Changes occurring in Iron Wire at a low red Heat. 475 



cooling is, as might be expected, much lessened, whilst the jerk 

 back is scarcely affected. Increasing the tension of the spring, 

 the forward jerk is correspondingly increased and the back- 

 ward jerk diminishes, and can be made to disappear. 



III. 



Is this anomalous action, then, due to a momentary change 

 in the cohesion of the wire ? If so, at a certain point during 

 the progress of heating, the molecules of iron have a sudden ac- 

 cession in elasticity, and at an approximately corresponding 

 point during cooling, they incur a sudden loss in elasticity. If, 

 however, this molecular change be entirely due to alteration in 

 cohesion, then the removal of the spring ought to cause the 

 anomalous behaviour to disappear. But it does not. Without 

 the spring, an iron wire can be seen by the naked eye to undergo 

 a momentary contraction during heating, and a momentary and 

 more palpable elongation during cooling*. Fixing one end of 

 the wire and bending the other extremity at right angles so that 

 it may dip into a trough of mercury, and thus preserve contact 

 with the battery, both actions can be seen ; the sudden outward 

 thrust on cooling is very conspicuous. Heating the wire by 

 gas-flames, the same result is given. 



All kinds of iron do not exhibit this behaviour ; and some show 

 it in a more or less marked degree. I have not been able to detect 

 any change, in heating or cooling, in certain specimens of good 

 soft iron wire ; but in hard iron wire, and notably in steel wire, 

 it is very apparent. The wire, moreover, requires to be raised 

 to a very high temperature before the jerk is seen on cooling. 

 I have not observed the momentary elongation on cooling when 

 the wire has only been heated to a point just beyond that at 

 which it would otherwise occur. The behaviour of iron wires of 

 different degrees of purity and of widely different thicknesses 

 are points I hope to examine in a subsequent inquiry. I may 

 here also mention that the precise magnetic condition of the 

 iron at the moment at which the jerk occurs, together with its 

 electric resistance and its thermo-electric position f> are ques- 

 tions upon which I have already made some experiments, but 

 not enough to justify the publication of any results at present. 



* A striking lecture experiment may be made by simply'stretching some 

 harpsichord-wire between two supports, and heating the wire to whiteness 

 by a current. On allowing the wire to cool, it gradually straightens itself 

 till just as it reaches the point of obscurity, when it suddenly drops for an 

 instant. It is extraordinary that this action has not been frequently 

 observed. 



t Professor Tait's remarkable investigation on this point is alluded to 

 subsequently. 



