie first 

 observed that the position of the magnetic needle, 

 with respect to the conducting wire, greatly 

 modified the effects produced. He ascertained 

 that the apparent attraction of the needle on one 

 side, and its consequent repulsion on the other, 

 did not occur in all circumstances ; but that 

 according as the wire was placed nearer to or 

 farther from the pivot of the needle, attractions 

 or repulsions were produced on the same side of 

 the wire. When the wire is made to approach 

 perpendicularly towards one pole of the needle, 

 the pole will pass off on one side, in that direc- 

 tion which the attraction and repulsion at the 

 extreme point of the pole would give. But if 

 the wire be made continually to approach the 

 centre of motion, by either the one or the other 

 side of the needle, the tendency to move in the 

 other direction diminishes; it then becomes null, 

 and the needle is quite indifferent to the wire ; 

 and ultimately the motion is reversed, and the 

 needle endeavours to pass the opposite way. 



From these facts, Mr Faraday concluded that 

 the centre of magnetic action, or the true pole of 

 the needle, is not placed at its extremity ; but in 

 its axis, at a little distance from the extremity, 

 and towards the middle, that this point has a 

 tendency to revolve round the wire ; or, which is 

 the same thing, the wire round the point. He 

 found, upon trial, that the conducting wire of a 

 battery, if at liberty to move, will revolve round 

 the poles of a magnet, while the battery is in 

 action. This discovery has given occasion to a 

 variety of little toys, in which small batteries are 

 made to revolve round the poles of a magnet or 

 magnets round a wire, uniting the opposite poles 

 of a battery in action. 



Mr Faraday has more lately discovered, that 

 when the two poles of a magnet are joined by & 

 piece of soft iron, when one of the poles is dis- 

 engaged from the iron a spark is perceived ; but 

 this spark does not appear when both poles are 

 disengaged together. 



From the phenomena of magnetism just enu- 

 merate I, there cannot be a doubt that electricity 

 and magnetism depend upon the influence of the 

 same principle. What that principle is, may 

 never be discovered. It has been usual to con- 

 sider it as a subtile and elastic fluid, which re- 



