1878.] 



Strain in a Glass Fibre. 



153 



plotted on paper, x t being the ordinate and log the abscissa ; if the 



t 



law be true we should find the points all lying on a straight line 

 through the origin. For each value for T they do lie on straight lines 

 very nearly for moderate values of t ; but if T is not small these lines 

 pass above the origin. "When t becomes large the points drop below 

 the straight line in a curve making towards the origin. This devia- 



A 



tion appears to indicate the form 0(£) = _ J a being less than, but 

 near to, unity. If a =0*95 we have a fairly satisfactory formula. 



x t = A'^T + t^—f^y where A' = A when T=121. 



In the following Table the observed and calculated values of xt when 

 T = 121 are compared, A' being taken as 0*032. 



t i 1 2 3 4 5 7 



^observed.... 0-00979 871 758 697 648 612 556 

 x t calculated. . . 0-00976 870 755 691 643 600 550 



t 10 15 30 65 90 120 589 



^observed.... 497 433 325 212 174 144 18 

 ^calculated... 493 429 320 218 176 147 42 



To show the fact that A' decreases as T increases if a be assumed con- 

 stant, I add a comparison when T = 20, it being then necessary to 

 take A = 0-03 7. 



t 1 2 3 4 5 7 10 



^observed.... 0-00580 470 398 358 327 276 234 

 x t calculated. . . 0-00607 485 422 370 337 285 283 



t 15 25 40 60 80 100 



^observed.... 188 140 311 085 072 066 

 x t calculated. . . 185 125 089 067 052 041 



A better result would in this case be obtained by assuming a = 0*92, 

 or =0'93 in the former case with A' = , 021. Probably the best result 

 would be given by taking A constant, and assuming that a increases 

 with T. 



Taking the formula 0(£)=_ these experiments give values of A 



ranging from 0*0017 to 0'0022. Boltzmann for a fibre, probably of 

 a quite different composition, gives numbers from which it follows 

 that A=0-0036. 



5. In my paper on "Residual Charge of the Leyden Jar" that 



