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We report on measurements and numerical simulations of the behavior of MgB2 superconductors when magnetic field components are applied along mutually perpendicular directions. By closely matching the geometry in simulations and measurements, full quantitative agreement is found. The critical state theory and a single phenomenological law, i.e. the field dependence of the critical current density Jc(B), are sufficient for a full quantitative description of the measurements. These were performed in thick strips of carbon nanotube doped MgB2 samples. Magnetization was measured in two orthogonal directions using a SQUID magnetometer. Magnetic relaxation effects induced by the application of an oscillatory perpendicular field were observed and simulated numerically. The measurements confirm the numerical predictions, that two relaxation regimes appear, depending on the amplitude of the applied magnetic field. The overall agreement constitutes a convincing validation of the critical state model and the numerical procedures used.

We report measurements of the hysteretic magnetization of YBCO under superimposed transport current cycles, together with numerical simulations of magnetization and current density profiles in the corresponding parallel configuration. Field cooled (FC) and zero-field-cooled (ZFC) experiments were carried out on polycrystalline YBa 2 Cu 3 O 7−x cylinders, with both the applied magnetic field and transport current in the axial direction, and the current cycled several times, around and above the dissipative threshold. As in previously reported multicomponent field configuration experiments, the magnetization is seen to collapse to a more stable state both in FC and ZFC, because of the interplay between the shielding and transport currents. The results of our numerical simulations are in good qualitative agreement with the measurements, and the competition between shielding and transport due to vortex-pinning interactions and equilibrium magnetization effects are shown to play an important role in the range of our experiments.

The voltage sensitivity and the measurement uncertainty due to the finite resolution in multi-decade and slide wire DC Wheatstone bridges have been reviewed and analysed in detail. The use of the term `precision' is discussed, and a criterion for maximum resolution with minimum power dissipation is shown. Resumen. Se examina en detalle la Sensibilidad (respecto del voltaje) y la incertidumbre de las mediciones debida a la resolución finita en el puente de Wheatstone de corriente continua de `hilo' y del tipo multi-década. Se discute el uso del término `precisión' y se muestra un criterio de máxima resolución con mínima disipación de potencia.

The pinning process within a hard superconductor in a two-component configuration was experimentally studied under a cycling transport current. The axial component of the magnetic moment of polycrystalline Y-Ba-Cu-O cylinders was directly measured in a SQUID magnetometer under longitudinal currents and simultaneously applied stationary parallel magnetic fields. The effect on the axial magnetization of the current cycling (i.e. the azimuthal field cycling), shows a strong pinning which reduces the shielding capability of the superconductor. Additionally, two different types of behaviour were found. For moderate current amplitude of the cycle (depending on the field penetration regime at a given temperature and applied field strength), the magnetization decreases in each cycle. In contrast, when the current amplitude is relatively high, 'butterfly' loops are observed. This evidence indicates that the cutting and cross-flow of nonparallel flux lines occur in the weak link regime of high-T c superconductors; the results can be analysed in the framework of a generalized critical state model.