Abstract
BOSTON, U.S.A. American Academy of Arts and Sciences, October 9.—Charles Francis Adams, president, in the chair.—Prof. W. A. Rogers read a paper on the limits of accuracy in measurements with the microscope, in which he stated that Prof. E. N. Morley and himself independently measured 195 spaces having a magnitude of about 1/500 of an inch, each space, however, varying slightly from this value. The measures were made with a glass eye-piece micrometer, a Beck's spider line micrometer, and with a screw attached to the stib-stage of the microscope. After the results were prepared for the press they were for the first time compared. It was found that the average difference between the results for a single space was 32 millionths of an inch, and the greatest difference was 12 millionths. There were only four cases in which the difference amounted to one hundred thousandth of an inch.—In a second paper Prof. Rogers gave a determination of the errors of the sub-divisions of a copy of the British yard known as Bronze No. 11 and of the metre of the U.S. Bureau of Weights and Measures and the production therefrom of an inch, which is one thirty-sixth of this particular yard, and of a centimetre, which is one-hundredth part of this particular metre, the temperature in both cases being 67° F.—Prof. John Trowbridge described a new electro-dynamometer for measuring strong electric currents without shunting them. The principle consists in cooling the two points in the revolving axis of the instrument where the current enters and leaves by means of a current of water and in using mercury pivots. The instrument can measure from a fraction of a Weber up to six hundred Webers. It is especially adapted for the measurement of currents produced by dynamo-electric machines.
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SOCIETIES AND ACADEMIES . Nature 18, 712 (1878). https://doi.org/10.1038/018712a0
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DOI: https://doi.org/10.1038/018712a0