Imperfect Symmetry Research Articles

Magnetic anomaly sequence in the central North Atlantic

Marine geophysical investigations in 1970 and 1971 have established the sequence of sea-floor-spreading magnetic anomalies within the corridor extending from Cape Hatteras in North America to Cap Blanc in Africa. The standard ‘Tertiary’ anomalies out to 25 or 26 (64–66 m.y. B.P.) as well as the J and K anomalies can be recognized and correlated among the four traverses. Displacements of the anomalies suggest a complex fault pattern that may be associated with the Kane fracture zone. A spreading chronology for the past 60–65 m.y. has been derived by modeling: the overall relative spreading rate is of the order of 1–1.2 cm/yr, the slower intervals ranging from 10 to 15 m.y. B.P. and being centered about 30 m.y. B.P.; a possible increase in spreading rate is marked at about 60 m.y. B.P. The imperfect symmetry of the magnetic anomaly profiles about the mid-Atlantic ridge as well as the low amplitude and the poor definition of the individual anomalies are, according to hypotheses of Matthews and Bath (1967) and Vine and Morgan (1967), consistent with formation of the ridge by dike injection over a relatively wide zone.

An experimental investigation of the stability of Poiseuille flow

An axially symmetric laminar flow of air was established in a long smooth pipe. This flow was steady up to Reynolds numbers of about 20,000, the capacity of the system. Small, nearly axially, symmetric disturbances were superimposed by longitudinally oscillating a thin sleeve adjacent to the inner wall of the pipe. Hot-wire anemometer measurements consisting of radial and longitudinal traverses were made downstream of the sleeve. These measurements indicated that within the Reynolds number range investigated (up to 13,000), the flow is stable to small disturbances. In general, the radial distribution of disturbance amplitudes was not independent of distance downstream; while the disturbances, as generated, exhibited imperfect axial symmetry, the non-symmetric part decayed more rapidly than the symmetric part. Results were interpreted in such a way that rates of propagation and rates of decay of the disturbances could be compared with those given by a recent theoretical stability analysis. It was found that the rates of decay are predicted fairly satisfactorily by the theory; however, the rates of propagation are not. In addition, it was found that transition to turbulent flow occurs whenever the amplitude of the disturbance exceeds a threshold value which decreases with increasing Reynolds number. Due to the departures from axial symmetry in the amplitude of the disturbance, it was not possible to obtain a quantitative measure of the threshold. A mathematical idealization of the disturbances, believed to be more akin to experimental perturbations than the classical model used in small-perturbation analyses, is proposed.

A New Technique for Preparing Monocrystalline Metal Surfaces for Work Function Study. The Work Function of Ag(100)

A new method of experimental attack upon the problem of the structural dependence of the work functions of the metals is described. The method utilizes the results of recent electron diffraction studies which show that many cubic lattice metals when thermally vaporized and condensed on heated cleavages of rocksalt form films which are in effect single crystals with a (100) plane parallel to NaCl (100). With silver as a typical metal of this class, there has been developed a technique of preparing an Ag(rocksalt) film in the measuring tube itself and of then determining its contact difference of potential against (1) polycrystalline silver films deposited on glass at room temperature, and (2) a reference metal of known work function, barium. The silver was fractionally distilled and only the middle fractions deposited on the glass and rocksalt targets, the barium fractionally and multiply distilled and its discarded fractions used to getter the tubes in which the measurements were made. The contact p.d. between Ag(rocksalt) and Ag(glass) has been found to range from 0.12 v in a preliminary measurement to 0.32\ifmmode\pm\else\textpm\fi{}0.03 v in final measurements made after the technique of preparing the Ag(rocksalt) films had been perfected. The contact p.d. for Ag(glass) ---Ba is found to be 1.95 v, in close agreement with our earlier results at liquid-air temperature. Work function values computed from these measurements and the work function of Ba, 2.52 ev, are 4.47 ev for Ag(glass) and 4.79 ev for Ag(rocksalt). Since two independent electron diffraction investigations, both especially complete for silver, indicate that Ag(rocksalt) films formed under the conditions of the present experiments invariably approximate closely to the ideal (100) structure, and since the contact potential method measures the average work function of a surface without weighting vestigial regions of imperfect symmetry, our Ag(rocksalt) work function value is identified tentatively with the work function of Ag(100). Our Ag(100) value is in good agreement with Farnsworth and Winch's recent photoelectric measurements on a massive single crystal but Winch's earlier value of 4.74 ev for the work function of a heat-treated silver wire is probably characteristic of the crystal faces of preferred orientation developed by thermal etching rather than of randomly oriented polycrystalline silver. The method developed in the present work for measuring Ag(100) appears to be directly applicable to the (100) orientations of Ni, Cu, Au, Pd and Al.

Symmetry of “Aurelia Aurita”

DURING the last few months I have seen countless thousands of living specimens of Aurelia aurila, and have paid special attention to abnormal varieties. I have found not only such as have throughout the five-fold symmetry, seen by Mr. Unthank at Brightlingsea, as named in NATURE for August 22, but have with me on the Glimpse specimens stained and mounted as lantern slides having entire three-fold and entire six-fold symmetry, and one in which it is partially two-fold. I think it may be said that in Suffolk and Essex, a few such abnormal varieties occur per thousand of the normal. An imperfect four-fold symmetry is much more common.