A PRELIMINARY STUDY OF SOME MODERN 

 MICROMETERS 



By MARSHALL D. EWELL. 

 {Read April ig, 190/'.) 



With the advent of the International Bureau of Weights and 

 Measures and our own National Bureau of Standards, each of 

 which has done and is doing a great work in the extension 

 of a uniform and correct system of standards throughout the 

 world, it was to be expected that this uniformity and precision 

 would be exemplified in the stage micrometers manufactured and 

 for sale by makers of scientific instruments in - this and other 

 countries. With a view to determining how far this expecta- 

 tion has been realized the writer has collected as many stage 

 micrometers as he conveniently could by purchase from dealers or 

 through the courtesy of friends, and has measured from five to 

 ten spaces on each with great care and herewith presents in part 

 the results of such measurements. The instruments used were an 

 excellent stand made especially for micrometry by the late W. 

 H. Bulloch, a very stable stand made by the Spencer Lens Co., 

 filar micrometers by Zeiss, The Spencer Lens Co., Bausch and 

 Lomb, Zentmayer and others, and a Bausch and Lomb opaque 

 illuminating one-inch objective, a Zeiss A A, and a Leitz No. 3 

 objective. From five to ten readings were taken at each end of 

 the space measured and the mean taken. The errors in total length 

 of the several spaces measured have not yet been determined, 

 though the observations have been made ; and the results here 

 given are the relative errors only or the differences between each 

 designated space and the mean of all the measured spaces. These 

 scales were all, except where otherwise designated, ruled on glass, 

 and in many instances the lines have deteriorated to a considerable 

 extent, though not so as to invalidate the measurements. A few 

 were photographs and one or two had very coarse lines, so coarse 

 as to render them unsuitable for any but low powers. 



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