Showing papers by "University of Patras published in 1970"

Hall effect in magnetite.

Abstract: Magnetite, a ferrimagnetic semiconductor of inverse spinel structure has attracted much attention due to a phase transition occurring at low temperatures. The transition temperature reported differs for the various properties under investigation. In the present paper measurements are reported of the Hall effect and resistivity in the range from 77 to 380°K. The 105°K Hall resistivity isotherm and the ones for higher temperatures follow the usual trend for ferromagnetic materials. Contrary, the lower temperature isotherms, up to 100°K, exhibit irregular behaviour, involving sign reversal. If we accept the average of the two temperatures, i.e. the value of 103°K, as our transition temperature we observe that it lies, by some degrees, below the values found in the literature. Only part of the difference can be attributed to a systematic error of our thermocouple. The previously mentioned sign reversal shifts to stronger magnetic fields as the temperature increases from 77 to 100°K. Magnetit, ein ferrimagnetischer Halbleiter mit inverser Spinelstruktur hat durch seinen Phasenubergang bei niedrigen Temperaturen starke Beachtung gefunden. Die veroffentlichten Ubergangstemperaturen unterscheiden sich bei der Untersuchung verschiedener Eigenschaften. In der vorliegenden Arbeit wird uber Messungen des Hall-Effekts und der Leitfahigkeit im Temperaturbereich zwischen 77 bis 380°K berichtet. Die Isothermen des Hallwiderstandes bei 105°K und hoheren Temperaturen folgen dem ublichen Verlauf fur ferromagnetische Materialien. Im Gegensatz dazu zeigen die Isothermen fur tiefe Temperaturen bis 100°K irregulares Verhalten, einschlieslich Vorzeichenumkehr. Wenn man den Mittelwert der beiden Temperaturen, d. h. den Wert von 103°K als in diesem Fall gultige Ubergangstemperatur akzeptiert, stellt man fest, das er etwas unterhalb der Werte liegt, die in der Literatur angegeben werden. Nur ein Teil dieser Differenz kann einem systematischen Fehler des Thermoelements zugeschrieben werden. Die oben erwahnte Vorzeichenumkehr verschiebt sich zu starkeren Magnetfeldern, wenn die Temperatur von 77 bis 100°K gesteigert wird.

Particle Resonance in a Spiral Field

Abstract: Plane galactic orbits near the particle resonance of a spiral field have been calculated numerically. Besides the ring type orbits, i.e. orbits filling a ring around the galactic center, there are orbits librating near the potential maxima at the co-rotation distance (Lagrangian points). This effect tends to disrupt the spiral arms at the co-rotation distance.

The symplectic property of matrizants

Abstract: The matrizants of simply and doubly symmetric orbits of the three-dimensional restricted problem of three bodies because of their symplectic character, can be transformed by means of multiplication with constant matrices into symmetric ones. As a result the bilinear relations between their elements, existing on account of the symplectic property, take linear and very simple forms. This fact is very useful in computer applications where these relations are used as criteria of accuracy.

Strengthening of historic masonry structures with fibre reinforced plastic composites

Abstract: The paper deals with the application of unidirectional fibre reinforced plastics for strengthening of masonry monuments. The materials are applied in the form of either externally applied circumferencial tendons to provide horizontal confinement, or laminates, which are epoxy-bonded to the facades of masonry buildings serving the role of tensile reinforcement.

Contributions Towards The Prediction Of Velocity Distribution In Open Channel Flows

Abstract: The steady state, depth-averaged hydrodynamic equations are used for the computation of transverse profiles of the longitudinal velocity in prismatic and natural channels. Turbulence closure of the comlete set of equations is accomplished (a) via a simle model in which vt is assumed proportional to the shear velocity and the local depth, and (b) the depth-averaged k-e model of Rastogi & Rodi. Subsequently, the governing equations are simplified to their purely parabolic form and the transverse distance is replaced by the cumulative discharge as the second independent variable. Turbulence closure in the latter set of equations is accomplished with model (a). All models are solved numerically via Patankar's scheme and the results are compared with literature data corresponding to laboratory and field measurements. It is shown that the purely parabolic, transformed-coordinates model is more efficient than, and equally accurate with, the complete model.