134 



NA TURE 



[December 8, 1904 



INVAR AND ITS APPLICATIONS. 



Preliminary. 

 /Description of Phenomena. — a new material requires 

 -^-^ a new name; that of " invar " has been adopted, on 

 the suggestion of Prof. Thury, to avoid the periphrase 

 " steel containing about 36 per cent, of nickel, which is 

 characterised by possessing an extremely small coefficient 

 of expansion or by the fact that its specific volume is 

 practically invariable when considered as a function of the 

 temperature." The name has been universally adopted, and 

 the title of this article is thus justified. 



The discovery of invar, as is the case with most dis- 

 coveries, was preceded by observations indicating the direc- 

 tion of the researches from which it had its origin. As early 

 as i88g the late Dr. John Hopkinson noted the singular 

 fact of the existence of a ferro-nickel containing about 

 25 per cent, of nickel, the density of which was found to 

 have diminished by about 2 per cent, after cooling to the 

 temperature of solid carbon dioxide; and in 1895 M. J.-R. 

 Benoit, director of the Bureau international des Poids et 

 Mesures, having to determine the length of a metre scale 

 composed of an alloy of iron with 22 per cent, of nickel 

 and 2 per cent, of chromium, was extremely surprised 

 to find that his measurements, made with an extreme range 

 of temperature of about 2 degrees C, gave concordant results 

 only on assuming for the alloy a totally abnormal coefficient 

 of expansion, equal to that of brass, and consequently half as 



Fig. I. — General form of the e.\pansion curve for a reversible nickel-steel. 



great again as that required by the law of mixtures generally 

 applicable to such cases. This alloy was not magnetic, and 

 thus resembled Hopkinson 's alloy before cooling, although 

 the latter after exposure to a low temperature became en- 

 dowed with magnetism. 



It was natural to coordinate these two anomalies and to 

 consider the non-magnetic iron of the second alloy as being 

 very expansible. At the time I considered that the alloy, 

 after being rendered magnetic by cooling, would possess a 

 normal coefficient of expansion ; but as the alloy studied by 

 M. Benoit did not become magnetic either in carbon 

 dioxide or in liquid air, I was forced provisionally to re- 

 nounce this hypothesis. For the liquid air I was indebted 

 to the kindness of Sir James Dewar at a time when liquid 

 air was not obtainable in Paris. I did not, however, 

 abandon this research, and it was in seeking for alloys 

 capable of a transformation similar to that observed by 

 Hopkinson that I was led to examine alloys possessing 

 a negatively abnormal coefficient of expansion. I may add 

 that I was able later perfectly to reproduce Hopkinson 's dis- 

 coveries and to extend them in various directions, but I am 

 unwilling to linger over the details in an article of a practical 

 character, these discoveries having hitherto been fruitless 

 of industrial applications. It will be sufficient to consider 



NO. 1832, VOL. 71] 



later in a brief manner the common cause of the anomaly 

 observed by Hopkinson and of the phenomenon which I have 

 studied. 



Reversible Alloys. — The alloys of iron and nickel which 

 contain more than 25 per cent, of the latter metal may or 

 may not be magnetic, according to the temperature at which 

 they are studied. The passage from one state to another 

 is gradual, the magnetism declining continuously as the 

 temperature is raised, whilst on lowering the temperature 

 the reappearance of the magnetism follows the same curve. 

 The temperature at which the magnetism totally disappears 

 depends on the composition of the alloy. For alloys con- 

 taining from 26 per cent, to 27 per cent, of nickel it is little 

 above 0° C. ; as the proportion of nickel increases it rises 

 very rapidly until a maximum, corresponding with 70 per 

 cent, of nickel, is reached at a temperature fixed by M. 

 Osmond at 550° C, when the curve falls to the transform- 

 ation point of nickel at 340°. This curve of variation is, 

 so to speak, an indicairix of the properties of the alloys ; 

 above the curve the expansion is abnormally great, but at 

 the moment of crossing it with descending temperature the 

 rate of the contraction diminishes, and a region is soon 

 reached in which the anomalous negative expansion exists. 

 Subsequently at a much lower temperature the normal 

 state is reached. The curve given in Fig. i shows the 

 general character of the variation for alloys of this class ; 

 its phases are more or less elongated, the different regions 

 more or less inclined, but the curve always consists of a 

 region of negative abnormality with two confluent curves, 

 one side being characterised by large expansions at high 

 temperatures, the other by a normal expansion. The 

 abnormal region covers generally several hundred degrees. 



The temperature indicated by the abscissa of the point A 

 corresponds sensibly with the ordinate of the indicatrix in 

 question at the point belonging to the same alloy ; in other 

 words, it is at this point that the magnetism finally dis- 

 appears as the temperature rises. 



Curve I shows that it is impossible to assign a general 

 value to the expansion of a particular nickel-steel ; the value 

 chosen must always apply to a definite region and to a more 

 or less extensive range of temperature. If we consider, for 

 instance, the temperatures 0° and 50° C, the two curves 

 of Fig. 2 can be traced, representing at these two tempera- 

 tures the inclination of the tangent to curve i for all the 

 reversible alloys of iron and nickel. It is the minimum of 

 this curve which corresponds with invar, strictly so-called. 

 This minimum will be displaced toward the left for alloys 

 considered at lower temperatures and conversely. 



It should be noted that beyond the minimum the curves 

 cross ; we are then in the region corresponding to the left- 

 hand side of curve i, where the true expansion diminishes 

 with rising temperature. This result of the measurements 

 is of interest because, independently of its being observed 

 for the first time, it has given rise to an interesting appli- 

 cation. 



Theoretical Views. — Without entering into the details of 

 a theory for the development of which I may refer to an 

 article in the Revue ginfrale des Sciences (July 15 and 30, 

 1003), I will indicate at least the source of the phenomena 

 which have been described. 



In the two transformations which take place successively 



