EFFICIENCY OF FOG CHAMBER. 51 



COLLOIDAL NUCLEI IN DUST-FREE AIR. EXHAUSTION PIPES AND 

 STOPCOCKS FOUR INCHES IN DIAMETER. 



44. Purpose. In the above experiments, the efficiency of the fog 

 chamber in regard to the capture of very small nuclei was successively 

 increased by enlarging the efflux pipe in diameter as far as 2 inches. 

 In the present experiments a further step is taken by increasing the 

 diameter to 4 inches. This final step, however, did not show the marked 

 improvement which might have been anticipated, while the difficulty 

 of manipulating a plug weighing 25 pounds is obvious. The limiting 

 coronas here, as above, were the large green-blue-purple type, and in no 

 apparatus of the present kind has it been certainly possible to exceed 

 this in angular aperture. The use of so large a stopcock introduces 

 other difficulties. It is in the first place nearly impossible to make it 

 quite tight. Provision against the influx of external air may be made 

 in perfection, by aid of an annular oil bath of the above form; but slow 

 leakage from the fog chamber to the vacuum chamber around the plug 

 could not be avoided. This is an inconvenience, though it need not 

 introduce serious error. With a tightly packed filter the air within the 

 fog chamber is never quite at atmospheric pressure, but a few milli- 

 meters below it, so that the exhaustion begins at a lower pressure than 

 76 cm. 



45. Apparatus. This is shown in fig. 26, where F is the fog chamber, 

 V the vacuum chamber, C the 4-inch stopcock between, P the air 

 pump. The pressure within the vacuum chamber is given by the gage 

 G, which also communicates with the air pump. The latter may be 

 shut off by the glass two-way stopcock c, which serves also for the 

 admission of dust-free atmospheric air when pressure is to be lowered 

 through the filter /. The gage g is in communication with the fog chamber 

 by the rubber pipe ab, which contains a lateral branch (not shown) 

 with a fine screw stopcock through which thoroughly dust-free air may 

 be admitted into the fog chamber from a long well-packed filter (not 

 shown) beyond the cock. 



The heavy plug of the stopcock is handled at h and counterpoised 

 by the spring p on a pulley. By properly adjusting the tension (taking 

 care to allow for excess and diminution of air-pressure), the plug may 

 be rotated as easily and quickly as a much smaller valve. There is no 

 evidence that increased speed in the rotation of the stopcock would 

 have increased the efficiency of the fog chamber. The limit reached 

 depends rather on the law of flow, the gradient of which eventually 

 vanishes. The projecting rims, m and n, of the stopcock and lower end 

 of the plug, form annular troughs into which oil or mercury may be 



