C. Barus — Large and Small Coronas. 349 



Art. XXXVII. — Alternations of Large and Small Coronas 

 observed in Case of Ldentical Condensations produced in 

 Dust-free Air saturated with Moisture /* by C. Barus. 



1. Apparatus. — By dust-free air, I mean air which has been 

 passed through a packed cotter filter. My filters are 16 inches 

 long, conical, tapering from about 2 inches in diameter at the 

 large end to about one-half inch at the other. They contain 

 absorbent cotton rammed in from both ends and kept in place 

 by wire. When filtered air is required, the stop-cock is only 

 just opened so that influx of dust-free air may be extremely 

 slow. This insures proper filtration and does not interfere 

 with the saturation of the air in the fog-chamber. In this paper 

 condensation was produced in a long glass cylinder, 16 inches 

 from end to end and 5^ inches in diameter, placed horizontally 

 and normal to the line of sight. It contained a rectangular 

 framework of copper wire covered with wet cotton cloth, except 

 on the two opposed broadsides through which the coronas were 

 observed. The distance between the bottom (water) and the 

 roof of the rectangular framework was about 9 centims. The 

 provisions for keeping the air saturated are thus ample. 



The vacuum chamber was a large boiler of galvanized iron, 

 having a capacity V, of over 100,000 cub cm , while the capacity 

 v, of the condensation chamber is about 6,700 cub cm , so that the 

 volume ratio, v/V, is but *063. The two chambers are con- 

 nected by about a foot of rubber tubing over one inch in bore, 

 usually containing a one inch plug gas cock. An instantaneous 

 clapper valve of the same dimensions and opened with a ham- 

 mer was often used for comparison. Later the glass fog-chamber 

 was advantageously replaced by one of waxed wood (cf. this 

 Journal, xlix, p. 175, 1905), with the opposed sides, through 

 which the coronas were observed, made of plate glass. The 

 internal dimensions in this case were 55xl0x20 cub cm , and the 

 volume ratio, v/V, in connection with the vacuum chamber 

 about -13. 



2. Manipulation. — The experiments were conducted as fol- 

 lows: Having selected a suitable pressure difference above 

 that at which condensation in dust-free air just begins (usually 

 termed the fog-limit in the present paper), the dust-free moist 

 air in the closed condensation chamber at atmospheric pressure 

 is suddenly exhausted and the corona measured. After all fog 

 has subsided the exhaustion cock is closed and the filtered air 

 very slowly admitted. The operations are then repeated allow- 

 ing time (about 2-3 min.) for saturation. Under all circum- 

 stances the treatment for large and small coronas was identical. 



In the given apparatus, condensation in dust-free moist air 



began at the pressure difference, 8^ = 22*5 for an atmosphere 



*Read to the Am. Physical Soc, Feb. 25, 1905. 



Am. Jour. Scl— Fourth Series, Yol. XIX, No. 113.— May, 1905. 

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