60 



PROCEEDINGS OF THE AMERICAN ACADEMY 



ing. I took a piece of the bar used in the preceding experiment, aud 

 heated it in boiling water for twenty minutes. The rod was then 

 tal^en out, wiped dry, placed horizontally across two wooden props, 

 and the thermo-electric junction of copper and iron was placed upon 

 the centre in a deep groove to insure a close connection. As the bar 

 cooled the deflections on the galvanometer scale were read every minute. 

 I performed the experiment four times, but the results were so nearly 

 alike that I take only the last. The following numbers give the ex- 

 cesses of temperature above that of the air every minute: — 



From these numbers we see that if any portion of the bar is at the 

 temperature 8.2 it loses 1.2 units of temperature in one minute, etc. 

 If these losses are laid off as ordinates right below the corresponding 

 temperature, (as given by the ordinates of the first curve,) we shall 

 have the second curve, whose area, for any definite portion, represents 

 the loss of temperature per minute from a<;orresponding portion of 

 the bar. If this second curve is drawn on co-ordinate paper, its area 

 can be found by actual measurement, and we need not obtain its equa- 

 tion. The errors of such a method of measurement will be no greater 

 than those which are liable to enter as experimental errors. 



Making the measurement in the manner indicated above, and using 



the equation, ir= —— ' 



I found for point 2, 



1.97 

 ^ = -2 2Tr4 = -^8. 



For point 3, 

 For point 4, 





From these results it appears that the conductivity decreases as the 

 temperature increases. It has al.so been shown, by Forbes, that the 

 conductivity of iron decreases as the temperature increases. 



