Hasil untuk "Electricity and magnetism"

J. Alan Chalmers, E. W. R. Little

Summary—An account is given of recordings of vertical electric currents in the atmosphere from November, 1938, to August, 1939. General results are given and some special phenomena analysed in detail.

Joseph G. Brown

The idea of an electrified atmosphere was generally accepted in Franklin's time, but the experiments of Ermann and Peltier led to the idea of a negatively charged Earth. Lord Kelvin,1 however, saw that both ideas were correct and pointed out that the electric field of the Earth is similar to that of a condenser. This analogy is not very close since only the negative plate is a conducting surface, while the positive plate is the first 8 or 10 kilometers of the atmosphere. The dielectric, moreover, which corresponds with the positive plate, is an ionized gas and for the potentials involved the “leaks” are very significant.

K. L. Sherman

The atmospheric‐electric program at College, Alaska, during the second International Polar Year extended from October 1, 1932, to August 31, 1933. (1) Continuous registration was made of air‐potential with a recording quadrant‐electrometer and a radioactive collector on a horizontal rod at the variation‐observatory and during most of the time also with a recording bifilar‐electrometer with radioactive collectors exposed by the stretched‐wire method for control at a standardization‐station. (2) Continuous registration was made of air‐conductivity due to both positive and negative ions, using semi‐portable apparatus in which the deflection is nearly proportional to the conductivity; in this apparatus the charge collected by the Gerdien condenser flows to earth through a resistance of about one electrostatic unit and the resulting voltage‐drop through this unit is recorded by a single‐fiber electrometer. (3) Determinations of the number of positive and negative small ions, usually made once each day, using a modified Ebert ion‐counter. (4) Determinations of the number of condensation‐nuclei, once daily from late in March through August, using an Aitken counter; during the cold weather the counter did not function satisfactorily.These data conform with those obtained in lower latitudes at stations on land that are relatively free from variable sources of pollution. The average gradient for all complete days for the eleven‐month period is 104 volts per meter with a maximum in January and a minimum in August. Harmonic analyses of the hourly values show that a 24‐hour component exists in gradient, in substantial agreement with Mauchly's findings. All complete days of positive and negative conductivity yield average values of 179 and 131 × 10−6 electrostatic unit, respectively. The variations are approximately the inverse of those in the gradient although their product—the electrical conduction current—shows regular daily and seasonal variations. The average number of positive small ions by months varied from a minimum of 620 per cc in July to a maximum of 980 in November. The number of negative small ions varied from 520 per cc in July to 660 in November and February. From the ion‐count and the corresponding values of conductivity a mean value of 1.56 cm2 volt−1 sec−1 for the mobility of both the positive and negative small ions was found, although different mobility‐groups are indicated by frequency‐curves. The average of 112 determinations of nuclei is 3760 per cc.The electrode‐effect appears to be quite important at this station. The ratio of positive to negative conductivity varies directly as the gradient, both in long‐period and short‐period changes. This and other features of the results are explicable on the basis of conditions as they would exist in quiet air. At times turbulence is effective in complicating the relations between the elements and so for a complete explanation of these results the effect of mixing must be considered in addition to conditions as they would exist in quiet air.

Franz Exner

Complaints can be made with justice at present that, although in every year new theories of atmospheric electricity are brought forth, the amount of useful observational data, however, increases very slowly. This is largely due to the difficulty experienced in measurements in atmospheric electricity, a difficulty greatly increased by the numerous disturbing influences encountered because of the impurities of the air in cities and in their vicinity.A further difficulty is due to the large variations in atmospheric electricity concomitant with the prevailing meteorological conditions of the atmosphere, in consequence of which it has been found necessary to separate the phenomena observed in clear, normal weather entirely from, those obtained during disturbed weather conditions, such as cloudiness, rain, snow, etc. The latter phenomena are to be regarded as disturbances of the usual normal electric field which prevails during fine weather.

GEORGE FORBES

Franz Exner

IV. VARIATION WITH ALTITUDE OF FALL OF POTENTIAL.That the Earth possesses a negative charge is shown by the generally positive fall of potential. Whether there exist also in the air electric charges, and, if so, whether they be positive or negative, can not be decided by observations made at the surface of the Earth; for this purpose observations in the upper regions alone suffice. If we premise a uniform distribution in the air of the charges in question along horizontal layers, then, in the absence of a charge of the air of its own, the fall of potential should remain constant for the same altitude; in the case of a positive charging of the air the fall of potential must decrease, and for a negative charge, the same should increase. I, therefore, some time ago pointed out the importance of such measurements, and have shown how it would be possible to carry them out in a balloon without conduction with the Earth. Parenthetically it may be remarked, that similar observations on high mountains, on account of the disturbances in the equipotential planes, can not replace, for this purpose, the balloon observations.

I.B. Cohen

N. M. BLIGH

L. F. BATES

James W. Rohlf

D T Edmonds

Abstract One important application of magnetism in biology is the magnetic compass, used by many animals for navigation while migrating. The form of the magnetic field of the Earth and some general facts about the animal compass are described. At this time the transducer used in the animal compass is not known but magnetic induction, the presence of magnetite crystals and the lifetime of free radicals are discussed as possible mechanisms for the compass.

James Clerk Maxwell

Abstract We have seen (art. 380) that the action of magnets on one another can be accurately represented by the attractions and repulsions of an imaginary substance called ‘magnetic matter.’ we have shewn the reasons why we must not suppose this magnetic matter to move from one part of a magnet to another through a sensible distance, as at first sight it appears to do when we magnetize a bar, and we were led to poisson’s hypothesis that the magnetic matter is strictly confined to single molecules of the magnetic substance, so that a magnetized molecule is one in which the opposite kinds of magnetic matter are more or less separated towards opposite poles of the molecule, but so that no part of either can ever be actually separated from the molecule (art.430)

James Clerk Maxwell

Abstract 155.] We have already shewn that when a conducting sphere is under the influence of a known distribution of electricity, the distribution of electricity on the surface of the sphere can be determined by the method of spherical harmonics. For this purpose we require to expand the potential of the influenced system in a series of solid harmonics of positive degree, having the centre of the sphere as origin, and we then find a corresponding series of solid harmonics of negative degree, which express the potential due to the electrification of the sphere.

M R Gavin

F J Pearson