Showing papers by "University of Cambridge published in 1971"

Surface energy and the contact of elastic solids

Abstract: This paper discusses the influence of surface energy on the contact between elastic solids. Equations are derived for its effect upon the contact size and the force of adhesion between two lightly loaded spherical solid surfaces. The theory is supported by experiments carried out on the contact of rubber and gelatine spheres.

Simple memory: a theory for archicortex.

Abstract: It is proposed that the most important characteristic of archicortex is its ability to perform a simple kind of memorizing task. It is shown that rather general numerical constraints roughly determine the dimensions of memorizing models for the mammalian brain, and from these is derived a general model for archicortex.

Axisymmetric Black Hole Has Only Two Degrees of Freedom

Abstract: A theorem is described which establishes the claim that in a certain canonical sense the Kerr metrics represent "the" (rather than merely "some possible") exterior fields of black holes with the corresponding mass and angular-momentum values.

Gravitational radiation from colliding black holes

Abstract: It is shown that there is an upper bound to the energy of the gravitational radiation emitted when one collapsed object captures another. In the case of two objects with equal masses $m$ and zero intrinsic angular momenta, this upper bound is $(2\ensuremath{-}\sqrt{2})m$.

Capitalism and modern social theory : an analysis of the writings of Marx, Durkheim and Max Weber

Abstract: Giddens's analysis of the writings of Marx, Durkheim and Weber has become the classic text for any student seeking to understand the three thinkers who established the basic framework of contemporary sociology. The first three sections of the book, based on close textual examination of the original sources, contain separate treatments of each writer. The author demonstrates the internal coherence of their respective contributions to social theory. The concluding section discusses the principal ways in which Marx can be compared with the other two authors, and discusses misconceptions of some conventional views on the subject.

The Evolution of the Indian Ocean since the Late Cretaceous

Abstract: Summary All available ship and aeroplane tracks across the Indian Ocean were searched for identifiable magnetic anomalies and transform faults, and hence the age and direction of motion at the time of formation of about two-thirds of the floor of the ocean established. The magnetic lineations show that India moved away from Antarctica at about 18 cm/y for 20 My in the Early Tertiary. This rapid motion ceased in the Eocene and was followed by a period in which little or no spreading took place west of the Ninety East Ridge. Australia separated from Antarctica during this period. The present spreading episode began about 36 My ago. This detailed study has permitted instantaneous poles of rotation to be obtained, and has established that Africa is now moving northward at 2cm/y relative to Antarctica in the South West Indian Ocean. The evolution of the triple junction between the South East, South West and Central Indian Ridges is clearly reflected in the topography and magnetic lineations. The depth of parts of the ocean formed since the Late Cretaceous increases with age in the manner expected from the temperature structure of a cooling plate, and supports the evolution determined from the magnetic lineations in a most remarkable way. Heat flow observations are more scattered but also consistent with the same thermal model. The proposed evolution agrees with the distribution of known continental fragments and with the Late Cretaceous palaeomagnetic poles from surrounding continents and one obtained from the shape of the magnetic lineations south of India. It is, however, not yet clear how to reconstruct Gondwanaland from the Late Cretaceous reconstructions.

A spherical envelope approach to ciliary propulsion

Abstract: In this paper, an attempt has been made to model the dynamics of ciliary propulsion through the concept of an ‘envelope’ covering the ends of the numerous cilia of the microscopic organism. This approximation may be made in the case when the cilia are close together, as can occur in the case of the symplectic metachronal wave (i.e. the wave travels in the same direction as the effective beat). For simplicity, a spherical model has been chosen, and the analysis which follows is a correction to Lighthill's (1952) paper on squirming motions of a nearly spherical organism. The velocity and efficiency compared to the work done in pushing an inert organism are obtained, and compared to that of a ciliated organism.

