STANDARDIZATION AND EFFICIENCY OF THE FOG CHAMBER. 21 



22. Apparatus. The fog chamber was of the usual pattern, cylin- 

 drical in form, with its axis horizontal. The clear walls, being of blown 

 glass, showed some refraction disturbances, not, however, of a serious 

 character. The fog chamber was connected with a large vacuum chamber 

 by a short, wide passageway, though width is of little consequence here. 

 The cylinder was lined with wet cloth, closely adhering, except at the 

 narrow horizontal windows for observation. 



For exhaustion the stopcock was suddenly opened at the beginning 

 of the first second, closed after 5 seconds, and the corona quickly measured. 

 Filtered air was then at once introduced and the next exhaustion made 

 at the beginning of the sixtieth second. This rhythm is essential. The 

 isothermal value of a drop of pressure [8p 2 ] was carefully predetermined. 

 It fixes the ratio y of the geometric progression of nucleations, since 



where p is the barometric pressure and n the vapor pressure at the given 

 temperature. If the cock were left open for a longer time than 5 seconds 

 [dp 2 ] would increase to the limit dp 3 . 



The goniometer was of the usual type, the eye being at the center 

 or apex, while two needles on radii 30 cm. long registered the angular 

 diameter of the coronal disks or annuli. Formerly the whole instrument 

 was placed 011 the near side of a fog chamber, the eye being about 30 cm. 

 from the nearest wall. It conduces to much greater sharpness of vision, 

 however, and admits of a measurement of larger coronas, caet. par., if 

 the eye is placed all but in contact with the nearer wall and the needles 

 (or in this case preferably the inner edges of round rods) beyond the 

 further wall. In such a case the refraction errors are also diminished. 

 In addition to these advantages I may mention the decidedly increased 

 (about 25 per cent) value of the aperture obtained. These excessive 

 apertures show, however, that the ordinary diffraction equation for 

 coronas is not fully applicable; for aperture varies with the position of 

 the eye along the line of sight. It is often surprising how large a corona 

 can be measured by the second method, in a small fog chamber scarcely 

 6 inches long. The distance between lamp and chamber is kept about 

 D= 250 cm. 



23. Equations. The equations needed in the present work are derived 

 in my last report* and need merely be summarized here. If y is the 

 exhaustion ratio, the nucleation n g of the 2th exhaustion in terms of the 

 original nucleation n Q will be 



where 5 is a subsidence constant and 5 the chord of angular diameter of 

 the coronal disk on a radius of 30 cm. 



*Carnegie Institution of Washington Publication No. 96, 1908, chapters I, in (equat. 

 i to 12). 



