December 8, 1904] 



NATURE 



135 



in iron in passing from the o condition to the 3 and 7 con- 

 ditions of Osmond, the metal undergoes different apparent 

 changes, of which the most characteristic are the transitions, 

 in two distinct stages, into the non-magnetic state and a 



MI. 



Fig. 3. — Expansion of iron. 



sudden diminution of the specific volume of the iron at the 

 moment it reaches the higher condition. The expansion of 

 iron up to high temperatures is indicated by a curve such 

 as ABCD, Fig. 3. The addition of 



a little carbon modifies this curve con- ^ 



siderably, as was observed especially 

 by M. Le Chatelier and MM. Charpy 

 and Grenet. The addition of nickel 

 begins to separate the change more 

 and more into two inverse transform- 

 ations, which commence at very 

 different temperatures {Hopkinson's 

 phenomenon) ; as the proportion of 

 nickel increases, the change again be- 

 comes simple, but instead of being 

 sudden, as with pure iron, it is spread 

 out over a wide interval of tempera- 

 tures, at each of which the reciprocal 

 solution of iron in its two extreme 

 states and of nickel strives to attain 

 a stable equilibrium. For the greater 

 part the attainment of equilibrium is 

 practically instantaneous ; it is much 

 more rapid, for example, than that 

 which is observed in an aqueous solu- 

 tion in which large crystals are 

 placed, and resembles rather that 

 which would occur in a saturated 

 solution containing an infinite 



number of crystalline nuclei of the same density as 

 the solution. In a medium thus constituted equilibrium 

 is reached almost instantaneously. The perfect dissemin- 



It is necessary to mention, however, a retardation in a 

 minor part of the change which follows very slowly the 

 principal instantaneous phenomenon. This retardation, 

 due perhaps to a migration of some of the molecules engaged 

 in the change, is rendered visible in the case of invar, 

 strictly so-called, by a gradual elongation with time. It is 

 enormously accelerated by heating the alloy, for example, 

 at 100° C Nevertheless, when a bar of invar has been 

 heated thus it still increases in length very slightly after 

 several years at the ordinary temperature. At the end of 

 five or six years the total elongation is nearly i/ioo mm. 

 per metre, but the subsequent lengthening each year does 

 not exceed a fraction of a micron. 



This phenomenon is of theoretical interest. Practically 

 it restricts the use of invar, and although, by systematic 

 heating, a much smaller limit of variation can be reached 

 than that above indicated, such a change prevents the alloy 

 from being employed in the preparation of standards of the 

 first order. It is necessary to point this out before proceed- 

 ing to consider the apparatus in which invar has introduced 

 decided elements of progress. For a consideration of other 

 qualities which may render it valuable I will refer to in- 

 formation already given in this Journal.- I can describe 

 here only a few of the uses of invar, and will choose three 

 of the most typical.' 



Applicafioiis. 



Standards of Length. — If the slight defect of stability 

 referred to above prevents the employment of invar in the 

 preparation of fundamental standards, the requirements of 

 which are infinite, a wide field of application still remains 

 in the construction of standards which can be referred from 

 time to time to fundamental units, and during these intervals 

 are employed at temperatures which are not readily ascer- 

 tained, as IS the case with the majority of measuring instru- 

 ments which cannot be maintained in a liquid bath. With 

 a brass scale, for instance, an uncertainty of 01 degree C. 



MMnattalMHii 



lUlLillllllUlli^llllllllllUlllllimiHUlllllllllrlllildlilllilliil 



Fig. 4.— Scale at the end of a wire (the divisions are millimeties). 



ation of iron throughout the nickel or the converse is 

 evidently a very important factor of the phenomenon. For 

 Hopkinson's phenomenon the same transformation is still 

 produced, but with an enormous thermal hysteresis. 



Fig. s.— Rolling of a 2 k 



in the temperature introduces an error little less than 2^ per 

 metre of length. But a rod of invar, thoroughly annealed 

 and aged, will not change to the same extent in an interval 

 of three years. The interpolation of definite values up to 

 five or six years can be made with even less uncertainty. 

 Measurements in which the instability of invar will intro- 

 duce an unacceptable error are very rare ; in the case of 

 standards prepared with the usual metals they would corre- 

 spond with errors of temperature which are exceeded in 

 nearly all ordinary measurements. 



But the greatest claim that invar can make to utility is in 

 its application to geodesy ; working in the open air under 

 extremely variable atmospheric conditions makes the deter- 



1 The variation of the rapidity of the change with temperature seems to 

 follow van 't HoflTs law of geometrical progression. 



■- Natuke, No. 1822, September 29, vol. Ixx. p. 527. 



3 A more complete description will be found in my recent work, " Les 

 Applications des Aciers au Nickel." 



NO. 1832, VOL. 71] 



