1,708 research outputs found
Reflections on a Measurement of the Gravitational Constant Using a Beam Balance and 13 Tons of Mercury
In 2006, a final result of a measurement of the gravitational constant
performed by researchers at the University of Z\"urich was published. A value
of G=6.674\,252(122)\times
10^{-11}\,\mbox{m}^3\,\mbox{kg}^{-1}\,\mbox{s}^{-2} was obtained after an
experimental effort that lasted over one decade. Here, we briefly summarize the
measurement and discuss the strengths and weaknesses of this approach.Comment: 13 pages, 5 figures accepted for publication in Phil. Trans. R. Soc.
Towards a Graphene-Based Quantum Impedance Standard
Precision measurements of the quantum Hall resistance with alternating
current (ac) in the kHz range were performed on epitaxial graphene in order to
assess its suitability as a quantum standard of impedance. The quantum Hall
plateaus measured with alternating current were found to be flat within one
part in 10^7. This is much better than for plain GaAs quantum Hall devices and
shows that the magnetic-flux-dependent capacitive ac losses of the graphene
device are less critical. The observed frequency dependence of about
-8x10^-8/kHz is comparable in absolute value to the positive frequency
dependence of plain GaAs devices, but the negative sign is attributed to stray
capacitances which we believe can be minimized by a careful design of the
graphene device. Further improvements thus may lead to a simpler and more
user-friendly quantum standard for both resistance and impedance
Violator Spaces: Structure and Algorithms
Sharir and Welzl introduced an abstract framework for optimization problems,
called LP-type problems or also generalized linear programming problems, which
proved useful in algorithm design. We define a new, and as we believe, simpler
and more natural framework: violator spaces, which constitute a proper
generalization of LP-type problems. We show that Clarkson's randomized
algorithms for low-dimensional linear programming work in the context of
violator spaces. For example, in this way we obtain the fastest known algorithm
for the P-matrix generalized linear complementarity problem with a constant
number of blocks. We also give two new characterizations of LP-type problems:
they are equivalent to acyclic violator spaces, as well as to concrete LP-type
problems (informally, the constraints in a concrete LP-type problem are subsets
of a linearly ordered ground set, and the value of a set of constraints is the
minimum of its intersection).Comment: 28 pages, 5 figures, extended abstract was presented at ESA 2006;
author spelling fixe
Base sequence dependent sliding of proteins on DNA
The possibility that the sliding motion of proteins on DNA is influenced by
the base sequence through a base pair reading interaction, is considered.
Referring to the case of the T7 RNA-polymerase, we show that the protein should
follow a noise-influenced sequence-dependent motion which deviate from the
standard random walk usually assumed. The general validity and the implications
of the results are discussed.Comment: 12 pages, 3 figure
Compendium for precise ac measurements of the quantum Hall resistance
In view of the progress achieved in the field of the ac quantum Hall effect,
the Working Group of the Comite Consultatif d'Electricite et Magnetisme (CCEM)
on the AC Quantum Hall Effect asked the authors of this paper to write a
compendium which integrates their experiences with ac measurements of the
quantum Hall resistance. In addition to the important early work performed at
the Bureau International des Poids et Mesures and the National Physical
Laboratory, UK, further experience has been gained during a collaboration of
the authors' institutes NRC, METAS, and PTB, and excellent agreement between
the results of different national metrology institutes has been achieved. This
compendium summarizes the present state of the authors' knowledge and reviews
the experiences, tests and precautions that the authors have employed to
achieve accurate measurements of the ac quantum Hall effect. This work shows
how the ac quantum Hall effect can be reliably used as a quantum standard of ac
resistance having a relative uncertainty of a few parts in 10^8.Comment: 26 pages, 8 figure
TDP-43 knockdown impairs neurite outgrowth dependent on its target histone deacetylase 6
<p>Abstract</p> <p>Background</p> <p>Trans-activation response element (TAR) DNA binding protein of 43kDa (TDP-43) is causally related to the neurodegenerative diseases frontotemporal dementia and amyotrophic lateral sclerosis being the hallmark protein in the disease-characteristic neuropathological lesions and via genetic linkage. Histone deacetylase 6 (HDAC6) is an established target of the RNA-binding protein TDP-43. HDAC6 is an unusual cytosolic deacetylase enzyme, central for a variety of pivotal cellular functions including aggregating protein turnover, microtubular dynamics and filopodia formation. All these functions are important in the context of neurodegenerative proteinopathies involving TDP-43. We have previously shown in a human embryonic kidney cell line that TDP-43 knockdown significantly impairs the removal of a toxic, aggregating polyQ ataxin-3 fusion protein in an HDAC6-dependent manner. Here we investigated the influence of TDP-43 and its target HDAC6 on neurite outgrowth.</p> <p>Results</p> <p>Human neuroblastoma SH-SY5Y cells with stably silenced TDP-43 showed a significant reduction of neurite outgrowth induced by retinoic acid and brain-derived neurotrophic factor. Re-transfection with TDP-43 as well as HDAC6 rescued retinoic acid-induced neurite outgrowth. In addition, we show that silencing of HDAC6 alone is sufficient to reduce neurite outgrowth of <it>in vitro </it>differentiated SH-SY5Y cells.</p> <p>Conclusions</p> <p>TDP-43 deficiency leads to impairment of neurite growth in an HDAC6-dependent manner, thereby contributing to neurodegenerative events in TDP-43 diseases.</p
A Measurement of Newton's Gravitational Constant
A precision measurement of the gravitational constant has been made using
a beam balance. Special attention has been given to determining the
calibration, the effect of a possible nonlinearity of the balance and the
zero-point variation of the balance. The equipment, the measurements and the
analysis are described in detail. The value obtained for G is 6.674252(109)(54)
10^{-11} m3 kg-1 s-2. The relative statistical and systematic uncertainties of
this result are 16.3 10^{-6} and 8.1 10^{-6}, respectively.Comment: 26 pages, 20 figures, Accepted for publication by Phys. Rev.
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