MAGNETISM AND TWIST IN IRON AND NICKEL. 527 



Up to this stage, the positive current is associated with the greater positive polarity ; 

 but when the next stage, —45°, is taken, it is the negative current that becomes 

 associated with the greater positive polarity. Somewhere between these two stages a 

 reversal of the current should produce no change at all in the intensity. An attempt 

 was made to catch this exact condition ; but this proved a difficult matter, there always 

 being some small change perceptible. It is evident, in fact, that the magnetic after-effect 

 will have a variable disturbing influence upon the position of this critical stage. In any 

 case we see that it cannot lie far from the stage in the twisting cycle at which the wire 

 changes polarity ; for the first and fourth columns change sign between the same stages. 

 Further, the greatest average polarity for the current reversal seems to occur at or near 

 the position at which the intensity due to the twisting cycle varies most rapidly. 



To sum up : When a nickel wire with a steady current flowing along it has been 

 brought into a cyclic magnetic state by to-and-fro twistings through a given range, the 

 reversal of the current when the wire is at either limit of twist is accompanied by a 

 reversal of the longitudinal intensity. If the current is reversed at any intermediate 

 stage of twist, the accurate reversal of the longitudinal intensity takes place only for 

 certain stages. It may perhaps be said that so long as the back-tivisting is of the 

 character of an un-twisting, there is a certain residual polarity in the wire which is 

 unaffected by the reversal of the current. In such cases in which a nearly symmetrical 

 reversal of longitudinal intensity accompanies the reversal of the circularly magnetising 

 force, we are clearly face to face with seolotropic magnetic susceptibility. No simple 

 theory of rotatable molecules will satisfy the phenomenon. For let it be granted that 

 the effect of twisting a wire circularly magnetised is to impose upon the molecules a 

 certain average pointing along the wire, what warrant is there for supposing that a 

 reversal of the circularly magnetising force could do other than bring back the molecules 

 to a circularly magnetised configuration ? It seems almost as if we should have to 

 assume an seolotropy in the molecules themselves. 



25. Iron and Nickel compared generally. — As regards the kind of polarity 

 established under a given combination of current and twisting, iron and nickel form a 

 contrast. In regard to other particulars, however, they present remarkably similar 

 characteristics. For small twistings the magnetic changes lag behind the changes of 

 strain, there is true magnetic lagging, or the return descending curve in the cyclic graph 

 lies above the ascending curve. For high enough twisting this relation is altered, the 

 lagging changes sign, so that the magnetic changes seem to run ahead of the changes of 

 strain. For a given twisting, the value of the current has very much the same effect 

 for both metals on the form and range of the cyclic graph, whose area tends to a 

 maximum for moderate values of current. The change of sign of this area when with 

 same twisting the current is gradually increased in strength — a phenomenon so beauti- 

 fully shown by the nickel — has not so far been obtained with iron. A careful and 

 prolonged search would probably, however, lead to its detection. For high twistings 

 the area of the cyclic graph, which measures the negative lagging, becomes very great. 



