DOVE OX THE ELECTRICITY OF INDUCTION. 109 



for instance in a closed brass tube, then the bundle of iron wires 

 will represent the magnet s n and the sheath will represent the 

 wire efg h. In a solid bar of iron its surface must be con- 

 sidered the surrounding sheath efg h: s n with its surrounding 

 wire e/ff h is such an electro-magnet. 



3. If by the side of c d a similar combination s' n' and e'f g' h' 

 be placed, then the equilibrium of the currents will be destroyed 

 in a twofold manner ; but from the direction of the resulting 

 current it will be evident which of the two disturbing causes of 

 equilibrium is the more powerful. If these are lessened, either 

 by modifying the stronger sn or the stronger efg h, the equili- 

 brium which had been destroyed may again be restored. The 

 apparatus then becomes a measuring instrument. 



For the purpose of increasing the action, it is convenient to 

 give the magnetizing wires a b and c d, as well as the wires a /3 

 and y 8 in which the inducing action is effected, the form of 

 spirals, the latter being wound in an insulated manner round the 

 former, whilst into the former are placed the bars of iron to be 

 magnetized, as well as that part of the apparatus representing 

 the conductor efg h. 



4. Some of the metallic rods placed within the spirals were 

 cylinders, others four-sided prismalic bay's. The cylinders were 

 of equal dimensions, namely 11 inches 7 lines long and 11^ lines 

 in diameter. There were thirteen of them, composed of brass, 

 tin, zinc, lead, hardened steel, gray cast iron from a crucible fur- 

 nace, gray cast iron from a cupola furnace Avith a hot blast, gray 

 cast iron from a cupola furnace with a cold blast, white cast iron 

 from a cupola furnace wdth cold blast, white cast iron from a 

 crucible furnace, and two cylinders of very soft wrought iron ; 

 besides these, gun-barrels, some cut open lengthways, others un- 

 cut, one brass tube cut open and another entire, a tube of lead, of 

 tin, of German silver, of nickel, a riveted tube of sheet-iron cut 

 open lengthways, — d\\ these had the same dimensions as the 

 cylinders. The ivires composing the bundles were of the same 

 length as the cylinders. Amongst these were four sorts of soft 

 iron wire, having diameters of 0"''70, l"'-02, l"'-46, 2"'-67 ; the 

 first sort was well varnished with shell-lac. Bundles were also 

 formed of soft steel wire 0"'-57 in diameter, of hard steel wire 

 of 0"'-87, and of varnished brass wire of 0"''70, of copper wire of 

 0"'75, of tin wire of l"'-10, of lead wire of 0"''80, of zinc wire of 

 0"''60 diameters; besides these, cylinders were constructed of 



