356 MR. A. E. TUTTON ON A COMPENSATED INTERFERENCE DILATOMETER. 



Mean values, a = O'OOO 008 599 8, b = O'OOO 000 002 278 6. 



The final result, therefore, for the mean coefficient of expansion a + bt of the 

 platinum-iridium of the tripod, between and t, is 



O'OOO 008 600 + O'OOO 000 002 28, or 10- 9 (8600 + 2'28). 



The true coefficient of expansion at t, or mean coefficient between any two 

 temperatures whose mean is t, a. = a + 2&, is 



a = O'OOO 008 600 + O'OOO 000 004 56t, or = 10~ 9 (8600 + 4-56). 



It will be interesting to compare the value for the expansion of the specimen of 

 platinum-iridium composing the author's tripod with the values found by BENOIT 

 for various specimens of JOHNSON and MATTHEY 10 per cent, alloys of iridium with 

 platinum. BENOIT'S values are as follows ; they refer to the mean coefficient between 



and t : 



Specimen 1, 10- 9 (8615 + 2'21). 



2, 10~ 9 (8593 + 2'40). 



3, 10- 9 (859S + 2'22). 



4, 10- (8575 + 2-38<). 



5, 10~ 9 (8575 + 2-54). 



It will be observed that the author's value agrees most satisfactorily with these 

 values of BENOIT. As the coefficient of iridium is much lower, namely, C'0000065 , 

 than that of platinum, 0'0000089, a slight difference in the proportions of the two 

 metals present in the alloy might be expected to produce just such slight variations 

 in the coefficient as are observed. 



DETERMINATION or THE EXPANSION OF THE ALUMINIUM COMPENSATOR. 



The aluminium compensating blocks were cut from the same casting of the purest 

 obtainable aluminium. The thickest block of the 25 millim. series, slightly over 

 12 millims. thick, was. used for the purpose of determining the expansion by the 

 differential Fizeau method. The platinum-iridium tripod screws were preliminarily 

 set to about 12'5 millims., on that side of the table which was furnished with 

 raised points. The aluminium block was placed upon the table, resting on 

 the three outer highest points. The tripod and block were then placed in the 

 interference chamber, and the large glass cover-wedge provided with the minute 

 central silver ring (not the cover-wedge silvered all over, which is used only for the 

 tripod determinations) was placed over the screws. The corresponding counter- 

 balancing wedge had previously been exchanged for the former one in the upper 

 aperture of the chamber, and the engraved direction chords were, as before, arranged 



