VIII. A compensated interference dilatometer

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Abstract
The instrument now described is a form of Fizeau interference dilatometer, in which the sensitiveness of the interference method is so largely increased as to render it unnecessary to employ a plate or block of the solid, whose expansion is to be measured, of greater thickness than 5 millimetres. The author has been led to adopt it for the accurate determination of the thermal expansion of the crystals of artificial chemical preparations, which it is frequently impossible to obtain sufficiently large, and at the same time homogeneous, to furnish plates a centimetre thick, as demanded by the forms of apparatus hitherto described. The exquisite method devised by Fizeau (‘Compt. Rend.,’ vol. 58, p. 923, and vol. 62, p. 1133; ‘Ann. Chim. Phys.,’ [4], vol. 2, p. 143, and [4], vol. 8, p. 335) depends essentially upon the determination of the difference of expansion, which accompanies rise of temperature, between the screws of a small metallic tripod and the object under investigation which is supported by it. In the form of apparatus now described the expansion of these screws is compensated and eliminated, thus rendering the total expansion of the object available for measurement. Hence, it is possible to obtain a result with a small crystal of as satisfactorily accurate a character as was formerly only to be obtained with a much larger crystal. Besides the introduction of this compensating principle, the new instrument combines, in the author’s opinion, the best features of the several forms of Fizeau dilatometer previously described; at the same time it is essentially different from any one of these previous forms, and includes many details of a novel character. The author is largely indebted to the Memoirs of Benoit (‘Trav. et Mémoires du Bureau Int. des Poids et Mesures,’ vol. 1, 1881, p. 1, and vol. 6, 1888, p. 106), concerning the Fizeau apparatus belonging to the Bureau International des Poids et Mesures, Paris, and the classical work which he has accomplished by the use of it ; and to the later one of Pulfrich (‘Zeitschrift fur Instrumentenkunde,’ 1893, p. 365) concerning an improved form of apparatus embodying the important modifications introduced into the method by Abbe in the year 1884 and described by Weidmann in 1889 (‘Wiedemann’s Annalen,’ vol. 38, p. 453).

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