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We describe a general procedure which can be used in order to obtain the spatial variations of the electrostatic potential in a crystalline semiconductor layer of finite thickness . Different kinds of variations are found, depending on the electrostatic potential values at each interface and on the semiconductor thickness. General analytic expressions are given in the framework of the depletion approximation.

Theoretical and experimental work on losses in superconductors has increased considerably during recent years. There is still, however, little information in the literature concerning the influence of thermal conditions on the ac losses of a superconducting coil.

Two aspects of the presentation of electromagnetism in three-dimensional space can be compared if a distinction is made between polar vectors P and axial vectors T. In option /spl alpha/, Baldomir and Hammond take B, E, D/spl alpha/, j/spl alpha/, H/spl alpha/ as basic vectors and need to use the *Hodge dual operator. Option /spl beta/ is presented based on the vectors B, E, D/spl beta/, j/spl beta/, H/spl beta/ where no special operator is required. For the presentation of electromagnetism in four-dimensional space the components of the group E B, common to both options, are brought together to form a tensor of 4/sup 2/ = 16 components. The B/sub ij/ are arranged within a 3 /spl times/ 3 square, with the E/sub k/ on the edge of this square. The slightly different forms of this arrangement correspond to the tensors identified symbolically by F/sub pq/ (B, E) for option /spl alpha/ and F/sub km/ (B, E) for option /spl beta/. Two Maxwell equations are related to the corresponding tensors for each option. For option /spl beta/, a tensor G/sup km/ directly linked to F/sub km/ (B, E) naturally leads to H/spl beta/ and D/spl beta/. The consideration of this tensor along with the four-dimensional current density vector J/sub /spl beta// (expressed using j/spl beta/ and /spl rho/) allows one to establish the two other Maxwell equations.

The transcendental equation whose solutions allow the computation of the exact electromagnetic time constants of a screen surrounded by a shield is at first derived. Then the use of Taylor and asymptotic expansions of the Bessel functions yields approximate expressions for the time constants. Numerical comparison between exact and approximate values shows the validity of the proposed method. Finally, a comparison of the frequency response curves obtained by the F.F.T. method [15] and by our method shows that the approximations lead to satisfactory results.

We show here how the application of the critical state model allows one to determine the magnetic flux density B in each point of a superconducting cylinder with an arbitrary cross section subjected to axial magnetic fields Hz; the B = 0 boundaries of the regions occupied by the vortices are so defined. We successively consider the cases where the critical current density Jc is either isotropic (constant or an arbitrary function of B) or tensorial, which means, for our problem, the use of two components Jcx and Jcy (either constant or depending on B but Jcx/Jcy remaining constant).

Investigations of the mechanics of metal fibres capable of carrying very high currents are presented. The role played by the Laplace force is emphasized. This force appears whenever a fibre carrying a current is subjected to a magnetic field whose direction is different from that of the fibre. The role played by the coefficient of friction is also taken into account. An exact non-linear differential equation is used to describe the problem, i.e. no assumptions are made about the magnitudes of the deflections. When the normal force exerted on the track by the fibre is an increasing function of the Laplace force, it is found that the maximum stress within the fibre has a non-linear dependence on the Laplace force. When the same normal force is a decreasing function of the Laplace force, it is shown that the normal force vanishes for a critical value of the Laplace force. A perturbation method is used to explain the behaviour of the fibre.

We determine a gauge allowing the order parameter to be real in the mixed state of type II superconductors although flux quantization is respected. The Ginzburg-Landau equations written in this peculiar gauge are then derived to obtain a new formulation of the vortex line free energy. The result is first tested in the simple case of an isolated vortex in an infinite superconductor and then used to compare the superheating field at a normal-superconducting boundary with that at a vacuum-superconducting boundary.

Resume Plusieurs auteurs ont suggere que les pertes dans les supraconducteurs de deuxieme espece, a l'etat mixte, parcourus par un courant, etaient dues au deplacement des vortex. Nous proposons une approche statistique permettant de traiter ce deplacement dans un materiau non ideal, c'est-a-dire contenant des defauts de toutes sortes: defaut de structure, inclusion d'impuretes, precipitation de phase. Grâce a cette analyse nous pouvons determiner la fonction de repartition des forces d'ancrage dans l'echantillon considere. Une comparaison est ensuite faite entre un modele issu de cette theorie et l'experience. Le bon accord obtenu semble confirmer la validite de notre analyse.

Au moyen de la theorie des phenomenes cooperatifs d'Yvon, nous avons obtenu l'expression de la susceptibilite paramagnetique des corps ferromagnetiques. Dans le cadre du modele d'Ising, cette expression nous a permis de voir l'influence des termes a deux noeuds, trois noeuds, quatre nœuds, etc... Nous pouvons egalement prendre en compte, non seulement les energies d'interaction entre premiers voisins, mais aussi entre deuxiemes, troisiemes voisins, etc... Pour le cobalt nous avons obtenu un tres bon accord entre notre courbe theorique et les resultats experimentaux.

