310 Mr. G. Owen on Condensation-Nuclei jyrodaced in 



sulphate placed on it just melted. From these three deter- 

 minations the coefficient B in Callendar^s "^ formula 



was calculated. Knowing S, the value o£ the centigrade 

 temperature corresponding to any value of the platinum 

 temperature TT could be calculated from the above formula. 



Owing to the conditions of the experiment, howcA'er, the 

 temperatures could not be measured to any very great degree 

 of accuracy. For it was found that when a current was sent 

 through the wire, the latter did not attain a steady tem- 

 perature for some time, ranging from a few seconds at the 

 lower temperatures to two or three minutes at temperatures 

 of the order of 1000 degrees, this being due to the air in 

 the cloud-chamber gradually getting heated. To wait after 

 cutting off the current until the air in the cloud-chamber 

 cooled down would involve a delay of some minutes before 

 the expansion could be made, and in this interval the nuclei 

 produced by the heating of the wire would have all dis- 

 appeared by diffusion to the walls of the vessel. 



By using, however^ a dead-beat galvanometer, it was found 

 possible after some practice to so manipulate the resistances^ 

 in the variable arm that the bridge was balanced after the 

 current had been passing for two or three seconds. At the 

 moment of balance the current was cut ofp, and the expansion 

 made a few seconds later. In this way the wire could be 

 heated to different temperatures, which could be fairly accur- 

 ately determined without the air in the cloud-chamber getting 

 heated to any appreciable extent. 



Results for Platinum Wire in Air. 



With the method described above it was found that the 

 wire must be raised to a fairly definite temperature in order 

 that condensation may result on expanding, this minimum 

 temperature depending upon the value of the expansion used 

 to catch the condensation-nuclei produced, the relation being 

 that the lower the temperature of the wire, the greater the 

 value of the expansion required to get any condensation. In 

 other W'Ords, the smaller the expansion used, the higher must 

 the temperature of the wire be raised in order that the 

 production of condensation-nuclei may be observed. 



This also means that the higher the temperature to which 

 the wire is raised, the bigger are the condensation-nuclei 

 resulting from the heating. 



* H. L. Oallendar, Phil. Mag. vol. xlviii. p. 519 (1899). 