A note on the image system for a stokeslet in a no-slip boundary

Abstract: The velocity and pressure fields for Stokes's flow due to a point force (‘stokeslet’) in the vicinity of a stationary plane boundary are analysed, using Fourier transforms, to obtain the image system required to satisfy the no-slip condition on the boundary. The image system, which is illustrated by diagrams, is found to consist of a stokeslet equal in magnitude but opposite in sign to the initial stokeslet, a stokes-doublet and a source-doublet, the displacement axes for the doublets being in the original direction of the force. The influence of the wall on the near and far fields is discussed. In the far field it is found that a stokeslet aligned parallel to the wall produces a stokes-doublet far-field, whereas a stokeslet normal to the wall produces a combination of a source-doublet and a stokes-quadrupole far-field. Although results can be alternatively derived by the method of Lorentz (7) using a reciprocal theorem, the present method yields much more clearly the form of the image system.

The stress generated in a non-dilute suspension of elongated particles by pure straining motion

Abstract: In a pure straining motion, elongated rigid particles in suspension are aligned parallel to the direction of the greatest principal rate of extension, provided the effect of Brownian motion is weak. If the suspension is dilute, in the sense that the particles are hydrodynamically independent, each particle of length 2l makes a contribution to the bulk deviatoric stress which is of roughly the same order of magnitude as that due to a rigid sphere of radius l. The fractional increase in the bulk stress due to the presence of the particles is thus equal to the concentration by volume multiplied by a factor of order l2/b2, where 2b is a measure of the linear dimensions of the particle cross-section. This suggests that the stress due to the particles might be relatively large, for volume fractions which are still small, with interesting implications for the behaviour of polymer solutions. However, dilute-suspension theory is not applicable in these circumstances, and so an investigation is made of the effect of interactions between particles. It is assumed that, when the average lateral spacing of particles (h) satisfies the conditions b [Lt ] h [Lt ] l, the disturbance velocity vector is parallel to the particles and varies only in the cross-sectional plane. The velocity near a particle is found to have the same functional form as for an isolated particle, and the modification to the outer flow field for one particle is determined by replacing the randomly placed neighbouring particles by an equivalent cylindrical boundary. The resulting expression for the contribution to the bulk stress due to the particles differs from that for a dilute suspension only in a minor way, viz. by the replacement of log 2l/b by log h/b, and the above suggestion is confirmed. The relative error in the expression for the stress is expected to be of order (log h/b)−1. Some recent observations by Weinberger of the stress in a suspension of glass-fibre particles for which 2l/h = 7·4 and h/2b = 7·8 do show a particle stress which is much larger than the ambient-fluid stress, although the theoretical formula is not accurate under these conditions.

Spatial-Frequency Channels in Human Vision*

Abstract: Psychometric functions were determined concurrently for detection of simple gratings (luminance sinusoidally modulated with spatial frequency f) and complex gratings (luminance modulated by the sum of two sinusoids, with frequencies f and f′). Results were used to test the hypothesis that the two components of a complex grating may be detected independently. In an extensive experiment with f = 14 cycles/deg, the independence hypothesis was consistently rejected only when f/f′=54 or 45, but rarely rejected when the value of f/f′ lay outside this range. In other experiments, f was between 1.9 and 22.4 cycles/deg. All results are compatible with the assumption that the human visual system contains sensory channels, each selectively sensitive to different narrow ranges of spatial frequencies, whose outputs are detected independently.

On the frequency dependence of conductivity in amorphous solids

Abstract: The literature on experiments on a.c. conductivity in various disordered semiconductors is briefly summarized. A theory of the a.c. conductivity, suitable for hopping conduction in the pseudogap of amorphous semiconductors, is developed, and expressions applicable under various conditions are derived. Comparing some of the results with available experimental data, the concentration of localized states in the pseudogap of several materials is estimated.

Identification of 1,25-Dihydroxycholecalciferol, a New Kidney Hormone controlling Calcium Metabolism

Abstract: A vitamin D metabolite, 25-dihydroxycholecalciferol, is further hydroxylated by kidney before acting as a hormone on target tissues. The structure of this kidney metabolite is now described.

Survival of mouse embryos after freezing and thawing.

Abstract: COMPARED with the successful storage of spermatozoa and other tissue cells at sub-zero temperatures, attempts to preserve frozen mammalian eggs have been disappointing (reviewed in refs. 1 and 2), although a few pregnancies have resulted from the transfer of previously frozen fertilized rabbit eggs3 and unfertilized mouse eggs4 to recipient foster mothers. I now wish to describe a technique for obtaining a high proportion of viable embryos after subjecting them to sub-zero temperatures.

