549 research outputs found
Short communication: improvements to INRIM Johnson noise thermometer
No full textThis paper presents a progress report for the Johnson noise thermometry experiment which is under development at the Istituto Nazionale di Ricerca Metrologica. In order to aim at an uncertainty level better than 10−5 and to reduce the measurement time, a new setup has been developed. In particular, several modifications have been applied to the experiment described by Callegaro et al. (Metrologia 46: 409,
2009) to improve the traceability to voltage, resistance and frequency standards, and new amplifiers have been designed in order to expand the working frequency range up to 20 kHz, with a sensing resistor of 1 kohm, while maintaining amplifier induced systematic errors to an acceptable level
Automatic compensation system for impedance measurement
No full textThis paper deals with the realization of the four-pair terminal definition of impedance standards. A simple, though reliable, system is described that allows an automatic compensation of the voltage at the low potential port of impedance standards to be obtained. Such a system employs a commercial data acquisition board and a signal generator with adjustable-phase capability, which acts as the phase reference for the generator that feeds the impedance standard. A standard PC controls the whole system and implements the demodulation and the control algorithms. Preliminary tests have been performed in the frequency range of 50 Hz to 20 kHz with different kinds of impedance standards (resistive, inductive and capacitive), obtaining a residual voltage at the low potential port of less than 5 μV
COHOMOLOGY OF BRAIDS, PRINCIPAL CONGRUENCE SUBGROUPS AND GEOMETRIC REPRESENTATIONS
No full textThe main purpose of this article is to give the integral cohomology of classical principal congruence subgroups in SL(2, Z) as well as their analogues in the 3-strand braid group with local coefficients in symmetric powers of the natural symplectic representation. The resulting answers (1) correspond to certain modular forms in characteristic zero and (2) the cohomology of certain spaces in homotopy theory in characteristic p. The torsion is given in terms of the structure of a ‘p-divided power algebra’.
The work is an extension of the work in Callegaro et al. [The cohomology of the braid group B3 and of SL2(Z) with coefficients in a geometric representation, Quart. J. Math. 64 (2013), 847–889] as well as extensions of a classical computation of Shimura to integral coefficients. The results here contrast the local coefficients such as that in Looijenga [Stable cohomology of the mapping class group with symplectic coefficients and of the universal Abel–Jacobi map, J. Algebraic Geom. 5 (1996), 135–150] and Tillmann [The representation of the mapping class group of a surface on its fundamental group in stable homology, Quart. J. Math. 61 (2010), 373–380]
On strategies for automatic bridge balancing
No full textAutomatic balancing of coaxial ac impedance bridges asks for simple, robust and efficient strategies suitable to be easily implemented on PC controllers. The paper considers the problem of balancing a bridge with n detectors to be equivalent to the problem of finding the root of a complex-valued vector function F. To avoid electrical modeling of the bridge, F is considered generally unknown except at specific points evaluated by experiment. In this framework, efficient root-finding strategies can be employed., in particular, secant method and two generalizations of it to the case n > 1 are proposed. Experimental results on a test circuit are given
Time-related variation of volatile contents of Western Ghats volcanic formations, Deccan, India
No full textDeccan volcanism in India covered more than 1 million square km and reached a maximum thickness of about 3 km, as presently preserved in the Western Ghats volcanic lava piles. Volcanic activity started at about 66.4 Ma (Jawhar formation) and ended at about 65.5 Ma (Mahabaleshwar unit; Renne et al., 2015). Deccan volcanism straddled the Cretaceous-Paleogene boundary (ca. 66.0 Ma) and possibly contributed to the end-Cretaceous mass extinction event through emission of gases such as SO2, CO2, Cl, F that may have triggered global climate changes. Severe pollution by volcanic gases is supported by the high S and Cl contents (up to 1400 and up to 900 ppm, respectively; Self et al., 2008) measured in a few olivine- and plagioclase-hosted melt inclusions from the Jawhar, Neral, and Thakurvadi Formations (early lava flows, ca. 66.3-66.4 ± 0.1 Ma; Renne et al., 2015) and by magmatic S contents (up to 1800 ppm; Callegaro et al., 2014) calculated from S measurements in clinopyroxenes from the Mahabaleshwar unit (ca. 65.5 ± 0.1; Schoene et al., 2015). Here, we present new analyses of S, Cl, and F, obtained by ion-probe and synchrotron light micro-fluorescence analyses on clinopyroxenes and plagioclase phenocrysts from ?al? lava flow units of the Western Ghats. The volatile contents of the host magmas have been calculated from recently published clinopyroxene/basalt partition coefficients. These new data will describe the time-related variation of volatile elements hosted and eventually emitted by Deccan lavas and shed light on their environmental impact. References: Callegaro S. et al. (2014). Geology 42, 895-898. Renne P.R. et al. (2015). Science 350, 76-78. Schoene B. et al. (2015). Science 347, 192-184. Self S. et al. (2008). Science 319, 1654-1657
On the cohomology of Artin groups in local systems and the associated Milnor fiber
No full textLet W be a finite irreducible Coxeter group and let X-W be the classifying space for G(W), the associated Artin group. If A is a commutative unitary ring, we consider the two local systems L-q and L'(q) over X-W, respectively over the modules A[q, q(-1)] and A[[q, q(-1)]] ,given by sending each standard generator of G(W) into the automorphism given by the multiplication by q. We show that H*(X-W, L'(q)) = H*(+1) (X-W, L-q) and we generalize this relation to a particular class of algebraic complexes. We remark that H*(X-W, L'(q)) is equal to the cohomology with trivial coefficients A of the Milnor fiber of the discriminant bundle of the associated reflection group
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