122 Mr. J. Buchanan on the 



ference between the curves obtained by heating and by 

 cooling is partly due to change taking place in the properties 

 of the carbon as the temperature changes. Bearing on this 

 there is an observation I made. Having calculated the neutral 

 point of a carbon-iron couple as about +208° C, a glyce- 

 rine bath was used to verify this result. Having raised the 

 temperature to about 290° C. and cooled again to 220° C. 

 several times, I found the B.M.F. of the carbon-iron couple 

 rose gradually, until at the time of stopping the experiment 

 the maximum E.M.F. had increased by more than 50 per 

 cent. As is well known, a thermo-couple reaches a maximum 

 E.M,F. when the temperature of one joint is that of the 

 " neutral point." It was this maximum that was found to 

 increase with the time during which heat was applied. 



The conclusion is, that the thermoelectric properties of 

 carbon alter with temperature. Principally for this reason I 

 did not deem it of utility to begin a new series of observa- 

 tions, and by "loading" the hot joint with metal try to 

 decrease the irregularities in the readings like those in curves 

 A and B. Moreover, the results calculated from these curves 

 agree as well as could be expected. Here are the details: — 



Curve A. 

 Assuming the equation to the curve as 



I find that T== _ 147 o q ^ K = 00470 



satisfy best the observations as represented by the curve A. 

 # = temperature of hot bath, 

 a= „ cold bath, 



T= neutral point. 

 The agreement of calculated and observed values of y is 

 exhibited by Table I.; these values, it will be recollected, 

 represent centimetres of the bridge-wire. To calculate the 

 E.M.F. of a carbon-platinum thermo-couple as given by 

 curve A, we have the following data : — 



Resistance of primary circuit =303*6 ohms. 



Observed E.M.F. of Daniell used . . . . =1*03 volt. 

 Resistance of 1 centim. of the graduated wire= '00246 ohm. 

 Denoting the E.M.F. required by E A , we get 



-^ AAi^A -. ^o i/^ , 00246 / N A .„ w + a\ 

 E A = -00470 x l-03x W—^*-a)\ltf + -£-J 3 



.-. E A =3-92(^-a)(u7+^y^) C.G.S. units. 



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