206 



Mr. C. Barus on Maxwell's Theory of the 



graphically shown for the divers tempers, An. 25°, An. 100°, 

 An. 190°, An. 370°, An. 450°. It so happened that the 

 normal is less viscous than the other (An. 450°) rods selected. 

 Hence the negative values of Table II. The axis of abscissa 

 in figure 1 represents Nos. 2 and 3, thus avoiding negative 

 values. Regarding the data for 0=20°, it is to be noted 

 that An. 25° are new wires, whereas An. 100° to An. 450° 

 have undergone twisting in earlier work. Hence the experi- 

 ment exaggerates the size of the interval slightly. The same 

 irregularity affects the distribution of the other degrees of 

 temper An. 100° to An. 450°. 



Returning to Table II., it is clear, inasmuch as viscous 

 deformations are measured differentially, that ($ — </>0/ T an ^ 

 Sq — sJ are to be compared. It appears that these quantities 

 increase and decrease together. This is more easily dis- 

 cernible when rods free from strain are compared*. The 

 exceptions of Table II. are due to the fact that latent strains 

 influence (<£— </>')/ T to a relatively much greater extent than 

 So - 5 o f « Again, if Sxqq—s^' and ((/> — <//)/t be compared at 100°, 

 Table II. shows that in this case also the two quantities in- 

 crease and decrease together. Indeed the data for 100° are 

 the more uniform, a result due to the fact that at 100° much 

 of the latent torsion is made to vanish because of the 

 annealing effect of 100°. Data of even greater uniformity, 

 ccet. par., are to be looked for at higher temperaturef. 



If a comparison is made between the mean rates at which 

 viscous deformation and resistance increase together with 

 temper, at 20° ((<£-<£')/t and *<,-*</) and at 100° ((<£-</>')/V 

 and s l0 o — *2(/)> it i s seen that the mean rate of increase of 

 (<£ — 0')/t relatively to s — s' is about ten times as great at 100° 

 as at 20°. This is the phenomenon in virtue of which the 

 viscous behaviour of steel, regarded as a test of MaxwelPs 

 theory, is almost crucially important. I shall endeavour to 

 explain it. 



10. Dr. Strouhal and I defined the glass-hard state of steel 

 as the stage of temper which undergoes incipient annealing 

 at mean atmospheric temperature:):. Inasmuch, therefore, as 

 annealing is demonstrably accompanied by chemical decom- 

 position §, even at temperatures slightly above mean atmo- 

 spheric, it is clear that the molecular configuration of glass- 



* B. and S., Am. Journal [3] xxxiii. pp. 26, 27, 1887. 



f The words of Maxwell (I. c.) are : — " . . . but if, on account of rise 

 of temperature ... the breaking up of the less stable groups is 

 facilitated, the more stable groups may again assert their sway, and tend 

 to restore the body to the shape it had before deformation." — Maxwell, /. c. 



t Wied. Ann. xi. pp. 962, 963, 1880. 



§ B. and S., Am. Journ. [3] xxxii. p. 276, 1886. 