Via a cellular model of the mixed state we study the spreading of a vortex normal to a superconducting boundary. For any value of the induction in the superconductor we derive the expressions of the magnetic microscopic field near the surface and of the extra energy added by the boundary. This energy could be responsible for a decrease inHc2, but our calculations show that this is unlikely to occur except for very thin films.

Sommaire.—Dans la realisation experimentale des experiences de diffusion des rayons X a de tres faibles angles, on limite generalement les faisceaux par des fentes au lieu de trous fins. Ce dispositif a pour effet de modifier la variation avec l'angle de l'intensite diffusee. L'importance de cet effet est calculee dans des cas varies. Une methode de corrections absolument generale est donnee, ainsi que quelques applications de celle-ci.

In spite of the increasing use being made of superconducting coils, experimental data on a.c. losses are still sparse. Using a calorimetric method, the authors have obtained results for a.c. power losses in a number of coils wound with niobium-zirconium wire, and have studied the dependence of losses on current and field.

Les auteurs etudient experimentalement les instabilites magnetiques (sauts de flux) dans un cylindre massif de supraconducteur, impur de seconde espece. En prenant pour la densite de courant critique une fonction quelconque j(B, T) de l'induction magnetique et de la temperature, ils determinent theoriquement le champ electrique qui apparait en tout point du cylindre supraconducteur lorsque celui-ci est place dans un champ magnetique exterieur parallele aux generatrices du cylindre et croissant lentement avec le temps. Ils montrent que les expressions obtenues permettent d'atteindre, a partir des valeurs experimentales du champ electrique, la densite de courant critique et la repartition de l'induction magnetique dans le supraconducteur apres un saut de flux. Des experiences ont ete realisees sur un barreau de Nb-40 % Ti baignant dans l'helium liquide (2 K ≤ T0 ≤ 4,2 K) ou dans l'helium gazeux a faible pression (4,2 K ≤ T 0 ≤ 6 K). Les profils d'induction apres saut de flux mettent en evidence l'importance de l'echange thermique entre le supraconducteur et le milieu exterieur. Les experiences permettent d'autre part de determiner le seuil de declenchement des premiers sauts de flux Hfj et la repartition de la temperature maximale atteinte en tout point du cylindre au cours d'un saut de flux limite. Les resultats obtenus sont compares aux diverses previsions theoriques proposees jusqu'a present.

1. 1. Changes due to urea. The action of urea on haemoglobin, haemocyanin, and serum albumin has been followed by examining the variation in size of the particles by the method of low angle X-ray scattering. The time required for dissociation depends not only upon the concentration of urea, but also upon the relative quantities of urea and protein present in the solution. As far as the action of urea is concerned, serum albumin is less stable than haemoglobin and haemocyanin. 2. 2. Changes due to copper. Addition of copper hydroxide to a solution of serum albumin, causes the appearance of large particles, in addition to the ordinary serum albumin particles. These large particles seem to arise from the association of the elementary particles in groups of four.

L'article est a peu preb uniquement consacre aux cas des fluides (solutions de macromolecules par exemple). Il est montre qu'il est necessaire de conuaitre plusieurs courbes de repartition de l'intensite du rayonuement diffuse en fonction de l'angle, relatives a plusieurs concentration de la matiere, pour obtenir le rayon de giration des macromolecules et des renseignements sur leur potentiel d'interaction.

Il est montre que les variations de flux dans un supraconducteur de deuxieme espece sont associees a deux types d'effets thermiques qui sont, d'une part, une production irreversible d'entropie due au travail des forces de piegeage et d'autre part, un apport reversible d'entropie par la creation ou la destruction des vortex. Les resultats des mesures calorimetriques de ces deux effets sont confrontes, premierement, aux resultats des mesures electriques des courants critiques et, deuxiemement, aux expressions theoriques de la variation d'entropie que nous deduisons d'un modele cellulaire de l'etat mixte. Le terme reversible est tres sensible a l'imperfection du supraconducteur. Une forte heterogeneite des forces de piegeage est mise en evidence par cette etude.

In an attempt to clarify the controversial views about the general validity of the rather frequently and successfully used critical state mode, we consider an explicit formulation of this model and present a careful analysis of the boundaries of the regions where the field B is zero. As a result, it seems that the arguments recently published that call into question the general validity of this mode are not acceptable.

Resume Le deplacement des vortex engendre un champ electrique qui, agissant sur les electrons normaux, provoque des pertes. A l'aide d'un modele nous calculons les pertes relatives a l'exterieur du coeur des vortex.