Gaba uptake in rat central nervous system: comparison of uptake in slices and homogenates and the effects of some inhibitors

Abstract: — Fifty-two substances were tested as inhibitors of the uptake of [3H]GABA in slices of rat cerebral cortex. Among GABA analogues tested, only the 2-fluoro, 3-hydroxy and 2-amino compounds had affinities for the uptake mechanism comparable to that of GABA. [3H]GABA uptake was also potently inhibited by p-chloromercuriphenylsulphonate, N-ethylmaleimide, chlorpromazine and haloperidol. No inhibitors were found to act in a competitive manner with respect to GABA. [3H]GABA uptake was also examined in homogenates of cerebral cortex and other regions of CNS. There was a rapid uptake of [3H]GABA into particles when homogenate samples were incubated with the labelled amino acid; this uptake had similar kinetic properties and inhibitor sensitivity to that observed in slices of intact tissue. Density gradient centrifugation experiments indicated that the particles responsible for the uptake of [3H]GABA in homogenates were probably synaptosomes. Uptake of [3H]GABA also occurred in slices and homogenates of rat spinal cord, and evidence was obtained by the simultaneous labelling of homogenates with [14C]glycine and [3H]GABA that these two amino acids were taken up by different nerve terminals in this region.

The measurement of amino groups in proteins and peptides

Abstract: A technique is examined for determining amino groups with 2,4,6-trinitrobenzenesulphonic acid, in which the extinction at 420nm of sulphite complexes of the trinitrophenylated amino groups is measured. The sensitivity of the method is 5-200nmol of amino group. The method is especially suitable for checking the extent of blocking or unblocking of amino groups in proteins and peptides, owing to the short time required for reaction (5min at room temperature). The reaction of the reagent with thiol groups has been studied and was found to proceed 30-50 times faster than with in-amino groups of model compounds. The in(420) of a trinitrophenylated thiol group was found to be 2250m(-1).cm(-1). The reaction with several amino acids, peptides and proteins is presented. The in(420) of a typical alpha-amino group was found to be 22000m(-1).cm(-1) and that of an in-amino group, 19200m(-1).cm(-1). Difficulties inherent in the analysis of constituent amino group reactions in proteins are discussed.

Aeronautics before 1919

Abstract: Professor Taylor describes his aeronautical experiences during the First World War and just afterwards.

The thickness, composition and structure of some lipid bilayers and natural membranes.

Abstract: It has been shown that the capacitance, thickness and composition of black lipid films may depend strongly on the hydrocarbon solvent used in their formation. By the use of n-hexadecane, films have been formed which contain effectively no solvent and which are comparable to the leaflets of the mesomorphic phase of the pure lipid. These films have capacitances of ca. 0.6 μF/cm2 and hydrocarbon thicknesses of ca. 31 A. Thinner black films of higher capacitances are also described.

An Ultrasonic Study of the Area of Contact between Stationary and Sliding Surfaces

Abstract: This paper describes a novel attempt to determine quantitatively the area of contact between stationary and sliding contacts by means of an ultrasonic technique. The basic idea is to generate an ultrasonic wave in one body and measure the impulse passing into the second body through the interface. Analysis shows that the transmitted pulse depends on the elastic stiffness of the interface. According to theory the stiffness of a single circular contact is pro­portional to the diameter of the contact and not to its area and this has been confirmed by simple scaled-up experiments. Because of this the stiffness of multiple contacts is not determined unequivocally by the true area of contact. The scaled-up experiments show that for widely separated contacts the stiffness is the sum of the stiffnesses of the individual con­tacts; if they are close together it is determined primarily by the size of the embracing circle of contact. Only when the apparent area of contact is more than ten times the true area does the effect of the empty spaces on the stiffness become appreciable. The ultrasonic experiments confirm all these conclusions and the technique has been applied to the static contact between metals, between smooth and rough glass, between rubber and between polymers. In some cases the results are able to distinguish between elastic and plastic deformation in the contact zone. The technique has been extended to sliding contacts and the results show that with ductile materials junction growth may occur. Although the ultrasonic method suffers from the major disadvantage in that it cannot unequivocally distinguish between the true and the apparent area of contact it has the great advantage that, in contrast to electrical resistance measurements, it can be used for non-metals and is scarcely affected by the presence of oxide films. Again in contrast to optical methods, it can be used with opaque materials.

