A new, differential method is described, suitable for detecting small changes in the conductivities of dilute, aqueous ionic solutions at a frequency of 3x10⁹c/s. The method essentially depends on measuring the change in 'q' of a resonant cavity containing a fine quartz tube, when the solution under investigation and water are passed through the tube in a continuously alternating stream, at a frequency of one complete change-over in eight seconds. The corresponding change in signal, picked up by a probe detector penetrating into the cavity, is viewed on a Cathode Ray Oscilloscope, after passing through suitable amplification and filtering processes. Measurements are described on a range of dilute solutions of potassium, sodium and lithium chlorides over a temperature range from 18°C to 25°C. For calibration purposes, the method has been used, in a modified form, for more concentrated solutions up to concentrations of 0.5 Normal. Assuming, from published results of other authors, that at high concentrations there is no difference between the ultra high frequency values of the ionic conductivities of these salts and the conductivities measured at low frequencies, the measured U.H.F. conductivities at low concentrations were compared with the corresponding D.C. values. They were found to exceed these by as much as 30%. This difference is in excess of the difference predicted by the theory of Falkenhagen, which has hitherto not been investigated at these frequencies. A new theory, which would explain the results obtained, is discussed in the final chapter; some justification for the new assumptions is obtained by using them to interpret the results of other workers for the variation of low-frequency dielectric constant of saline solutions with concentration.

PART I contains a description of the writer's further work on the absolute measurement of electrical resistance by the new rotating coil method of Nettleton and Balls. The preliminary investigation was published in 1934. In the fundamental equation, K, a constant, depends upon where M is the mutual inductance between rotor and stator at angle of rotation from the conjugate position where the brush is at the commutator sector's edge. K is shown to be most accurately determined at and with the field coils in the Helmholtz position. To satisfy these conditions and to render constant at the sector edges, it is necessary to construct a differential rotor consisting of two coplanar coils in series and opposition. The theory of this rotor necessitated the calculation of mutual inductances and Tables of Functions, given in Appendix II. With such a rotor, resistances of the order of one ohm are measured absolutely. above, is the angular velocity of revolution and is measured by the method described in Appendix I. Part II is a preliminary account of the writer's investigation, suggested by Dr Nettleton, on the absolute measurement of resistance by the method, proposed by Rosa in 1909, using the e.m.f. of a commutating generator. No data by any form of this method have ever been published. The basic equation of this method, has the great advantage that a frequency and the maximum mutual inductance only are required. A remarkable flat of maximum mutual inductance between rotor and stator was obtained, permitting commutation without loss of generated e.m.f. Rosa's difficulties, due to self-inductance and to break at commutation in the steady e.m.f. drawn from the resistance to be measured, are overcome by a short-circuiting device covering each commutation.

A magnetically controlled total immersion hydrometer is described. Changes in upthrust due to changes in density of the solution are balanced by adjusting the current in a suitably placed coil. A simple relationship between the coil current and the density increment permits the density of the solution as a function of concentration of solute to be investigated to a high degree of accuracy. The electrolytes of 1:1, 2:1, 1:2 valence types and four tetra-alkyl ammonium salts are investigated. In all these cases the density shows a linear function of concentration over the concentration range of 5.53 x 10N to 3.32 x 10N. The molar volumes at infinite dilution calculated from our experimental results show slightly higher values in most of the electrolytes investigated. However, in cases of KF, Na2CO3, Mgcl2, Sncl2 and KOH the values of the molar volumes deviate considerably from the presently available values. The electrostriction estimated on the basis of the densities of the molten state of the electrolytes is comparable to that estimated by Mukherjee. In cases of 1:1 electrolytes the electrostriction varies from 3 ml/moleto 11 ml/mole and in cases of 1:2 electrolytes from 44 ml/moleto 53 ml/mole with an exception of Na2CO3. The electrolytesof 2:1 valence type show electrostriction around 30 ml/mole with a number of exceptions. In cases of all the four tetra-alkyl ammonium salts the electrostriction is found negative around - 20 ml/mole.

