A short history of measurement standards at the National Physical Laboratory

Measurement science has been one of the major interests of the NPL since the creation of the Laboratory in 1900. This article gives a historical review of NPL work on the establishment, maintenance and improvement of the basic standards of measurement for mechanics, electro-magnetics, thermometry, photometry and radiology. Many early NPL standards and essential components of classioal measuring equipment have now been assembled into a permanent collection for preservation at the Laboratory and illustrations of some items in this collection are reproduced in the article.     

Geometric optics analysis of non-circular,graded-index fibres

The cross-sections of practical optical fibres do not have perfect circular symmetry. The appropriate mathematical representation of the refractive index profile is discussed and it is shown that the basic power law profile in circular fibres should be replaced by a grading functionF which is a homogeneous function of thex andy coordinates. Multimode optical fibres of this type are analysed using geometric optics. General properties of ray paths are described. The fundamental quantities of interest in fibre optics (power acceptance, ray transit time, impulse response) are shown to depend on the degree of homogeneity ofF but not on its specific form. Thus, fibres need not have circular symmetry in order to retain the desirable properties of the circular power law fibres. Ray paths and classifications are analysed in detail for elliptical, parabolic-index fibres. Splicing losses are examined and are shown to be not drastically dependent on deviations from circular symmetry.     

Highly diastereoselective nitrone cycloaddition onto a chiral ketene equivalent: asymmetric synthesis of cispentacin

[reaction: see text] A highly diastereoselective intramolecular nitrone cycloaddition onto a chiral ketene equivalent, obtained by Horner-Wadsworth-Emmons olefination of either enantiomer of bis-sulfinyl phosphonate 6, is described. Cycloaddition gave 5,5-disubstituted isoxazolidine 10 in good yield as a single diastereomer. Catalytic hydrogenolysis of 10 furnished either enantiomer of optically pure cis-2-aminocyclopentane-1-carboxylic acid.     

Highly enantioselective oxidations of ketene dithioacetals leading to trans bis-sulfoxides

Ketene dithioacetals undergo a Sharpless-type asymmetric oxidation using (+)-DET, Ti(O(i)()Pr)(4), and cumene hydroperoxide to give the trans bis-sulfoxides 4a-f with essentially complete control of enantioselectivity and diastereoselectivity. The high enantioselectivity is a consequence of carrying out two asymmetric processes on the same substrate. However, this should lead to the formation of a small amount of the meso isomer but none was isolated. From monitoring the enantioselectivity of the monoxide over time, it was concluded that small amounts of the meso isomer must be formed. The inability to isolate this compound could be because it acted as a ligand on titanium and remained tightly bound even upon workup.