An air suspension to demonstrate the properties of torsion balances with fibers of zero length.

We report on the design and characterization of an air-bearing suspension that has been constructed to highlight the properties of torsion balances with fibers of zero length. A float is levitated on this suspension, and its rotational and translational motion in the horizontal plane of the laboratory is controlled using magnetic actuators. We demonstrate the in situ electromagnetic tuning of the float's center-of-buoyancy to an accuracy of ±0.3 mm, which was limited by the noise in the air bearing. The rotational stiffness of the float, which is approximately zero by design, was also measured. We compare the observed behavior of the float with the predictions of a detailed model of the statics of the float-actuator system. Finally, we briefly discuss the application of these ideas and results to the construction of sensitive devices for the measurement of weak forces with short ranges.

A method for reducing the adverse effects of stray-capacitance on capacitive sensor circuits.

We examine the increase in voltage noise in capacitive sensor circuits due to the stray-capacitance introduced by connecting cables. We have measured and modelled the voltage noise of various standard circuits, and we compare their performance against a benchmark without stray-capacitance that is optimised to have a high signal-to-noise ratio for our application. We show that a factor limiting sensitivity is the so-called noise gain, which is not easily avoided. In our application, the capacitive sensor is located in a metallic vessel and is therefore shielded to some extent from ambient noise at radio frequencies. It is therefore possible to compromise the shielding of the coaxial connecting cable by effectively electrically floating it. With a cable stray-capacitance of 1.8 nF and at a modulation frequency of 100 kHz, our circuit has an output voltage noise a factor of 3 larger than the benchmark.

A cryogenic optical feedthrough using polarization maintaining fibers.

Polarization maintaining optical fibers can be used to transmit linearly polarized light over long distances but their use in cryogenic environments has been limited by their sensitivity to temperature changes and associated mechanical stress. We investigate experimentally how thermal stresses affect the polarization maintaining fibers and model the observations with Jones matrices. We describe the design, construction, and testing of a feedthrough and fiber termination assembly that uses polarization maintaining fiber to transmit light from a 633 nm HeNe laser at room temperature to a homodyne polarization-based interferometer in a cryogenic vacuum. We report on the efficiency of the polarization maintaining properties of the feedthrough assembly. We also report that, at cryogenic temperatures, the interferometer can achieve a sensitivity of 8 × 10(-10) rad/√Hz at 0.05 Hz using this feedthrough.

Forces between conducting surfaces due to spatial variations of surface potential.

We describe analytical and numerical methods for calculating forces between conductors due to variations of electrostatic surface potential across their surfaces. In the simple case where the spatial variation of surface potential gives rise to uniform power spectra, we show that the electrostatic force can be large in comparison with, and scale in approximately the same way with distance of closest approach as, the Casimir force. Patch potentials that are consistent with existing experimental data could give rise to forces with a magnitude of 4% of the Casimir force at separations of 0.1 microm.

A new determination of G using two methods.

We present the results of a measurement of G made with a torsion-strip balance used in two substantially independent ways. The two results agree to within their respective uncertainties; the correlation coefficient of the two methods is -0.18. The result is G = 6.675 59(27)x10(-11) m(3) kg(-1) s(-2) with a standard uncertainty of 4.1 parts in 10(5). Our result is 2 parts in 10(4) higher than the recent result of Gundlach and Merkowitz.

nifH sequences and nitrogen fixation in type I and type II methanotrophs.

Some methane-oxidizing bacteria (methanotrophs) are known to be capable of expressing nitrogenase and utilizing N2 as a nitrogen source. However, no sequences are available for nif genes in these strains, and the known nitrogen-fixing methanotrophs are confined mainly to a few genera. The purpose of this work was to assess the nitrogen-fixing capabilities of a variety of methanotroph strains. nifH gene fragments from four type I methanotrophs and seven type II methanotrophs were PCR amplified and sequenced. Nitrogenase activity was confirmed in selected type I and type II strains by acetylene reduction. Activities ranged from 0.4 to 3.3 nmol/min/mg of protein. Sequence analysis shows that the nifH sequences from the type I and type II strains cluster with nifH sequences from other gamma proteobacteria and alpha proteobacteria, respectively. The translated nifH sequences from three Methylomonas strains show high identity (95 to 99%) to several published translated environmental nifH sequences PCR amplified from rice roots and a freshwater lake. The translated nifH sequences from the type II strains show high identity (94 to 99%) to published translated nifH sequences from a variety of environments, including rice roots, a freshwater lake, an oligotrophic ocean, and forest soil. These results provide evidence for nitrogen fixation in a broad range of methanotrophs and suggest that nitrogen-fixing methanotrophs may be widespread and important in the nitrogen cycling of many environments.