Bridge Constrictions Research Articles

Discussion on “Bridge constrictions for Discharge Measurement in Channels”

Discussion on “Bridge constrictions for Discharge Measurement in Channels”

Effect of hexylene glycol-altered microtubule distributions on cytokinesis and polar lobe formation in fertilized eggs of Ilyanassa obsoleta.

Some effects of gravity on early morphogenesis are correlated with microtubule locations within cells. During first cleavage in Ilyanassa obsoleta embryos, a transitory polar lobe constriction forms and then relaxes, allowing the polar lobe to merge with one daughter cell. If the polar lobe is equally divided or removed, morphogenesis is severely disrupted. To examine microtuble locations during early Ilyanassa development, eggs were fixed and stained for polymerized alpha-tubulin during first cleavage. The mitotic apparatus assembles at the animal pole. The cleavage furrow forms between the asters, constricting to a stabilized intercellular bridge encircling midbody-bound microtubules, whereas the polar lobe constriction forms below and parallel to the spindle, constricting to a transitory intercellular bridge encircling no detectable microtubules. At metaphase an alpha-tubulin epitope is distributed throughout the spindle, whereas a beta-tubulin epitope is present predominantly in the asters. Incubation in hexylene glycol, a drug that increases microtubule polymerization, during mitosis causes the polar lobe constriction to tighten around polymerized alpha-tubulin and remain stably constricted. If hexylene glycol is removed, alpha-tubulin staining disappears from the polar lobe constriction, which relaxes, whereas microtubules remain in the cleavage furrow, which remains constricted. These observations suggest that asymmetric distribution of microtubules affects early Ilyanassa cleavage patterns, and that continued presence of microtubules extending through an intercellular bridge is important for stabilization of the bridge constriction prior to completion of cytokinesis. These data provide the basis for further analysis of the role of microtubules in possible microgravity disruptions of Ilyanassa development.

Analysis of bank erosion on the Merced River, Yosemite Valley, Yosemite National Park, California, USA

Channel changes from 1919 to 1989 were documented in two study reaches of the Merced River in Yosemite National Park through a review of historical photographs and documents and a comparison of survey data. Bank erosion was prevalent and channel width increased an average of 27% in the upstream reach, where human use was concentrated. Here, trampling of the banks and riparian vegetation was common, and banks eroded on straight stretches as frequently as on meander bends. Six bridges in the upper reach constrict the channel by an average of 38% of the original width, causing severe erosion. In the downstream control reach, where human use was minimal, channel widths both decreased and increased, with a mean increase of only 4% since 1919. Bank erosion in the control reach occurred primarily on meander bends. The control reach also had denser stands of riparian vegetation and a higher frequency of large woody debris in channels. There is only one bridge in the lower reach, located at the downstream end. Since 1919, bank erosion in the impacted upstream reach contributed a significant amount of sediment (74,800 tonnes, equivalent to 2.0 t/km2/yr) to the river. An analysis of 75 years of precipitation and hydrologic records showed no trends responsible for bank erosion in the upper reach. Sediment input to the upper reach has not changed significantly during the study period. Floodplain soils are sandy, with low cohesion and are easily detached by lateral erosion. The degree of channel widening was positively correlated with the percentage of bare ground on the streambanks and low bank stability ratings. Low bank stability ratings were, in turn, strongly associated with high human use areas. Channel widening and bank erosion in the upper reach were due primarily to destruction of riparian vegetation by human trampling and the effect of bridge constrictions on high flow, and secondarily to poorly installed channel revetments. Several specific recommendations for river restoration were provided to park management.

Observation of ultrahigh critical current densities in high-Tc superconducting bridge constrictions.

Narrow bridge structures were precisely fabricated (using novel ion-beam-milling techniques) on pulsed-laser-deposited ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ films. The bridges have a length scale down to L\ensuremath{\simeq}500 \AA{}. Supercurrents were observed to flow across the bridge structures with critical current densities up to ${\mathit{J}}_{\mathit{c}}$\ensuremath{\simeq}1.3\ifmmode\times\else\texttimes\fi{}${10}^{9}$ A/${\mathrm{cm}}^{2}$, the highest critical current densities yet reported. The current-density magnitudes are consistent with the Onsager-Feynman vortex-ring-creation mechanism (for producing voltages across narrow constrictions) in the length scale region of \ensuremath{\xi}\ensuremath{\ll}L\ensuremath{\le}\ensuremath{\Lambda}, with \ensuremath{\xi} the coherence length and \ensuremath{\Lambda} the London penetration depth. The highest ${\mathit{J}}_{\mathit{c}}$ is in good agreement with the critical depairing mechanism.

Model for small superconducting variable-thickness bridge constrictions

We present a simple model to simulate small three-dimensional superconducting constrictions of variable thickness (VTBs) for which the cross-sectional dimensions are of the same order as the length, and both are less than ξ(T). We study the behavior of the modulus of the order parameterf and the supercurrent densityJs as a function of the various dimensions. We find that the extent to whichf is depressed in the banks depends on the ratio of width to length of the constriction. We show that even for very abrupt geometries,Js is not zero andf does not reach its equilibrium value immediately outside the constriction. On the basis of our results, we propose a more general expression for the effective length valid also when all dimensions are of the same order. The calculated values ofdIc/dT are consistent with experimental data from well-characterized VTBs. We suggest that the discrepancy between the calculated and experimental values ofIcRn may be due to the nonuniformity of the normal current density in the constriction.

Variation with microwave power of the current steps of superconducting microbridges

Observation of periodic variation with power of the current steps in the I-V characteristics of small superconducting bridge constrictions (Anderson-Dayem bridges) in a microwave field is reported. The results are compared with the predictions of the dynamical equations for a Josephson junction; agreement is found near the transition temperature.