DOVE ON THE ELECTRICITY OF INDUCTION. 117 



series, making use of another sort of well-varnished wire, 0*70 

 line in thickness, and employing one of Bunsen's carbo-zinc 

 batteries for the production of the magnetizing current. Of 

 wires of this thickness 170 could be placed within the hollows, 

 but they were more than compensated by the cylinder of soft iron, 

 by the gun-barrels, and by the slit tubes of sheet iron, whilst a 

 welded tube of double sheet iron just held them in equilibrium. 

 The reason of the excess of power in the cut tubes above that of 

 the welded tube, may be sought in the greater amount of ex- 

 ternal surface which their elasticity obliges the former to assume. 

 By this mode of experiment the following series was obtained : — - 



Number of wires 0"'*70 

 Substances. thick required for galvano- 



metric compensation. 



Cylinder of soft iron 1 70 -h a? 



Gun-barrel 170 + <» 



Slit tube of double sheet iron 170 -f a? 



Welded tube of double sheet iron 1 70 



Cylinder of soft steel 150 



Cylinder of gray pig iron from a crucible furnace 140 



Cylinder of gray pig iron from a cupola furnace, 



with hot blast 86 



Cylinder of white pig iron, crucible cast • • \ q 



Tube of sheet iron j 



Cylinder of white pig iron from cupola furnace, 



with cold blast 83 



Cylinder of hard steel "] 



Cylinder of gray pig iron from cupola furnace, V 67 



with cold blast J 



Four-sided rod of nickel (4"'- 75 thick) ... 10 



Tube of nickel 



PUe of iron discs separated by paper . 



Pile of steel discs separated by paper .... 2 



Pile of tinned iron discs separated by paper . "| 



Tube of German silver > 1 



Cylinder of fine iron borings J 



15. The galvanometric arrangement of the different kinds of 

 iron I determined in a more direct manner by placing one of the 

 iron cylinders in one spiral, and counteracting its effects by the 

 other eight successively placed in the other spiral ; by the direc- 

 tion given to the needle of the galvanometer, it was ascertained 



:} ^ 



