ABSOLUTE THERMAL CONDUCTIVITY OF NICKEL. 



381 



we deduct 1'23 from the gradient, 5 66, at 120-65° C. in the latter experiment (date 



1st December 1897), we find the gradient (the remaining 4-43) which would cause the 



me heat to pass the cross-section at 120 '6 5° C. as passes the cross-section at 63 "2° C. 



ith its gradient at 4*33. In other words 4'43 and 4/33 would be corresponding values 

 i the gradients at 120-65° and 63 - 2° respectively if no heat were lost by radiation from 

 • e bar. The conductivities at these two temperatures are inversely as these numbers. 



lis shows a diminution of conductivity of 2^ per cent., with la rise in temperature of 

 i'out 60°. This is within the limits of experimental error. The assumption that the 

 rerage emissivities for temperature excesses of 67'45° and 76° are the same is not 

 kely to be correct. The emissivity in the latter case will be greater, probably by 

 .mething of the order of 2 per cent. The effect of the increase of emissivity with 

 nnperature will be to reduce the apparent diminution of conductivity with rise of 



rnperature, and might even change it into an increase, but in any case it would be 



ry small and within the limits of experimental error. 



The following tables give two sets of data obtained from the curves drawn from the 

 <rrected readings already given in tabular form. ; -. . . 





Corresponding Values of 





 873 





 18-23 



63-2 

 632 



108-5 

 108-5 



cf'e/dx 





 4-33 





 5-335 



Average 

 Temperature 



Excess. 



Corresponding Values at another Section of BarTQf 



67-45 

 76-0 



125-0 

 1308 



38-0 

 20-45 



30-0 



30-55 



120-65 

 120-65 



188-7 

 188-7 



de/dx 



3-55 

 5-66 



6-00 

 8-13 



From these are deduced the following 



orresponding values of 6 and dOjdx which 

 would be found if no heat were lost from 

 surface of bar, 



Ditto, 



63-2 

 120-6 



108-5 

 188-7 



ddjdx 



4-33 

 4-43 



5-335 

 5-55 



These figures indicate a diminution of conductivity of the amount "000066 per rise 

 o temperature of 1° C. The conductivity cannot fall so much as this, and in any case 

 t i change of conductivity with temperature is within the limits of error of such experi- 

 ments up to a temperature of 200° C. J - 



§ 13. Conclusion. — The conductivity of nickel found by the direct method is -132. 

 'lere is some doubt about the third figure after the decimal point, and that figure is the 

 <> y one affected by changes of temperature up to 200°C. It is interesting to note that 



VOL. XXXIX. PART II. (NO. 12). 3 M 



