122 DOVE ON THE ELECTRICITY OF INDUCTION. 



the inversion of their polarity, exceed in power all kinds of cast 

 iron ; if however the polarity of these latter be reversed, they 

 act more powerfully than soft and hard steel. So, when difFei'- 

 ent sorts of cast iron are compared with each other, the strongest 

 action is always in favour of the kind of cast iron the polarity of 

 which has been reversed. Similar results were obtained with 

 bundles of w'ires. 



23. The foregoing experiments appear to me to explain a cir- 

 cumstance which has frequently been adduced in support of the 

 view, that a retardation of the current increases its physiolo(jical 

 action. The circumstance is this, that the shock is greater when 

 the battery is discharged by sliding the wire than when it is 

 effected by immediate separation. I find that the shock from 

 the induction-spiral is more powerful when the circuit is broken 

 very shortly after it has been closed, and I explain it in this man- 

 ner : the first current which is produced by closing the circuit 

 is not completely gone when the second begins,, as the produc- 

 tion and disappearance of magnetism in iron requires a certain 

 time. The second current therefore meets with a conductor 

 which is being traversed by a current in an opposite direction, 

 and probably this change of direction in the current takes place 

 more quickly in this case than if the conductor had not been 

 previously traversed by any current, as the tendency of the con- 

 ductor to return to its natural unelectric state is aided by the 

 action of the second opposing current. Sliding is nothing more 

 than a quick, often-repeated closing and breaking of the cix'cuit, 

 as may cleai'ly be seen in the dark, and hence its increased phy- 

 siological action. 



2. The action of the current in magnetizing steel compared with 

 its action in magnetizing soft iron- 



24. One hundred feet of wire, surrounded with silk and well 

 varnished, was coiled 200 times round a wooden frame, in 

 which were placed the needles (darning needles) to be magnet- 

 ized, at right angles to the magnetic meridian to which the wire- 

 coils were parallel. The free ends of the induction-spirals, 

 which had been joined crossways, were connected by means of 

 cups containing mercury with the ends of the wire of the frame, 

 and in such a manner that when the galvanic circuit was closed 

 this connection was not established, but always on the repeated 

 bi-eaking of the circuit. Hence the magnetizing action was al- 



