Vertical Potential Gradient Research Articles

III. Investigations on lighting discharges and on the electric field of thunderstorms

The method and apparatus used in the measurements are substantially those described in a paper "On Some Determinations of the Sign and Magnitude of Electric Discharges in Lightning Flashes." The induced charge on an exposed earthed conductor (test-plate or sphere) is used as a measure of the electric field. The testplate virtually forms part of a flat portion of the earth’s surface, and the vertical electric force or potential gradient at ground level is equal (in electrostatic measure) to 4 π Q/A, where Q is the charge on its exposed surface and A is its area. The charge Q on the earth-connected sphere of radius R, when exposed at a height h , great compared with R, is a measure of the potential at that height; the zero potential of the sphere being the resultant of the undisturbed atmospheric potential V at the height h and of the potential Q/R due to the charge on the sphere, so that Q/R = - V. The earthed conductors can be shielded from the earth’s field: the test-plate by means of an earth-connected cover, the sphere by lowering it into a conducting case resting on the ground. The quantity of electricity which flows to earth through the connecting wire on exposing or shielding the test-plate or sphere, is measured by a special type of capillary electrometer in which the readings indicate the total quantity of electricity which has traversed the instrument ; the sign and magnitude of the charge on the exposed conductor, and thus of the potential gradient, at the beginning and end of an exposure are thus determined. The sign and magnitude of sudden changes of potential gradient which occur while the conductor is exposed are indicated by the direction and magnitude of the resulting displacements of the electrometer meniscus. The total flow of electricity between the atmosphere and the test-plate or sphere during an exposure is also measured —being given by the difference between the electrometer readings before and after the exposure. The principal improvement introduced has been the provision of apparatus for giving a photographic trace of the electrometer readings; rapid changes in the field occupying less than one-tenth of a second are in this way recorded. In the observations described in the previous paper the sphere was supported in a manner which did not admit of absolute measurements being made, as the charge measured included that on the upper part of the support as well as that on the sphere itself; in these earlier measurements therefore the sphere was standardised by comparison with the test-plate. The method of supporting the sphere is now such that the charge on the sphere alone is measured, while the disturbing effect of the earthed supporting rod is small, and thus the potential at the level of the earthconnected sphere can be calculated from the charge upon it. The new method of mounting the sphere is shown in fig. 1.

Societies and Academies

LONDON. Royal Society, June 3.—Sir William Crookes, president, in the chair.—Prof. C. H. Lees: The shapes of the equipotential surfaces in the air near long buildings or walls, and their effect on the measurement of atmospheric potential gradients. The shapes of the equipotential surfaces are determined, and the equipotential lines drawn to scale in the following cases:—(1) A thin vertical wall; (2) a retaining wall separating a lower from a higher horizontal plane; (3) a series of equidistant parallel vertical walls. In each case the normal vertical potential gradient may be calculated from observations of the potential at any point. A point on each wall is indicated at which the horizontal potential gradient is identical with the normal vertical gradient.—Prof. O. W. Richardson: The influence of gases on the emission of electrons and ions from hot metals. As is well known, the thermionic saturation current i is expressed accurately and quite generally over wide ranges of temperature by the equation i = ATie-/T. In the case of metals, in particular, the equation is satisfied when the metals are contaminated by the presence of a gaseous atmosphere, as well as when the surfaces of the pure metals are tested. In general, however, the effect of the contaminating gas is to cause large changes in the. values of the constants A and b. The changes which are thus brought about in these constants are considered in the present paper. So far as it may be considered trustworthy, the available evidence shows that A and b for a given metal always change together in such a way that the change in log A is proportional to the change in b. This linear relation is very closely satisfied by the results of all Langmuir's observations with tungsten, for which substance different gases change A by as large a factor as 1012. A similar relation, with an almost equal coefficient of proportionality, is required by the best observations on the negative emission from platinum. In the case of tungsten, contaminants cause an increase in A and b, whereas with platinum a diminution occurs. All the known data point to the existence of a similar law governing the steady emission of positive ions from platinum. By applying thermo-dynamic considerations to the emission of electrons from contaminated surfaces, it is shown to follow from the linear relation between log A and b, tjiat the contact potential difference between the pure and the contaminated metal, is of the form a0(i - aT), where the constant a0 has opposite signs for tungsten and platinum, and a has approximately the same value for both metals. T is the absolute temperature.—Prof. J. W. Nicholson: The band spectrum associated with helium. Fowler has concluded recently that the heads of the bands in the new spectrum associated with, and perhaps due to, helium follow laws of the type hitherto peculiar to line-series. A further examination of some points which were in doubt has been made with the, following results:—(1) The paper supports the conclusion that the heads of the bands in the spectrum of Goldstein and Curtis follow ordinary series laws by showing that the doublet separations actually tend to zero at the limits of the series; (2) both the doublet series isola ted by Fowler are strictly analogous to principal series in line-spectra; (3) the generalised Rydberg formula, in which the wave number is a function of m +, gives the most suitable representation of these series as well as of line-series.—C. Dobell and A. P. Jameson: The chromosome cycle in Coccidia and Gregarines. The authors have investigated the chromosomes of a coccidian (Aggregata eberthi) and a gregarine (Diplocystis schneideri). They have found that the chromosomes are present in the haploid (“reduced”). number-six in Aggregata, three in Diplocystis-at every nuclear division in the life-history except that of the zygote nucleus. This nucleus contains the diploid number of chromosomes-twelve in Aggregata, six in Diplocystis-and its division is a reduction division which halves the chromosome number. Reduction thus occurs in these organisms immediately after fertilisation, and not during gameto-genesis.