492 On the Secular Softening of Cold Hard Steel. 



for a period of 180 months has produced about the same 

 degree of hardness as 66° acting for the period of 180 minutes. 

 One may infer with certainty, however, that 100° C. acting 

 for a period of minutes produces much greater softening than 

 atmospheric temperature acting for a like period of months. 

 In order to estimate the secular change of a resistance-standard 

 of any metal (cf. § 2), one should previously expose the wire 

 to the tempering effect of 100°, for a number of hours. One 

 may then estimate the secular change of resistance of the 

 untempered standard for any period of months to be very much 

 less than the change of the tempered wire (100 c ) for a like period 

 of minutes. Two hours of annealing at 100° would thus 

 furnish the superior limit of change of the resistance-standard 

 within ten years. The same rule is applicable if the resistance- 

 standard (as is preferable) is made of wire tempered at 100° 

 or more, as far as the limiting degree of softness*. This 

 tempering should be applied even after coiling, in order that 

 all molecular instability may be wiped out as far as 100° at 

 least. 



6. I will conclude with a word on the causes of the changes 

 of temper set forth in the chart. 



The simplest explanation would assume that in hardening 

 a strain has been imparted to the steel rod. This strain 

 vanishes through intinite time more rapidly in proportion as 

 the viscosity of the metal is more fully reduced by tem- 

 perature. At each temperature, therefore, there is a limiting 

 or residual strain corresponding to the reduced value of the 

 viscosity of the rod at this temperature. 



Again, in consequence of the occurrence of recognizable 

 chemical debris within the rod as the result of tempering, we 

 may interpret these curves with reference to Wilhelmy's law, 

 and conclude that they are an expression of increased rate of 

 reaction or of chemical decomposition with increased tem- 

 perature. There is some difficulty, however, in assuming that 

 the limiting softness for each temperature is a case of chemical 

 equilibrium : for whereas the hard molecule during tempering 

 passes easily into the soft molecule, it is not probable that the 

 soft molecule can at low temperatures again change into the 

 hard molecule. 



If, however, the occurrence of the hard molecule is con- 

 ditioned by the strain imparted to steel by sudden cooling, 

 and vanishes contemporaneously with this strain, the expla- 

 nation in terms of the viscosity of the metal is again at hand, 

 and is now in such shape as not to ignore the chemical 



* Tempered at too high a temperature, the wire may receive fresh 

 strain iu cooling. 



