Christian Huygens | EPFL Graph Search

Christiaan Huygens (), Christianus Hugenius en latin et Christian Huygens en français, né le à La Haye (dans les Provinces-Unies) et mort le dans la même ville, est un mathématicien, astronome et physicien néerlandais. Il est considéré comme un alter ego de Galilée, notamment pour sa découverte de Titan qu'il décrit dans Le Système de Saturne (1659) où il fait une première description exhaustive du Système solaire à six planètes et à six lunes, avec une précision alors inégalée. Pour la première fois, il est possible d'avoir une idée de la dimension du système, de l'éloignement des étoiles et de la position exacte de la Terre en son sein, ainsi que de sa dimension exacte, nettement plus grande que Mars ou Mercure, à peine plus grande que Vénus, mais nettement plus petite que Jupiter et Saturne. Il construit également la première horloge à pendule, qui améliore la précision des horloges existantes de à par jour (1656). Huygens est généralement crédité pour son rôle fondamental dan

Rocker oscillator time bases

Simon Nessim Henein, Ilan Vardi

Since the invention of the pendulum clock by Christiaan Huygens in 1657, precision timekeepers have been regulated by oscillators. Although the pendulum is not an isochronous oscillator, its introduction as a time basis for clock regulation led to a significant increase in accuracy. Indeed, numerous clocks were subsequently retrofitted with pendulums to replace the foliot time base. Oscillators have defined revolutions in accuracy, as seen by the terms quartz watch and atomic clock. Observation of a rocking chair and its mathematical modelling shows that it provides oscillatory motion and can therefore theoretically be used as a clock time base. This led to the study, design, and fabrication of a rocker oscillator for the regulation of a table clock. The demonstrator consists of a rocker oscillator designed to replace the original pendulum of a commercial precision table clock. Two identical clocks were used to compare the operation of the modified clock with that of the original one. The rocker oscillator was dimensioned so that its natural frequency as well as its quality factor fit those of the original pendulum. The performance of both clocks was then the subject of an experimental study. This project serves as a demonstration of how new oscillators can be retrofitted to existing timekeepers.

2021

Al-Cu-Fe quasicrystalline coatings and composites studied by mechanical spectroscopy

Mechanical spectroscopy measurements were performed on two types of materials: decagonal quasicrystalline Al-Cu-Fe-Cr coatings deposited on a mild steel substrate and aluminium or magnesium matrix composites reinforced with icosahedral quasicrystalline Al-Cu-Fe particles. The internal friction spectra of the substrate with three different thicknesses of the coating indicate that the internal friction of such composites is mostly caused by the quasicrystalline coating and that the contributions of the steel substrate and of the interface are small. The shear modulus measured in torsion increases with temperature, while the Young's modulus measured in flexion behaves normally. This shear modulus anomaly is interpreted as due to a solid friction between cracked segments of the quasicrystalline coating. This phenomenon can also explain the broad athermal maximum, which was found to occur in isochronal internal friction measurements. A quantitative model successfully reproducing the observed behaviour was developed. Finally, the reversible high-temperature exponential background was interpreted as due to the onset of the brittle-to-ductile transition, which may be associated with dislocation motion controlled by collective phason flips in the quasicrystalline coating. The measured activation enthalpy is similar to the value that was deduced from compression tests performed on icosahedral Al-Cu-Fe bulk material. Aluminium or magnesium matrix composites reinforced with icosahedral Al-Cu-Fe particles were processed by gas pressure infiltration and characterised by X-ray diffractometry, electron microscopy observations, micro-hardness measurements, and compression tests. The internal friction spectra of these composites also show a high-temperature exponential background, while measurements of the matrix alone or of the matrix with Al2O3 short fibres exhibit a different behaviour. The difference can be explained by a partial phase transformation of the matrix due to the presence of the quasicrystalline particles. The exponential background is probably caused by dislocation motion in the matrix, however, the effect of the quasicrystalline reinforcement can be neither excluded nor confirmed with certainty.

EPFL2003

COSMOGRAIL: the COSmological MOnitoring of GRAvItational Lenses XV. Assessing the achievability and precision of time-delay measurements

Vivien François Bonvin, Frédéric Courbin, Thibault Adrien Kuntzer, Georges Meylan, Malte Tewes

COSMOGRAIL is a long-term photometric monitoring of gravitationally lensed quasars aimed at implementing Refsdal's time-delay method to measure cosmological parameters, in particular H-0. Given the long and well sampled light curves of strongly lensed quasars, time-delay measurements require numerical techniques whose quality must be assessed. To this end, and also in view of future monitoring programs or surveys such as the LSST, a blind signal processing competition named Time Delay Challenge 1 (TDC1) was held in 2014. The aim of the present paper, which is based on the simulated light curves from the TDC1, is double. First, we test the performance of the time-delay measurement techniques currently used in COSMOGRAIL. Second, we analyse the quantity and quality of the harvest of time delays obtained from the TDC1 simulations. To achieve these goals, we first discover time delays through a careful inspection of the light curves via a dedicated visual interface. Our measurement algorithms can then be applied to the data in an automated way. We show that our techniques have no significant biases, and yield adequate uncertainty estimates resulting in reduced chi(2) values between 0.5 and 1.0. We provide estimates for the number and precision of time-delay measurements that can be expected from future time-delay monitoring campaigns as a function of the photometric signal-to-noise ratio and of the true time delay. We make our blind measurements on the TDC1 data publicly available.

Edp Sciences S A2016

Al-Cu-Fe quasicrystalline coatings and composites studied by mechanical spectroscopy

EPFL2003