The electronic band structure of a face-centred monatomic crystal is calculated by a plane wave method. A model pseudopotential is used depending on two adjustable parameters. The object of this work is mainly to investigate how the change of the potential affects the band structure, for the case of a potential similar in some respect to pseudo-potentials which have been Used for real metals. A detailed calculation is made for the points of symmetry and along the axis of symmetry within the Brillouin Zone. Five different sets of parameters for the pseudopotential are considered and the corresponding Energy v. Wave vector graphs are plotted. Also the coefficients of the Fourier expansion of the wove function, the so-called momentum eigen functions, are plotted against the wave vector k in an extended zone scheme. The features of the band structure which are independent of the potential and those which are sensitive to any change of it are found. Finally the results given by this model are compared with the band structure of actual elements having a face-centred cubic lattice.

Le travail porte sur la modélisation et l’optimisation d’un micro-réseau multi-sources autonome dans le cadre d’une application résidentielle. Le micro-réseau d’étude est un système multisources à énergie renouvelable comprenant des panneaux photovoltaïques (PV) comme source de production principale, une pile à combustible (PàC) comme source de production auxiliaire, des supercondensateurs et une batterie Lithium-ion comme système hybride de stockage, l’ensemble interfacé via des convertisseurs statiques pour l’adaptation des niveaux de puissance. Dans un premier temps, ces travaux de recherche se concentrent sur la modélisation multiphysique des sources et des éléments de stockage de l’énergie ainsi que leur validation expérimentale. En effet, l’étude du vieillissement des différents organes du système nécessite la modélisation de l’intégralité du système, étant donné que le vieillissement dépend principalement des paramètres électriques et thermiques. Dans un second temps, nous avons élaboré une stratégie de gestion d’énergie basée sur le principe de la séparation fréquentielle. Cette stratégie a pour objectif de réduire la consommation du carburant (Hydrogène) par la pile à combustible et d’améliorer la durée de vie des systèmes de stockage et de production de l’énergie. Une étude technico-économique est ensuite effectuée, afin d’identifier le meilleur compris entre le coût total du système et la quantité de l’énergie produite. Finalement, une analyse de sensibilité est réalisée pour identifier les paramètres les plus influents sur le coût total du système. / This research focuses on the multi-physic modeling and the optimization of an autonomous multi-source microgrid supplying a residential application. The studied micro-grid is a renewable energy system composed of photovoltaic panels PV as a main production source, a fuel cell as an auxiliary production source, supercapacitors and a Lithium Ion battery as a hybrid storage system and static converters for power levels adjustment. As a first step, this research focuses on the multi-physics modeling of energy sources and storage elements by considering electrical, thermal and aging domain and their experimental validation. In fact, the aging study of the system’s elements requires the modeling of the whole system, since aging depends mainly on electrical and thermal parameters. Secondly, multiphysical energy management strategy is developed based on frequency separation. Thus, the main strategy objectives are reducing the fuel consumption (Hydrogen) and improving the storage and production elements lifetime. Therefore, a technical-economic study is carried out, in order to identify the best agreement between the system total cost and the energy produced. Finally, a sensitivity analysis is conducted to identify the most influential parameters on the total cost of the system.

The ferromagnetic semiconductor gallium manganese arsenide is an important test-bed material for spintronics applications. Whilst a Curie temperature anywhere close to room temperature has yet to be demonstrated, the excellent micromagnetic properties, simple band structure and unusual combination of having both low moment densities and high spin-orbit coupling make this an interesting material to study from both theoretical and experimental perspectives. This thesis reports some experimental studies into the magnetic and magnetoresistive anisotropies in gallium manganese arsenide. In the first main chapter a study of the Anisotropic Magnetoresistance in thin (Ga,Mn)As films is reported, based on transport measurements of micro-scale devices, contributing to the first systematic study in this material. The Anisotropic Magnetoresistance comprises crystalline and non-crystalline components; this study shows that a uniaxial crystalline component can dominate over the whole range of temperatures from 2K up to the Curie temperature, the first time this has been seen in any material system to our knowledge. The following chapter shows that the magnetic anisotropy of gallium manganese arsenide thin films can be engineered by lithographically patterning the material into structures on length scales of a micron or less. Using electron beam lithography to define the structures and SQUID magnetometery to study the resulting magnetic configuration, it is shown that the magnetic anisotropy can be greatly modified, even resulting in a switching of the easy- and hard-axis directions. Finally a new technique based on Anisotropic Magnetoresistance measurements is presented to locate the crossover of competing magnetic anisotropy coefficients in the temperature domain. Conventionally performed by SQUID magnetometry, this new technique is cheaper and simpler whilst qualitatively reproducing the main features of the SQUID measurements.

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