Air and Hijdroaen hf/ heating a Platinum Wire. iUI 



For a given expansion the number of nuclei caught increases 

 rapidly with the tenii)erature of the wire after reaching the 

 niininunn temperature at which the nuclei are caught with 

 that expansion, the condensation changing from the form of a 

 small shower, through denser and denser showers, into a fog. 

 Similarly, for a given temperature of the wire, the number of 

 nuclei caught increases with the expansion. 



This shows that the effect of heating the platinum wire in 

 air is to produce condensation-nuclei of all sizes up to a 

 certain size — the maximum size depending upon the tem- 

 perature of the wire. 



The curves shown in PL XL diagrams 1, 2, 3 were got for 

 platinum wire in air by proceeding in the following way. 



An expansion of a definite amount was arranged, and the 

 wire heated for two or three seconds up to a certain tem- 

 perature ; the expansion was then made and the result of the 

 same noted. If no condensation resulted, the same process 

 was repeated several times, wdth the w^ire raised each time to 

 a slightly higher temperature, until condensation took place 

 in the form of a small shower. A similar series of observa- 

 tions were made for a different value of the expansion. The 

 curves show^ the relation between the lowest temperature 

 to which it is necessary to raise the wire in order that 

 condensation may commence for expansions of different 

 amounts. 



The points on the curves corresponding to expansions 

 greater than 1'25 were got by noting at what temperature 

 AVilson's rain-like condensation w^as increased in density by 

 the heating of the wire. 



It is seen from the curves that the production of con- 

 densation-nuclei by the heating of platinum wire in air to 

 about 160° C. can be detected with large expansions. When 

 the temperature of the wire is as high as 300° C, con- 

 densation-nuclei are detected with expansions as small as 1*1. 



If the temperature of the wire be raised to about 400° C, 

 the nuclei are so large and numerous that dense fogs can be 

 obtained with the smallest expansions. 



Residts for Platinum Wire in Hydrogen. 



When hydrogen was first substituted for air, no special 

 precautions were taken to ensure its purity. The result 

 obtained, however, is interesting. At the first heating of the 

 wire nuclei were produced at a tem2:)erature not very different 

 from what was obtained in the case of air ; but after making 

 the wire red-hot for one minute it was found necessary to 

 raise the wire to a much higher temperature (to about 



