ELECTRO-DYNAMICS ELECTRO-MAGNETISM. 



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the gable. In this state of things, if the piece of 

 wood in the square hole, g, is placed in such a posi- 

 tion that the diagonal wire shall connect the upper 

 and lower wires on the gable ; then if the lower ball 

 of the wire, c e, be brought near the ball, d, a connex- 

 tion will be formed between the outer and inner 

 coating of' the jar, and a discharge will take place, 

 but the gable will remain unaltered. If, however, 

 before the discharge is made, the piece of wood in 

 the hole,g-, be turned, so that the diagonal wire shall 

 not connect the upper and lower wires of the gable, 

 then when the discharge takes place, it will be found 

 that the piece of wood in the hole, g, will be thrown 

 out to a considerable distance. Unscrew the ball, d, 

 which will leave the wire pointed, and repeat the 

 experiment with the piece of wood, carrying the 

 diagonal wire in the two positions above described, 

 and it will be found that no cliange takes place in 

 either case, that is, the piece of wood remains sta- 

 tionary in the hole. These experiments clearly show 

 that a house may be protected by a conducting rod of 

 metal, which leads from the top to the bottom of the 

 building, and also that the conducting rod is the 

 more secure from being terminated in a point. 

 Somewhat similar to the foregoing is the thunder 

 house, fig. 13. In this figure the side and part of 

 the roof next the eye are taken away, in order to show 

 the interior. The conducting rod A, a, c, at the 

 gable, differs from that formerly described, in this, 

 that the under part connecting the two windows with 

 the bottom is in one line. The sides and gables are 

 connected with the bottom by means of hinges, and 

 they are kept upright by means of a ridge at the top. 

 The tube resting on the stand, c, is filled with gun- 

 powder, having wires in at each end, and arranged 

 so that when the discharge of the jar takes place, the 

 shock shall pass through the tube, and inflame the 

 powder, in consequence of which the house will be 

 thrown down. 



ELECTRO-DYNAMICS; the science which 

 treats of electricity in motion through a system 

 of conductors ; a name used in contradistinction to 

 electro-statics, or the science of electricity in equilibria. 

 For the facts belonging to this science, see Electro- 

 Magnetism, Electricity, and Galvanism. 



ELECTRO-MAGNETISM ; the name applied to 

 a very interesting class of facts, principally deve- 

 loped by professor Oersted, of Copenhagen, in the 

 year 1819. The power of lightning in destroying 

 and reversing the polarity of a magnet, and of com- 

 municating magnetic influence to iron previously 

 not magnetic, had long been observed, and had led 

 to the supposition that similar effects might be pro- 

 duced by the common electrical or galvanic appara- 

 tus. The first observation of professor Oersted was, 

 that an electrical current, such as is supposed to pass 

 from the positive to the negative pole of a voltaic 

 battery, along a wire which connects them, causes a 

 magnetic needle, placed near it, to deviate from its 

 natural position, and to assume a new one. the di- 

 rection of which depends upon the mode of conduct- 

 ing the experiment. The metallic wire to be made 

 use of, in this experiment, should be two or three 

 feet in length, in order to allow of its being bent 

 or turned by the hands in various directions, and 

 is called the conjunctive wire. When the wire is 

 extended horizontally in the line of the magnetic 

 meridian, with a freely suspended compass needle, 

 whose centre is directly under the wire, the needle 

 instantly deviates from the magnetic meridian, and 

 declines towards the west, under tlrat part of the 

 conjunctive wire which is nearest the negative elec- 

 tric pole, or the copper end of the voltaic apparatus, 

 the amount of declination depending upon the 

 strength of the electricity, and the sensibility of the 



needle. If we change the direction of the conjunc- 

 tive wire out of the magnetic meridian towards the 

 east or the west, no change in the above result takes 

 place, except that of its amount. But if the wire 

 be disposed horizontally beneath the needle, the 

 effects take place in an inverse manner ; i. e. the 

 pole of the needle, under which is placed the portion 

 of the conjunctive wire, which receives the negative 

 electricity of the battery, declines towards the east. 

 When the conjunctive wire is stretched alongside of 

 the needle, in the same horizontal plane, it occasions 

 no declination, either to the east or west : but it 

 causes it merely to incline in a vertical line, so that 

 the pole adjoining the negative influence of the bat- 

 tery on the wire, dips when the wire is on its west 

 side, and rises when it is on the east. If we stretch 

 the conjunctive wire, either above or beneath the 

 needle, in a plane perpendicular to the magnetic 

 meridian it remains at rest, unless the wire be very 

 near the pole of the needle ; in which case it rises 

 when the entrance takes place by the west part of 

 the wire, and sinks when it takes place by the east 

 part. When we dispose the conjunctive wire in a 

 vertical line opposite the pole of the needle, and 

 make the upper extremity of the wire receive the 

 electricity of the negative end of the battery, the 

 pole of the needle moves towards the east ; but if 

 we place the wire opposite a point betwixt the pole 

 and the middle of the needle, it moves to the west. 

 The phenomena . are presented in an inverse order, 

 when the proper extremity of the conjunctive wire 

 receives the electricity of the positive side of the 

 apparatus. The foregoing observations induced 

 professor Oersted to believe that the electric action 

 is not enclosed within the conducting wire, but that 

 it has a pretty extensive sphere of activity around 

 it. He also concluded that this influence acts by re- 

 volution ; for, without such a supposition, it is im- 

 possible to conceive how the same portion of wire, 

 which, placed beneath the magnetic pole, carries the 

 needle towards the east, should, when placed above 

 this pole, carry it towards the west. Such was the 

 nature of the first discovery in electro-magnetism. 

 It was no sooner announced, than the experiments 

 were repeated and varied by philosophers in all 

 parts of the world ; and a multitude of new facts 

 were soon brought to light through the labours of 

 MM. Ampere, Arago, and Biot, in France, and Sir 

 H. Davy, and Mr Faraday, in England. Two very 

 important tacts were ascertained by Ampere and 

 Davy, that the conjunctive wire becomes itself a 

 magnet, and that magnetic properties might be 

 communicated to a steel needle not previously pos- 

 sessing them, by placing it in the electric current. 

 The former of these facts is proved by throwing 

 some iron filings on paper, and bringing them under 

 the wire, when they will immediately adhere to it, 

 forming a tuft around it ten or twelve times the dia- 

 meter of the wire : on breaking the connexion with 

 the battery, however, they immediately fall off, 

 proving that the magnetic effect depends entirely on 

 the passage of the electricity through the wire. The 

 degree of force of this magnetic property thus com- 

 municated to the uniting wire was imagined, by Sir 

 H. Davy, to be proportional to the quantity of elec 

 tricity transmitted through it. Hence the finer the 

 wire, the more powerfully magnetic was it rendered ; 

 and hence, also, a battery of very large plates, such 

 as is used for producing intense heat, and light, was 

 found to give the strongest magnetism to the wire 

 connecting its poles. Accordingly we find that the 

 calorimotor of doctor Hare (see Galvanism), a gal- 

 vanic arrangement, in which the plates are nearly 

 two feet square, exhibits the strongest magnetic 

 effects, and this notwithstanding the powerful heating 



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