DOVE ON THE ELECTRICITY OF INDUCTION. 121 



magnetic needle here accord with those of sensation : with the 

 harder kinds of cast iron the effect of inversion is so great, that 

 with two cylinders which compensate each other, when the one 

 is reversed a shock is the consequence. A few examples will 

 show how necessary it is to take into consideration this increase 

 of power by the reversion of position. 



21. The inducing action of soft iron is superior to that of soft 

 steel, and this again exceeds that of hard steel when the excita- 

 tion continues uniform. If no very great difference exists in 

 the action of these latter, and into one spiral of the differential 

 inductor the cylinder of soft steel is placed, whilst the other 

 contains the cy Under of hard steel in a reversed position, the re- 

 markable phaenomenon is observed, that the galvanometer needle 

 deviates in the same direction when the circuit is closed as when 

 it is broken. By inverting the polarity of the hardened steel on 

 closing the circuit, the current which it excites becomes stronger 

 than that caused by magnetizing the soft steel in the same direc- 

 tion as before. On breaking the circuit, however, the soft steel 

 cylinder loses more of the magnetism communicated to it than 

 the hardened cylinder, and hence exerts a more powerful indu- 

 cing action. But as the direction of the current induced by the 

 evanescent magnetism on breaking the circuit is opposed to the 

 direction of the current induced by the magnetism resulting on 

 closing the circuit, the current passes in both cases in a like di- 

 rection through the connected induction-spirals. Similar re- 

 lations were observed with cast iron, with this difference only, 

 that a repeated c\o?,\n^ and breaking of the circuit was requisite to 

 produce this phaenomenon, which, in the case of hardened steel, 

 takes place when the circuit has been once closed for a short 

 time ; and hence it appears that white cast iron in particular 

 offers a greater hindrance to the inversion of its polarity than 

 steel. 



22. The series adduced at a former page would have been very 

 different had not attention been paid to this principle of the 

 increase of power. For, whilst all kinds of cast iron, when 

 the polarity is unchanged, exert a weaker action than mal- 

 leable iron, yet on inversion the action of gray and white iron 

 from the cupola furnace with cold blast is the more powerful; 

 white iron, crucible cast, remains about the same, and gray iron 

 from the crucible furnace and from the cupola furnace with hot 

 blast are below these in power. Thus soft and hard steel, on 



