16 



OCEAN ATMOSPHERIC -ELECTRIC RESULTS 



the active carriers entering the outer cylinder are 

 brought to the central cylinder. The saturation current 

 produced in the ionization chamber by the active deposit 

 collected on the central cylinder in a given time is then 

 measured. This, combined with a knowledge of the air 

 flow during the collection of the deposit, permits the 

 amount of active material per cubic meter of the air to 

 be estimated, if one assumes a knowledge of the nature 

 of the deposit, which latter can be obtainedfrom the form 

 of the decay curve. 



The Carnegie apparatus was designated radioactive 

 content apparatus 4. The ionization chamber was the 

 same as used on previous cruises, but it was overhauled 

 and put in good working condition before cruise VII began. 

 The high potential generator of previous cruises was taken 

 along on cruise VH and in addition a unit incorporating 

 new and improved features was supplied. Both these were 

 superseded in September 1929 by a still further Improved 

 generator tinit, and this final unit was the one with which 

 successful measurements of radioactive content were 

 obtained. 



The collecting apparatus consisted of an air-flow 

 tube and a central collecting system. The air-flow tube 

 was a copper cylinder 64 cm long and about 20 cm in 

 diameter, with an anemometer at one end and a fan at 

 the other. The fan was driven by an electric motor op- 

 erated from the ship's power plant. The central system 

 consisted of an insulated wooden cylinder 12 cm long and 

 12 cm in diameter, supported by a rod passing through 

 its axis and insulated from it by sulphur. When the col- 

 lecting apparatus was in use, the surface of the wooden 

 cylinder was covered with a sheet of copper foil, held on 

 by rubber bands, and it was on this foil that the deposit 

 was collected. Earthed metal caps, attached to the cen- 

 tral rod, covered the top and bottom ends of the central 

 cylinder without touching it, to insure that the negative 

 charge, and consequently the active deposit, would be 

 confined to the copper foil. 



The collecting apparatus, high potential generator, 

 and accessories, were mounted together as a portable 

 unit which could be moved to suitable positions for col- 

 lecting. The collecting apparatus was operated on the 

 bridge in such a position as to be exposed to the prevail- 

 ing wind. For charging the central cylinder of the col- 

 lecting apparatus a potential of about 2000 volts was re- 

 quired to be supplied by the high potential generator. A 

 form of Kelvin water dropper was used on earlier cruis- 

 es as the high potential generator. In the simplest form 

 of water dropper, a jet of water issues from the nozzle 

 of a water tank, the droplets falling through an insulated 

 metal cylinder mounted below the tank. The cylinder is 

 connected with one terminal of a battery of, say, 100 

 volts, the other terminal of which is earthed. Below the 

 cylinder is mounted a funnel or container which becomes 

 charged by the droplets falling into it, and in practice the 

 potential of the funnel or container rises until the rate of 

 electrical leakage over the supporting insulators equals 

 the rate of supply of electricity by the drops. The origi- 

 nal Carnegie water dropper had, instead of one cylinder 

 and one jet of water, a group of seven smaller cylinders 

 each with a separate jet, and the effect of the multiple 

 jets was shown to be additive, so that the efficiency of 

 the apparatus accordingly was enhanced (12). 



The other type of high potential generator provided 

 at the beginning of cruise Vn had, instead of several jets 

 from which water issued under force of gravity, a single 

 nozzle (atomizer) from which water was forced by com- 



pressed air, thus giving a fine spray of innumerable very 

 small droplets. These droplets, as in the earlier appa- 

 ratus, passed through a charged insulated cylinder and 

 then communicated their charges to an insulated funnel 

 or container below the cylinder. This type of generator 

 was found very efficient under laboratory conditions. 

 Under sea conditions, however, with rolling and pitch- 

 ing of the ship, the spray contrived to reach the insula- 

 tion of the funnel or container and further modification 

 was necessary. 



The final modified form of generator was supplied 

 at San Francisco in August 1929 (fig. 25). On this unit 

 the insulation for the cylindrical container to which the 

 droplets gave up their charge was at the upper end of 

 three supporting posts, within an upper compartment 

 into which no water was sprayed. Drying material in 

 this compartment assisted in keeping the insulators dry. 

 In the figure the generator unit is shown removed from 

 its outer case. The connections for water hose and air 

 hose are readily seen at the top of the unit. One of two 

 black binding-posts on the top plate makes connection 

 with the charged metal cylinder through which the water 

 is sprayed; the other is an earth connection. A third 

 binding-post on the top plate has a metal hood protecting 

 the insulating material, and this post makes connection 

 with the container collecting the charge. 



The ionization chamber of the radioactive content 

 apparatus has been shown in figure 5. The chamber was 

 a cylindrical copper container about 12 cm in diameter 

 and about 25 cm long. The insulated central system of 

 the ionization apparatus was a long thin rod, and it was 

 attached to the fiber system of a unifilar electrometer. 

 The sensitivity of the electrometer was adjusted to be- 

 tween 5 and 10 divisions per volt. When the copper foil 

 was removed from the collecting apparatus, it was 

 formed into a cylinder with the active surface inward, 

 and placed as a lining against the inner wall of the ioni- 

 zation chamber. Installed this way, the foil did not con- 

 tribute to capacitance of th? apparatus. The height of the 

 chamber was made about twice that of the foil cylinder, 

 so that the latter would cover only the middle part of the 

 chamber wall; in this way it was arranged that none of 

 the a particles would strike the top or Iwttom of the 

 chamber. Thus, although the range of some of the a 

 particles would be cut short by their traversing, for ex- 

 ample, a short chord of the cylinder, the average reduc- 

 tion of range brought about in this way would be a defi- 

 nite and calculable function of the radius of the cylinder 

 and of the true range of the cc particles. It would be in- 

 dependent of the distribution of the active material on 

 the foil- -a point of some importance, since the distribu- 

 tion of active deposit on the foil would not be uniform. 



In the measurements with the ionization chamber, 

 the method of allowing for insulation-leak was exactly 

 analogous to that adopted in the case of the eye-reading 

 conductivity apparatus, except that no test could be made 

 after a measurement because the whole internal surface 

 of the ionization chamber would be likely to be covered 

 with the disintegration products of the material origi- 

 nally collected. The observations were recorded on a 

 suitable form (Department form 103). The symbol, tj, 

 used on this form represents the number of pairs of ions 

 produced per second in the ionization chamber, because 

 of active material which would be deposited on the col- 

 lecting cylinder in an air flow of 1 cc per second, tj is 

 recorded on the form for various periods of time after 

 the completion of the deposition, and serves as a 



