Summary In order to determine the origin and nature of the Earth's magnetic field and to test the various hypotheses which have been advanced to explain the field, it is desirable to determine the history of this field throughout geologic time and to investigate more carefully its spatial variations, both inside and outside the Earth's surface. This research is concerned with the determination of the history of the Earth's field as it can be deduced from the present polarization of crustal material. Unconsolidated fresh‐ and salt‐water sediments have been investigated. These sediments are in the form of clays and offer one of the simplest types of polarization, since the clays can be redeposited under laboratory conditions. A particularly lengthy investigation of the polarization of glacial varves has been made, together with measurements on core samples of sediments from the Pacific. From a study of anomalous deposits in the glacial clays, the geologic stability of the polarization of these clays has been established over geologic time. From the measurements of the glacial clays, it is concluded that the Earth's field has not changed substantially in direction or intensity during the last 15,000 years. From measurements of the Pacific cores, it is tentatively concluded that the direction and intensity of the Earth's magnetic field has probably remained substantially constant during the last million years. A much more complete investigation is necessary to verify these tentative conclusions. It would be desirable to extend the measurements to periods of the order of one billion years. These results are consistent with the “fundamental” theory proposed by Schuster, Babcock, and Blackett, but do not provide positive evidence to support this theory.
By means of automatic recorders, the small-ion and large-ion content of the atmosphere at ¾Vashington, D.C., have been secured. The number of large-ions have been continuously recorded over a period of nineteen months and the small-ions over a period of twelve months. Curves of diurnal variation of large ions and of small ions for the various months of the year are drawn. During the cold season of the year, the large ions pass through maxima in the morning and in the evening. During the warm season, only the evening maximum is present which shows a seasonal variation in time of occurrence. The small-ion variation through the day is more or less reciprocal to the large-ion variation, but is considerably smaller when regarded on a percentage basis. This is largely attributable to the fact that a portion of the current in the small-ion counter is contributed by intermediate ions present in the atmosphere.The large ions and relative humidity vary directly during the cold season and in an inverse manner during the warm season. This change in character from one season to another will be a contributing factor to the daily and yearly changes that occur. The mobility of the large ions, as determined in this investigation, is noticeably greater than that ascribed to them by Longerin, thus indicating that the ions in the two places are not identical in size, unless the difference may be attributed to a difference in experimental conditions.
In July and August, 1931, observations were made on the total and uncharged condensation‐nuclei in the free atmosphere on the grounds of the Department of Terrestrial Magnetism of the Carnegie Institution of Washington, in northwest Washington. The nuclei were measured before and after passing through a large‐ion counter between the tubes of which sufficient potential was applied (about 900 volts) to remove all charged nuclei with mobility of 1/3000 cm per second per volt per cm. From 21 sets of measurements, the ratio of uncharged to total nuclei (N0/NA) is found (after applying corrections for loss of nuclei while passing through the apparatus) to be 0.66, the mean value of N0 being 6,300 and the computed value of NA being 9,500. With potentials of 535, 890, and 1890 volts interchanged on the large‐ion tubes, practically no change occurred in No/NA, showing the mobility of the large ion to be not less than 1/1700 cm per second per volt per cm. In October and November, 1931, 77 measurements were made with the large‐ion counter on the number of electronic charges of one sign (positive and negative alternately) per cc in the atmosphere, simultaneously with measurements of total and uncharged nuclei. Small ions were removed from the air entering the large‐ion counter by a small‐ion counter attached to the intake. From these data N0/NA is 0.72, N±/NA is 0.14, N0/N± is 5.8 and the charge per large ion 1.10, the last figure being in good support of the generally accepted view that each large ion carries only one electronic charge. It is seen that NA = N0 + 2N±. The values of N0 and NA are more than double those found in July and August, being 15,000 and 21,000 nuclei per cc, respectively. These results are compared with the work of J. J. and P. J. Nolan, Scholz, Hess, and Israël, and evidence is found for believing that the ratio N0/NA increases in magnitude with increasing nuclei‐content of the air. A quite different analysis of the data obtained in October and November than was made for the above results is undertaken on the basis that N±/NA is not 0.14 but has instead two values, since 23 individual values fall closely around 0.20 and the remaining 54 around 0.10. The immediate conclusion is that the nuclei are at times doubly charged. From this point of view, the data are retabulated and re‐examined and it is found that the results do not show any inconsistencies which would preclude the possibility that doubly‐charged large ions were present almost exclusively on certain occasions and singly‐charged ones almost exclusive on other occasions during the observations here discussed.
Magnetic polarization measurements were made on 99 samples of flat‐lying, undisturbed sedimentary rocks collected at eight sites in the States of Colorado, Utah, Idaho, Wyoming, Washington, and South Dakota. The majority of samples ranged in age from Pliocene to Eocene, being thus from 5 or 10 million to 50 or 60 million years old. A few samples were obtained from Jurassic formations 125 to 150 million years old. The direction of polarization in the plane of bedding, or declination, was found in two‐thirds of the samples to fall in a narrow range centering on geographic rather than present geomagnetic north. Present geomagnetic north at the eight sites ranges from 13° to 22° east of geographic north. The dip or inclination of polarization in two‐thirds of the samples falls in a small range centering on 63°, which compares closely with the value of 69° for the average inclination of the earth's present field at the‐sites sampled. The results, though few, are consistent with the idea that for 50 million years or so the polarity of the earth's magnetic field has been the same as now and the magnetic axis has had an average orientation that coincides with the earth's geographic axis.
Measurements of condensation‐nuclei near the ground were made daily with an Aitken pocket counter from June 1, 1934, to December 31, 1936. Simultaneous observations were made of potential‐gradient and conductivity of the atmosphere, and daily records were kept of rainfall. The data are grouped in monthly means for discussion. In the dry season, when there are only a few days on which rain falls each month, the nuclei are numerous, averaging more than 20,000 per cc, while in the wet season, when there is some rain nearly every day, the average is less than 10,000 per cc. The conductivity of the atmosphere responds to the change in nuclei‐concentration, being conspicuously higher when nuclei are few than when they are numerous. Potential‐gradient shows less response than conductivity to changes in nuclei‐concentration, but in general shows a variation similar to that for nuclei. Computed values of air‐earth current show definite response to changes in nuclei‐content, being smallest when the nuclei are most numerous. This suggests that nuclei exist in sufficient quantities and to such heights above the Observatory as to significantly affect the total resistance of the atmosphere there. Detailed study of the daily observations in the dry month of June in all three years give added support to these conclusions. The data for the three June months are subjected to analysis on the basis of formulas proposed by F.J.W. Whipple and H.L. Wright for determining the size of the nucleus and the combining‐coefficients, η1 and η0, for charged and uncharged nuclei with small ions. The rate of production of small ions is assumed to be ten pairs per cc per second. The radius of the nucleus varies only between 1×10−6 and 3×10−6 cm when nuclei are more numerous than 10,000 per cc. For few nuclei, 1000 or 2000 per cc, the radius becomes 12×10−6 cm. These values of the radius are uncertain to 20 or 30 per cent, depending on the validity of various assumptions in the calculations. Values of η1 and η0 decrease rapidly with increase in nuclei up to 10,000 nuclei per cc. Thereafter η0 is comparatively small (less than 1×10−6) and η1 may be regarded as constant at about 2.5×10−6. Less than one‐fifth of the nuclei are charged, of one sign, when the concentration is greater than 10,000 per cc.
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