314 Mr. G. Owen on Condensation Nuclei produced in 



The apparatus was used in the following way : — The piston 

 was allowed to ascend while the two bulbs A and B were in 

 communication. The tap was then closed, and the heating- 

 current sent through the wire, the resistance of which was 

 measured when a steady state had been attained. The effect 

 of the heating is to increase the pressure in B, so that if 

 connexion be suddenly made between the bulbs by opening 

 the tap, a puff of gas carrying with it condensation-nuclei will 

 enter A out of B. The tap is then closed and the expansion 

 made. The small puff of heated air will not appreciably 

 heat the gas in A. 



A further advantage possessed by this apparatus was that 

 the wire not being in the cloud-chamber, no drops fell on the 

 wire. As the current passed through the wire for a longer 

 time than in the other experiments, it was thought that 

 nuclei might possibly be detected at still lower temperatures* 

 The reverse, however, was found to be the case, the expansion 

 and minimum temperature curve for air lying on one occasion 

 between the limits 300° C. and 500° C. This is probably due 

 to the nuclei disappearing as fast as produced, owing to the 

 presence of the hot wire as mentioned before. For this 

 reason the experiments with this method were discontinued. 

 The results got with this apparatus, however, go to jDrove 

 that the production of nuclei depends not so much upon the 

 length of time the wire is heated as upon the maximum 

 temperature attained. 



Effect of an Electric Field. 



As a platinum wire at a dull red heat is known to radiate 

 positive ions, and at higher temperatures negative corpuscles, 

 the effect of putting an electric field on the wire was tried. 

 The cloud-chamber used is shown connected to the expansion- 

 apparatus in PL XL fig. 1. The platinum spiral, connected to 

 earth, was fixed at a mean distance of about 1*5 cm. from the 

 water- surface. By means of a battery of small storage-cells 

 the potential of the water could be raised in steps of 2 volts 

 from ±2 volts to +120 volts. 



It was thought that putting on an electric field between 

 the wire and water-surface while the wire was being heated 

 might lower (or raise) the minimum temperature at which 

 nuclei are caught with any given expansion. It would 

 follow from this that the condensation got (when there is no 

 field) by raising the wire to a temperature a little higher 

 than the minimum temperature would be altered in densit}- 

 if the wire were heated to that temperature with the field on. 



No such effects, however, were observed in air or in 



