1877.] 



and Steel during Heating and Cooling. 



129 



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

 a very dull red heat, a sudden accession of temperature occurred, so that 

 it glowed once more with a bright red heat. It was found that the 

 reheating of the wire occurred simultaneously with the momentary elon- 

 gation." 



In studying these phenomena it seemed, in the first place, desirable to 

 construct apparatus of much greater delicacy and heating-power than that 

 used by either of the previous observers, so that, if possible, the shorten- 

 ing during heating could be shown with the same facility as the elonga- 

 tion 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 with- 

 out the use of mirrors or batteries, and that would admit of experiments 

 being performed either with or without strain. This latter has been 

 accomplished by placing the wire vertically, so as to get rid of all droop- 

 ing when heated. With apparatus thus constructed, we found no diffi- 

 culty in displaying the phenomena in the most perfect manner, the 

 heating-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 pro- 

 duction 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 over- 

 looked with certain kinds of apparatus) to be observed. It i& 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, would 

 fa?our the exhibition of the act ; but it is equally clear that it would ope- 

 rate in an opposite manner when the tendency was to contract, as in the 

 heating ; and therefore, although it would favour the cooling, it would be 

 prejudicial to the heating-kick. 



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

 duction of false quantities, prejudicial to the quantitative comparison of 

 the several effects, it has been carefully avoided in these experiments, suf- 

 ficient weight alone being used to keep the thread tense upon the pulley, 

 and never in any case to elongate the particular wire under use, even 

 when maintained for some time at its highest temperature. 



It is desirable that these facts should be borne in mind ; for no sooner 

 do we enter upon this research with more delicate and reliable apparatus, 

 than we find that the kicks are but a small part of the phenomena we are 

 called upon to observe and explain. We ascertain at the very outset that 

 iron and steel wire presents itself to our examination under very different 

 conditions, which demand a rigid analysis. Thus, apart from its inherent 

 quality, it may come to us for experiment : — 

 (a) In its usual commercial state. 



(6) After having been subjected to various degrees of annealing, 

 (c) In various degrees of induced hardness. 



k 2 



