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.

Some advantages of video conferencing over high-quality audio conferencing

Abstract: There are many commercial systems capable of transmitting a video image of parties in a conversation over a digital network. Typically, these have been used to provide facial images of the participants. Experimental evidence for the advantages of such a capability has been hard to find. This paper describes two experiments that demonstrate significant advantages for video conferencing over audio-only conferencing, in the context of a negotiation task using electronically shared data. In the video condition there was a large, high-quality image of the head and upper torso of the participant(s) at the other end of the link and high-quality sound. For the audio-alone condition the sound was the same but there was no video image. The criteria by which these two communication conditions were compared were not the conventional measures of task outcome. Rather, measures relating to conversational fluency and interpersonal awareness were applied. In each of the two experiments, participants completed the same task with data presented by a shared editor. In Experiment 1, they worked in pairs and in Experiment 2 they worked at quartets with two people at each end of the link. Fluency was assessed from transcripts in terms of length of utterance, overlapping speech and explicit questions. Only the latter measure discriminated between the two communication conditions in both experiments. The other measures showed significant effects in Experiment 2 but not in Experiment 1. Given this pattern of results it is concluded that video can result in more fluent conversation, particularly where there are more than two discussants. However, in the case of dyadic conversation auditory cues to turn taking, etc., would seem to suffice. In both experiments there was a large and significant effect on interpersonal awareness as assessed by ratings of the illusion of presence, and most clearly, awareness of the attentional focus of the remote partner (s). In Experiment 2, the ratings for the remote partners were similar to those for the co-located discussants, demonstrating the effectiveness of the video link with regard to these subjective scales.

Methodologies for determining the dynamic ranges and signal-to-noise ratios of terahertz time-domain spectrometers.

Abstract: We discuss procedures for calculating the signal-to-noise ratios and the dynamic ranges of terahertz time-domain spectrometers. We also offer recommendations for standardized calculations for time-domain and frequency-domain data.

The Scotopic Visibility Function

Abstract: The scotopic (dark-adaptation) visibility of radiation through the spectrum has been determined for fifty observers under 30 years of age. A modified photometric matching method was used at a very low brightness (3×10-6 candles/ft2 or 3.2×10-9 stilbs). A subsidiary investigation showed that the ultimate scotopic curve was approximated closely. Another demonstrated the effect of age, showing a progressive decrease in sensitivity at the blue end of the spectrum with increasing age, the effect first becoming noticeable at about 30 years of age. A detailed comparison of the present results with those of previous workers is made and reasons for discrepancies are discussed.

The physics of high temperature creep in metals

Abstract: The slow deformation of metals, known as creep, first came clearly into focus about half a century ago As a problem in metal use it has grown steadily in importance because engineers have persistently raised their operating temperatures in many fields, until now it is one of the half-dozen most important of these problems Besides the abundance of technical data that has necessarily been accumulated, there have been many studies in the last fifteen years of the physics of creep, and it is with these that this article is concerned These physical studies have shown that there are several different creep r?gimes depending mainly on the temperature If TM is the melting point of the metal in question, the different r?gimes roughly cover the temperature ranges 0-0?3 TM, 0?3-0?5 TM, 0?5-0?9 TM and 0?9-1?0 TM The bottom range includes the so-called logarithmic creep and the top range creep by diffusion, which is somewhat similar to flow in liquids Both are quite well understood but neither is particularly important and they are dealt with briefly It is the middle two temperature ranges in which creep worries engineers; they have therefore received much more attention and the creep behaviour in them is described more fully in this article In both of these temperature bands the rate of creep strain is very dependent on temperature, stress applied and composition of the metal The temperature dependence is no surprise, since it is quite clear that creep is a thermally activated phenomenon and its rate is therefore governed by an Arrhenius factor In many pure metals the temperature dependence is quantitatively close to that of self-diffusion, particularly in the 0?5-0?9 TM band, and the agreement grows better the more refined the measurements become In the last ten years this result has guided most theories, and has encouraged attempts to be made with partial success to calculate the absolute rates from first principles Calculations have been founded on both the two basic theoretical models One assumes that the factor controlling creep rate lies in the deformation process, or glide movement of dislocations, itself The other assumes that the rate-controlling factor lies in the continuous annealing that takes place at the high temperatures involved The two models are really two sides of the one penny, since both slip and recovery take place simultaneously and are unavoidably interconnected When the two models are properly joined they explain the influence of composition, which may affect creep rate by well over a millionfold, and the great influence of stress as well as the temperature effect The physics of creep deformation in the middle temperature ranges is therefore quite well understood Indeed, the important parameters like diffusion rate, stress and stacking fault energy combined in a single equation give the creep rates of many simple metals with fair accuracy A consequence of the growing success in producing alloys that deform very slowly even at high stress and temperature is that the problem of creep fracture has loomed larger There is a fracture mechanism quite distinctive to creep, in which tiny holes nucleate and grow by some means until they are so large, or sufficiently linked together, that the metal breaks The speed of this fracture process increases with temperature and stress, and evidently also depends on composition in a complicated way, which has made possible a certain degree of manufacturing control if not of understanding In essence, the formation and growth of the holes is a phase change in which stress provides the driving free energy since the holes enlarge the overall dimensions Both the nucleation and growth rates help to determine the time to fracture, which is what really matters There are several obscure points such as the nature of many of the nucleating sites and the strong influence of deformation rate during nucleation and growth Both theoretically and practically, understanding of the youthful problem of creep fracture is less mature than that of the older problem of creep deformation