La première partie de cet article est consacrée à l'étude des écoulements magnétohydrodynamiques plans parallèles, turbulents, de fluide newtonien incompressible, dans le cas où les parois limitant le canal de métal liquide sont mobiles l'une par rapport à l'autre dans le sens de l'écoulement. Une induction magnétique extérieure est appliquée perpendiculairement aux parois. La mise en mouvement du fluide est due à la fois à la mobilité des parois, supposées parfaitement lisses, et à la présence d'une source d'énergie électrique extérieure. Nous déterminons une expression analytique des dissipations en étudiant les variations de l'entropie de l'unité de masse du fluide en mouvement (en absence de phénomènes de thermoconduction). Une étude qualitative de la turbulence jointe à des considérations dimensionnelles (semblables à celles utilisées par Landau et Lifschitz dans le cas d'écoulements hydrodynamiques à parois fixes) nous a permis de proposer, d'une part, un critère de transition (laminaire-turbulent) pour ce type d'écoulement et, d'autre part, une loi régissant les dissipations. La deuxième partie de cette étude décrit brièvement les solutions apportées aux principaux problèmes de M. H. D. posés pour l'utilisation des métaux liquides dans les machines homopolaires à inducteur supraconducteur. La troisième partie permet de comparer les résultats expérimentaux aux résultats théoriques obtenus à l'aide d'une méthode de calcul basée sur l'étude décrite en première partie.

Transfusional malaria is a disease in continual increase because of the greater number of international journeys and because of the insufficient prophylaxis at the present time. The authors report a new case which occurred in Limoges due to Plasmodium falciparum. The blood-donor was cambodgian and specific immunological tests were positive on this serum. Any fever, without evident cause, happening after blood or blood-derivative transfusion must suggest transfusional malaria. This diagnosis must be quickly confirmed by parasite detection in the blood and a good and efficient treatment will be easy. Maybe soon a culture of human Plasmodium in human red cells will make possible an efficient prophylaxis using a cheap fluorescent antibody test.

The electronic transport in hydrogenated amorphous silicon (a-Si:H) is studied by means of a time-of-flight experiment performed on samples presenting a classical sandwich configuration. Both macroscopic and microscopic behaviors are analyzed. From the experimental results we determine, when the transport is nondispersive or slightly dispersive and with a minimum of assumptions, the density of carriers reaching the rear collection electrode. By means of a Fourier transform we can compare this quantity to the solution of the general transport equations of the ``conventional'' multiple-trapping problem in the Fourier space. The determination of the time constants controlling the interaction of the conduction-band tail with the extended states during the transit of the carriers results from the comparison of these two expressions. Therefore we can derive the shape of the density of states of the conduction-band tail in the energy range 0.1${E}_{c}$-E0.3 eV. We have applied this method to the results of various numerical simulations. The range of validity of the method is given: The diffusion has simultaneously a negligible effect on the transport mechanism in the conduction band and a negligible enlargement of the packet width.From the computed transits the introduced densities of states were found again. As these tests were positive, we have carried out experiments on different samples. These time-of-flight experiments were performed at various applied fields ${(10}^{3}$\ensuremath{\le}\ensuremath{\Vert}scrE\ensuremath{\Vert}\ensuremath{\le}3\ifmmode\times\else\texttimes\fi{}${10}^{4}$ V/cm) and various temperatures (200\ensuremath{\le}T\ensuremath{\le}350 K) with a very low injected charge (\ensuremath{\simeq}${10}^{\mathrm{\ensuremath{-}}12}$ C) and reasonably thick samples (\ensuremath{\simeq}4.5 \ensuremath{\mu}m). Our method applied to our experimental results shows that the density of states of the conduction-band tail can be roughly approximated by an exponential distribution, the characteristic temperature of which is 200 K for the states located in the (0.2--0.3)-eV energy range. Near the conduction-band edge, the density of states can be approximated by another exponential distribution, the characteristic temperature of which is mainly related to the assumed mobility and the attempt-to-escape--frequency values. This temperature is typically 400 K.

La diffusion des rayons X par les fluides et les substances amorphes a deja fait l’objet de nombreux exposes detailles [2], [6],[11], [12], [13], [15] et [16]. Nous avons profite de cette monographie pour developper quelques idees nouvelles et perfectionner leur presentation.

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The concept of a flux jump is now well known and several authors1–4 have reported experimental results on values of H fj (the lowest magnetic field for which a flux jump can occur). Theoretical calculations, using various hypotheses about the orders of magnitude of D th and D m (thermal and magnetic diffusivities) were also made4–7 and gave rather similar expressions for H fj.

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The behaviour of pressure contacts between a superconductor and a normal metal is studied here for high intensities. The V-I characteristics can show a bend or a break for a certain I of order 10 A. A simple model is given which explains shape of the curves.

Noise measurements have been carried out by other authors and we propose to explain their results in making use of a previous statistical model for the velocities of the vortices. In this way we assign to each vortex a propervelocity instead of considering a single one for all the vortices. The resulting noise spectrum is calculated and is found equal to the sum of elementary spectra produced by each velocity. Therefore this spectrum does not reach zero value, and this fact seems to be in fairly good agreement with the experimental data.