Transport functions of nitrogen up to 26,000 K

The current field strength characteristic, E(1), and a large number of radial temperature distributions, T(r,I), measured in a 5 mm N2 cascade arc at normal pressure are used to evaluate the transport properties of nitrogen up to 26,000 K. The electrical conductivity sigma (T) and the Coulomb cross section are determined directly from the E(I) and several T(r,I) curves. Between 10,000 and 15,000 K the radiative energy flux for different arc current, the thermal conductivity, and from this the charge exchange cross section are determined in a good approximation utilizing the large number of measured temperature distributions. It turns out, that at the highest measured arc current, i.e., 570 A, in the axial region of the arc about 95% of the supplied energy is carried off by radiation

Taking the lead: post-2012 climate targets for the North: towards adequate and equitable future climate commitments for industrialised countries

Any definition of adequacy consistent with the objective of the Climate Convention will require increased mitigation efforts in industrialized countries far beyond those levels agreed in Kyoto. This paper, therefore, focuses on future reduction targets for industrialised (Annex I) countries. It starts with an assessment of mid- and long-term targets already adopted in industrialised countries against the backdrop of required emission cuts to keep climate change within tolerable limits. Taking into account their heterogeneous national circumstances, the main part of the paper presents a method for differentiating Annex I countries with a view to assigning future mitigation and financial transfer commitments. This differentiation exercise is based on an analytical approach that was developed in the project “South-North Dialogue – Equity in the Greenhouse". Slightly modifying the original approach the level of reduction targets is determined by the two criteria of “responsibility" and “potential" to mitigate. Obligations to provide financial and technological resources to developing countries, on the other hand, are to be assigned according to the “capability" criteria. --

Interpreting two-photon imaging data of lymphocyte motility

Recently, using two-photon imaging it has been found that the movement of B and T cells in lymph nodes can be described by a random walk with persistence of orientation in the range of 2 minutes. We interpret this new class of lymphocyte motility data within a theoretical model. The model considers cell movement to be composed of the movement of subunits of the cell membrane. In this way movement and deformation of the cell are correlated to each other. We find that, indeed, the lymphocyte movement in lymph nodes can best be described as a random walk with persistence of orientation. The assumption of motility induced cell elongation is consistent with the data. Within the framework of our model the two-photon data suggest that T and B cells are in a single velocity state with large stochastic width. The alternative of three different velocity states with frequent changes of their state and small stochastic width is less likely. Two velocity states can be excluded

Ethical aspects of emissions trading: Contribution to the World Council of Churches Consultation on Equity and Emission Trading - Ethical and Theological Dimensions, Saskatoon, Canada, May 9-14, 2000

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Cutting edge: back to "one-way" germinal centers

The present status of germinal center (GC) research is revisited using in silico simulations based on recent lymphocyte motility data in mice. The generally adopted view of several rounds of somatic hypermutations and positive selection is analyzed with special emphasis on the spatial organization of the GC reaction. We claim that the development of dark zones is not necessary for successful GC reactions to develop. We find that a recirculation of positively selected centrocytes to the dark zone is rather unlikely. Instead we propose a scenario that combines a multiple-step mutation and selection concept with a "one-way" GC in the sense of cell migration

EU environmental principles: Implementation in Germany

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Analysis of B cell selection mechanisms in the adaptive immune response

The essential task of a germinal centre reaction is the selection of those B cells that bind the antigen with high affinity. The exact mechanisms of B cell selection is still unknown and rather difficult to be accessed in experiment. With the help of an already established agent-based model for the space-time-dynamics of germinal centre reactions [1,2] we compare the most important hypotheses for what the limiting factor for B cell rescue may be. We discuss competition for antigen sites on follicular dendritic cells, a refractory time for centrocytes after every encounter with follicular dendritic cells, competition for the antigen itself, the role of antigen masking with soluble antibodies, and competition for T cell help. The unexpected result is that neither competition for interaction sites nor competition for antigen nor antigen masking are in agreement with present experimental data on germinal centre reactions. We show that these most popular selection mechanisms do not lead to sufficient affinity maturation and do not respect the observed robustness against changes of initial conditions. However, the best agreement with data was found for the newly hypothesized centrocyte refractory time and for competition for T cell help. Thus the in silico experiments point towards selection mechanisms that are not in the main focus of current germinal centre research. Possible experiments to test these hypotheses are proposed

An analysis of B cell selection mechanisms in germinal centres

Affinity maturation of antibodies during immune responses is achieved by multiple rounds of somatic hypermutation and subsequent preferential selection of those B cells that express B cell receptors with improved binding characteristics for the antigen. The mechanism underlying B cell selection has not yet been defined. By employing an agent-based model, we show that for physiologically reasonable parameter values affinity maturation can be driven by competition for neither binding sites nor antigen—even in the presence of competing secreted antibodies. Within the tested mechanisms, only clonal competition for T cell help or a refractory time for the interaction of centrocytes with follicular dendritic cells is found to enable affinity maturation while generating the experimentally observed germinal centre characteristics and tolerating large variations in the initial antigen density

Diffusion of information among small-scale farmers in Senegal: the concept of Farmer Field Schools

Recent research on the Farmer Field School (FFS) approach in agriculture in developing countries has raised some doubts on the economic impacts of this concept and especially the knowledge diffusion effects from trained to non-trained farmers. Based on a study in Senegal this paper hypothesizes that the question of the project placement strategy is vital when analyzing knowledge diffusion effects of FFS in Africa. Results show that the share of trained farmers in a community is a decisive factor for adoption behavior and knowledge diffusion. It is concluded that when introducing an FFS, a critical mass of trained farmers is important to attain effective dissemination of information and to generate positive stimuli for adoption and learning among non-participants.Africa,Senegal,agricultural extension,Farmer Field School,diffusion