Fracture mechanics of interfacial cracks

Abstract: This paper contains a two-dimensional analysis of the stress field around a crack on the plane interface between two bonded dissimilar anisotropic elastic half-spaces. This analysis is then combined with the usual local form of the Griffith virtual work argument to give an explicit fracture criterion which involves a suitably defined ‘stress concentration vector’ and the specific surface energy of the bonded surfaces. This criterion has a simple structure and reduces to the conventional form of Irwin when the two half-spaces are isotropic and identical. The analysis is then extended to cracks moving uniformly and a local fracture criterion with the same structure as the static criterion is derived by an energy balance argument. The criterion is specialized to isotropic half-spaces for illustration, when it predicts that the speed of a crack on an interface between such media will be limited by a speed Vc which is slightly greater than the smaller of the two Rayleigh wave speeds. A by-product of the analysis is an expression for the displacement field of an arbitrary interfacial dislocation, either stationary or moving uniformly.

Localizing 3H-GABA in nerve terminals of rat cerebral cortex by electron microscopic autoradiography.

Abstract: CONSIDERABLE biochemical, physiological and pharmacological evidence supports the idea that GABA is an important inhibitory neurotransmitter substance in the mammalian central nervous system (CNS)1–3. Cytochemical techniques for demonstrating the cellular location of this amino-acid in the CNS have so far not been available.

A model for the boundary condition of a porous material. Part 1

Abstract: In problems where a viscous fluid flows past a porous solid it has frequently been assumed that the tangential component of surface velocity is zero. When the porous solid has an open structure with large pores the external surface stress may produce a tangential flow below the surface. Recently, Beavers & Joseph (1967) have assumed that the surface velocity UB depends on the mean tangential stress in the fluid outside the porous solid through the relation \[ \left[\mu\frac{d\overline{u}}{dy}\right]_{y=0} = \frac{\mu\alpha}{k^{\frac{1}{2}}}(U_B-Q), \] where Q is the volume flow rate per unit cross-section within the porous material due to the pressure gradient, k is the Darcy constant and α is a constant which depends only on the nature of the porosity. An artificial mathematical model of a porous medium is proposed for which the flow can be calculated both inside and outside the surface. This conceptual model was materialized and the experimental results agree with the calculations. The calculated values of α so found are not quite independent of the external means of producing the external tangential stress.

Glycine uptake in rat central nervous system slices and homogenates: evidence for different uptake systems in spinal cord and cerebral cortex

Abstract: — Evidence is presented that glycine is taken up by two different transport systems in rat CNS tissue slices; one system has relatively low affinity for glycine (Km= 300 μm) and predominates in cerebral cortex, cerebellum and mid-brain, the other has a higher affinity for glycine (Km= 40 μm) and is detectable only in spinal cord, medulla and pons. The low affinity transport system appears to be shared by other small neutral amino acids, whereas the high affinity system is very specific for glycine. Both transport systems were shown to be present in particles in homogenates of CNS tissue by incubation with glycine in vitro, and subcellular fractionation studies suggested that synaptosomes were partly responsible for such uptake. Various substances were tested as inhibitors of the high affinity uptake system for glycine in spinal cord slices; the most potent inhibitors were p-chloro-mercuriphenylsulphonate, N-ethylmaleimide, chlorpromazine, imipramine, desipramine, hydrazinoacetic acid and haloperidol. No competitive inhibitors of the high affinity glycine uptake were found. It is suggested that the high affinity transport system is associated with inhibitory synapses where glycine is a transmitter.

Hall conductivity of amorphous semiconductors in the random phase model

Abstract: Kubo's formula for the electrical conductivity is used to evaluate the Hall conductivity in the Random Phase Model of Amorphous Solids appropriate to the diffusive, short mean free path regime. The Hall mobility is found to be small in magnitude, temperature insensitive, and to exhibit the same anomalies in sign as found in the localized regime: namely, negative for a non-degenerate distribution of holes on electrons due to diffusive motion about a closed path of three sites.