Evaporation of different Liquids at their Boiling-points. 43 



7. Second Series of Experiments. June 1894. 



Up to this time the resistance of the platinum spiral 

 during the passage of the current had never been directly 

 measured, but was calculated from the cold resistance and the 

 coefficient of increase with rise of temperature, on the assump- 

 tion that the temperature of the spiral was the same as that 

 of the surrounding liquid. The truth of this assumption was 

 now called in question, and it was found to be unjustifiable. 

 The resistance of the spiral was directly measured, first when 

 surrounded by a liquid at its boiling-point, then when the 

 current was passed so that the liquid boiled. 



The resistance was always found to be greater when the 

 current was passing ; that is to say, the temperature of the 

 spiral is always higher than that of the liquid boiling round 

 it*. One does not exactly know what conditions govern the 

 difference of temperature between spiral and liquid. The 

 probability is that the ratio of the resistances when two liquids 

 are compared will be altered, as well as the absolute values of 

 the resistances themselves ; and this would tend to alter all 

 the results hitherto obtained. 



It should also be observed that the coefficient of increase of 

 resistance of platinum with rise of temperature is that given 

 by Callendar (1*338; Phil. Trans, clxxxii. A, p. 136) for a par- 

 ticular sample of wire employed by him ; but as this coefficient 

 is found to be different for different samples of wire, the value 

 adopted may very probably not hold for the particular wire 

 here used. 



It was therefore necessary to determine directly the re- 

 sistance of the platinum spirals in each experiment. The 

 resistance of the wire during the passage of the current was 

 now determined by PoggendorfFs method : — viz., comparison 

 with the resistance of a standard wire carrying the same 

 current. [For diagram of arrangements vide infra, p. 45.] 



The standard resistance was a piece of uninsulated german- 

 silver wire, immersed in cold water to keep its temperature 

 constant. 



Table III. shows the results obtained. 



* Griffiths, Phil. Trans. clxxxiv.A (1893), pp. 400-407. 



