in Iron and Steel during Heating and Cooling. 391 



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 backward jerk diminishes and 

 can be made to disappear. AVithout the spring, an iron wire can 

 be seen by the nalced eye to undergo a momentary contraction 

 during heating, and a momentary and more palpable elongation 

 during cooling. 



" 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 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 tem- 

 perature before the jerk is seen on cooling. I haA^e 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. 



" During the cooling of the wire it was found that just as it 

 reached a very dull red heat, a sudden accession of temperature oc- 

 curred, so that it glowed once more with a bright red heat. It 

 was found that the reheatiug of the wire occurred simultaneously 

 with the momentary elongation." 



In studying these phenomena it seemed, in the first place, de- 

 sirable to construct apparatus of much greater delicacy and heating- 

 power than that used by either of the previous observers, so that, if 

 possible, the shortening during heating could be shown with the 

 same facility as the elongation during cooling. This has been 

 achieved by means of the instrument a sketch of which is given 

 with the Charts. The main idea has been to construct an apparatus 

 which would exhibit the phenomena readily without the use of mir- 

 rors or batteries, and that would admit of experiments being per- 

 formed either with or luithout strain. This latter has been ac- 

 complished by placing the wire vertically, so as to get rid of all 

 drooping when heated. With apparatus thus constructed, we 

 found no difficulty in displaying the phenomena in the most 

 perfect manner, the heatiiig-kick not unfrequently being three or 

 four centimetres in length, and the cooling-kick sometimes as much 

 as 13 centimetres. 



It is necessary to state in limine that strain plays no part in the 

 production of the phenomena of the kicks or jerks. Its only 

 effect is to cause a very small influence in the cooling (which might 

 have been overlooked with certain kinds of apparatus) to be ob- 

 served. It is obvious that when a wire has a tendency to expand, 

 the presence of weight or tension, by assisting to overcome the 

 inertia of the apparatus, M'ould favour the exhibition of the act ; 

 but it is equally clear that it would operate in an opposite manner 

 when the tendency was to contract, as in the heating ; and there- 

 fore, although it would favour the cooling, it would be prejudicial 

 to the heating-kick. 



Strain, therefore, being unessential to the phenomenon, and, by the 

 introduction of false quantities, prejudicial to the quantitative com- 

 parison of the several effects, it has been carefully avoided in these 

 experiments, sufficient weight alone being used to keep the thread 

 tense upon the pulley, and never in any case to elongate the par- 



