ELECTRO-MAGNETISM. 



53 



excited. Induction, therefore, is one of 

 the phenomena of electro-magnetism, 



and must be enumerated among the 

 properties of electricity in motion. 



(U9.) It was discovered both by Sir 

 Humphry Davy and by Mr. Arago, 

 nearly at the same tirne^ that the con- 

 necting wire of a galvanic battery has a 

 sensible attraction for iron filings, and 

 that it will hold them suspended like 

 an artificial magnet, as long as the 

 electric current circulates through the 

 wire ; but the moment the galvanic cir- 

 cuit is interrupted, the action ceases, 

 and the filings immediately fall off. In 

 Sir II. Davy's experiments, the filings 

 adhered to the wire connecting the poles 

 of a voltaic apparatus, consisting of a 

 hundred pairs of plates of four inches, 

 in such considerable quantities as to 

 form a mass round it ten or twelve 

 times the thickness of the wire *. 



The transverse magnetic action of the 

 electric current upon iron in its vicinity 

 is beautifully illustrated by the following 

 experiments, which were devised by 

 Mr. Watkins, and which he was so 

 obliging as to show us. A copper wire 

 of considerable thickness is extended 

 between the poles of a voltaic battery ; 

 and upon sifting overit very gently some 

 fine iron filings, they are observed to 

 adhere to the wire all round its circum- 

 ference, in the form of distinct trans- 

 verse bands, the particles of which mu- 

 tually cohere, as long as the current is 

 maintained. When a broad and thin 

 copper ribbon is substituted for the wire 

 as the conductor of the voltaic electri- 

 city, and iron filings are carefully sifted 

 upon it in small quantities, they are 

 seen to arrange themselves in parallel 

 lines at right angles to the length of the 

 ribbon ; and their magnetic properties 

 are further evinced by the quick changes 

 of position and general disturbance oc- 

 casioned by the approach of a magnet 

 brought underneath the conducting 

 plate. 



(150.) It might naturally be expected 

 from these experiments with soft iron, 

 that steel, under the same circum- 

 stances, would receive a permanent 

 magnetism. Sh\Humphry Davy, hay- 

 ing fastened several steel needles, in 

 different directions, by fine silver wire, 

 to a wire of the same metal, of about 

 the thirtieth of an inch in thickness, and 

 eleven inches long, some parallel, others 

 transverse, above and below, in different 



* Philosophical Transactions for 1821, p. 1>, 



directions, placed them in the electrical 

 circuit of a battery of thirty pairs of 

 plates of nine inches by five, and tried 

 their magnetism by means of iron filings. 

 They were all magnetic ; those that 

 were parallel to the wire attracted 

 filings in the same way as the wire it- 

 self ; but those in transverse directions 

 exhibited each two poles. All the needles 

 that were placed under the wire when 

 the positive end of the battery was east, 

 had their north poles on the south side 

 of the wire, and their south poles on the 

 north side ; while those that were over 

 the wire had their south poles to the 

 south, and their north poles to the 

 north ; and this was the case whatever 

 was the inclination of the needles to the 

 horizon. On breaking the connexion, 

 all the steel needles that were on the 

 wire in a transverse direction retained 

 their magnetism, which was as powerful 

 as ever, whilst those that were parallel 

 to the silver wire appeared to lose it at 

 the same time as the wire itself. 



(151.) All the needles placed trans- 

 versely under the communicating wire, 

 the positive end being on the right hand, 

 had their north poles turned towards 

 the face of the operator; and those 

 above the wire, their south poles. Con- 

 tact with the wires was not at all neces- 

 sary for the magnetization of the needles ; 

 for this effect is produced instanta- 

 neously, by the mere juxtaposition of 

 the needle in a transverse direction, and 

 that through very thick plates of glass. 

 A needle that had been placed merely 

 for an instant in this transverse direc- 

 tion with regard to the wire, was ren- 

 dered as powerful a magnet as one that 

 had long been in communication with it. 



(152.) The intensity of the induced 

 magnetism was found to be propor- 

 tional to the quantity of electricity trans- 

 mitted through the wire in a given time. 

 Hence a wire electrified by a common 

 machine, however powerful, produces 

 no sensible effect ; a feeble magnetism 

 only is obtained by the reception of 

 large sparks : but on passing the dis- 

 charge from a Leyden battery through 

 the wire, the needles placed transversely 

 to the wire are rendered permanently 

 magnetic. The discharge of an electri- 

 cal battery of seventeen square feef, 

 highly charged, through a silver wire of 

 the twentieth of an inch in thickness, 

 rendered bars of steel two inches long 

 and from one-twentieth to one- tenth of 

 an inch in thickness, so magnetic, as to 

 enable them to attract small pieces of 



