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VIII. A Compensated Interference Dilatometcr. 



By A. E. TUTTON, Assoc. R.C.S. 

 Communicated by Capt. ABNEY, G.B., F.It.S. 



Received March 8, Read April 28, 1808. 



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 elimi- 

 nated, 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 com- 

 bines, 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 Memoires 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 PULFJRICH (' Zeitschrift fvir Instrurnentenkunde,' 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). 



VOL. CXCT. A. 2 S 21.7.98 



