National Physical Laboratory

About: National Physical Laboratory is a facility organization based out in London, United Kingdom. It is known for research contribution in the topics: Dielectric & Thin film. The organization has 7615 authors who have published 13327 publications receiving 319381 citations.

The change of visual sensitivity with time

Abstract: On changing instantaneously the external light stimulus applied to the eye, the latter does not also change instantaneously its state of adaptation to that corresponding to the new stimulus. No general definition of the state of adaptation of the eye has been given, or, perhaps, can be given. One can, however, imagine quite clearly that, when the eye is viewing some given external distribution of brightness, the concentration of photochemical substances in the various parts of the retina, the nervous messages sent from retina to brain and the consequent reactions in the latter will all be tending towards certain equilibrium values. When the external brightness distribution is changed, these equilibrium values will also change in the direction of a new set of equilibria. Unfortunately, it is difficult to measure any of these things directly and it is necessary to compromise. Amongst quantities more readily measurable the liminal or threshold responses of the eye occupy an important place. These quantities must be closely related to the actual physical state of the visual system and they are of considerable practical significance in themselves. The problem of the change of visual state with time has therefore been approached by measuring the change of visual sensitivity with time. The general experimental problem is to expose the eye to a certain primary or conditioning stimulus for a given period, then suddenly to change to a different stimulus, called for convenience the secondary stimulus, and to make at intervals thereafter a series of threshold measurements, the secondary stimulus remaining steady. This general specification of the problem may be subdivided as follows : 1—Primary and secondary stimuli the same in pattern. ( а ) Primary stimulus greater than secondary. ( b ) Primary stimulus less than secondary. 2—Primary and secondary stimuli different in pattern. This section may also be subdivided according to whether the secondary stimulation corresponds to ( a ) a higher, ( b ) a lower, or ( c ) the same equilibrium value of the visual test on the test area of the retina which is under investigation.

Lamb shifts and hyperfine structure in 6Li+ and 7Li+: Theory and experiment.

Abstract: High-precision laser-resonance measurements accurate to \ifmmode\pm\else\textpm\fi{}0.5 MHz or better are reported for transitions among the 1s2s $^{3}$${\mathit{S}}_{1--}$1s2p $^{3}$${\mathit{P}}_{\mathit{J}}$ hyperfine manifolds for each of J=0, 1, and 2 in both $^{6}\mathrm{Li}^{+}$ and $^{7}\mathrm{Li}^{+}$. A detailed analysis of hyperfine structure is performed for both the S and P states, using newly calculated values for the magnetic dipole and electric quadrupole coupling constants, and the hyperfine shifts subtracted from the measurements. The resulting transition frequencies are then analyzed on three different levels. First, the isotope shifts in the fine-structure splittings are calculated from the relativistic reduced mass and recoil terms in the Breit interaction, and compared with experiment at the \ifmmode\pm\else\textpm\fi{}0.5-MHz level of accuracy. This comparison is particularly significant because J-independent theoretical uncertainties reduce through cancellation to the \ifmmode\pm\else\textpm\fi{}0.01-MHz level. Second, the isotope shifts in the full transition frequencies are used to deduce the difference in rms nuclear radii.The result is ${\mathit{R}}_{\mathrm{rms}}$${(}^{6}$Li)-${\mathit{R}}_{\mathrm{rms}}$${(}^{7}$Li)=0.15\ifmmode\pm\else\textpm\fi{}0.01 fm, in agreement with nuclear scattering data, but with substantially improved accuracy. Third, high-precision calculations of the low-order non-QED contributions to the transition frequencies are subtracted from the measurements to obtain the residual QED shifts. The isotope-averaged and spin-averaged effective shift for $^{7}\mathrm{Li}^{+}$ is 37 429.40\ifmmode\pm\else\textpm\fi{}0.39 MHz, with an additional uncertainty of \ifmmode\pm\else\textpm\fi{}1.5 MHz due to finite nuclear size corrections. The accuracy of 11 parts per million is the best two-electron Lamb shift measurement in the literature, and is comparable to the accuracies achieved in hydrogen. Theoretical contributions to the two-electron Lamb shift are discussed, including terms of order (\ensuremath{\alpha}Z${)}^{4}$ recently obtained by Chen, Cheng, and Johnson [Phys. Rev. A 47, 3692 (1993)], and the results used to extract a QED shift for the 2 $^{3}$${\mathit{S}}_{1}$ state. The result of 30 254\ifmmode\pm\else\textpm\fi{}12 MHz is shown to be in good accord with theory (30 250\ifmmode\pm\else\textpm\fi{}30 MHz) when two-electron corrections to the Bethe logarithm are taken into account by a 1/Z expansion method.

Measurement of thermal and ultrasonic properties of some biological tissues

Abstract: The measurement of thermal and ultrasonic properties of biological tissues is essential for the assessment of the temperature rise induced in vivo by diagnostic ultrasound. In this paper, we present measurements of thermal conductivity, thermal diffusivity, speed of sound and ultrasonic attenuation of fresh ex vivo porcine tissue, namely 'muscle' (from abdomen and leg), 'skin with subcutaneous fat' (from abdomen and leg), 'abdominal fat' and 'bone'. The measurements of the thermal properties of biological tissue samples are based on a transient method. Thermal property measurements show that subcutaneous fat has the lowest thermal conductivity (0.23 W m(-1) K(-1)), while muscle gives the highest values (0.46 W m(-1) K(-1)). Thermal diffusivity of muscle tissue recorded the highest value among the studied tissues (0.16 mm(2) s(-1)) while that of skin with subcutaneous fat gave the lowest value (0.11 mm(2) s(-1)). A scanning acoustic macroscope was used to measure attenuation coefficient and speed of sound for the tissue samples. The results for the speed of sound are broadly similar to those reported in the literature. The power law dependence of the attenuation coefficient of the form eta = a f (b) as a function of frequency was found to be more appropriate than the linear fit in this study.

Suppression of low-frequency charge noise in superconducting resonators by surface spin desorption.

Abstract: Noise and decoherence due to spurious two-level systems located at material interfaces are long-standing issues for solid-state quantum devices. Efforts to mitigate the effects of two-level systems have been hampered by a lack of knowledge about their chemical and physical nature. Here, by combining dielectric loss, frequency noise and on-chip electron spin resonance measurements in superconducting resonators, we demonstrate that desorption of surface spins is accompanied by an almost tenfold reduction in the charge-induced frequency noise in the resonators. These measurements provide experimental evidence that simultaneously reveals the chemical signatures of adsorbed magnetic moments and highlights their role in generating charge noise in solid-state quantum devices.

On the Stability under Shearing Forces of a Flat Elastic Strip

Abstract: 1. The investigation relates to flat elastic strip, of uniform breadth, thickness and material, upon which a uniform shear is imposed by tangential tractions applied at its edges and in its plane. The tractions appear in the expression for the change of potential energy which occurs when the strip is bent, and they must therefore affect both the modes and the frequencies of its free transverse vibrations. If sufficiently intense, they will bring about a condition of limiting elastic stability, since they can neutralize, in certain types of distortion, the restoring effects of the flexural stresses. The results have some bearing on the stability of the webs of deep plate girders, which take the greater part of the total shear transmitted. The correspondence must not, however, be pressed unduly, because in a girder uniform shear will be accompanied by a varying bending moment which imposes additional stresses upon the web. It is more accurate to describe the sheared strip (of which the length, in this paper, has been assumed to be infinite) as the limiting case either of a narrow annular disc, or of a short tube, subjected to torsion. The similarity of the three problems is illustrated by the specimens shown in fig. 1, which have buckled under conditions of limiting elastic stability.