566 REPORT — 1894. 



iiif^ fjas, as is easily proved by calculation. In this wa}' tlie fail of temperature at 

 the ends of tlie strip is perfectly eliminated. The resistance of the strip amounted 

 to 87 ohms at the temperature of 17° C; the variations measured by us were 

 between 1 and 4 ohms. 



In order to find the lowest temperature T., reached by the gas while expanding 

 we have only to measure the smallest value which the resistance of the strip takes 

 during the expansion. We worked in the following way. First, when the 

 balloon is filled with compressed gas of the temperature T,, the Wheatstone bridge 

 is equilibrated so that no current is going through the galvanometer. This state 

 we call the first equilibrium. Then the galvanometer arm is opened and out of the 

 arm of the bridge op])()site to the strip a part of the resistance is taken away, so 

 tliat the resistance of this arm now is lower than that of the strip. Now the gas is 

 allowed to escape out of the balloon, the temperature of the gas is lowered, and the 

 resistance of the strip decreases and approaches that of the opposite arm. Imme- 

 diately after the end of the expansion, at the moment when the strip has the 

 .smallest resistance, we close the galvanometer circuit, and now the galvanometer 

 shows by its deflection if the resistance of the strip at this moment is higher or 

 lower than the resistance of the other arm. Only in the case when both resistances 

 are exactly equal does the galvanometer remain at rest. This state we call the 

 second equilibrium. This second equilibrium is always to be attained by a syste- 

 matic variation of the initial pressure of the gas, and when it occurs the galvano- 

 meter remains at rest for some seconds, sliowing that daring this time there is no 

 appreciable conduction of heat to the gas surrouudmg the platinum strip. 



In this way four corresponding values o{p^,p.,, T,, and T„ are found. 



A small error in these experiments is caused by the heat radiating from the 

 walls of the balloon to the strip cooled by the expanding gas, but it is possible to 

 determine the amount of this error by experiment. For this purpose we executed 

 the experiments described above, once with a simple platinum strip, the second 

 time with the same strip after having blackened it by platinum black. In the 

 second case the error due to radiation is increased in the same proportion, as the 

 absorbing power of blackened platinum is larger than that of the uncovered, and 

 therefore we found a smaller value of T than before. The relation of the two 

 absorbing powers was determined by special experiments, and so we found the 

 correction necessary to remove the error resulting from radiation. 



The gases on which we worked were atmospheric air, oxygen, carbonic acid, 

 and hydrogen. 



The results obtained ■with the naked strip were : 



Air O COj n 



1-3994 1-.394I 1-2940 1-40G3 



The probable error of the result : 



±000024 ±0-00024 ±0-00031 ±0-00020 



To these values we must add the correction caused by radiation : 



+ 00021 

 Consequently we get the values of 



T. 



Air O CO., II 



1-401.-. 1-3902 1-2961 1-4084 



That we found a much greater value for hydrogen than all former experimenters 

 is the best proof of the superiority of our method. All other methods failed with 

 hydrogen because the heat conduction of this gas is very great, and consequently 

 the experiments were not adiabatic. Our method gives also for hydrogen the 

 second equilibrium of the Wheatstone bridge lasting for more than a whole second, 

 showing that also here the expansion is quite adiabatic. 



