MR. A. E. TUTTON ON A COMPENSATED INTERFERENCE DILATOMETER. 363 



THE DETERMINATION OF THE EXPANSION OP A CRYSTAL. 



It only remains now to briefly indicate the mode of employing the compensator 

 for the determination of the expansion of a parallel-faced block of a crystal or any 

 other substance, whose thickness between the parallel plane surfaces need not exceed 

 5 millims. 



In case the method is followed of employing the compensator below the crystal, one 

 of the two thinnest aluminium compensating blocks, respectively 4 and 6 millims. 

 thick, is employed, together with one of the 2 millims. thick black glass discs 

 of 10 millims. diameter. The exact length of platinum-indium screw which expands 

 to the same amount as each of these, is readily found from the knowledge now 

 available of the coefficient of expansion of each. Knowing, therefore, the length 

 of screw whose expansion would be exactly counterbalanced by the added expansion 

 of the particular aluminium compensator and crystal-covering-glass, the screws are 

 to be set to this length above the raised points by the aid of the thickness 

 measurer. As regards choice of compensator, that one is selected whose thickness, 

 added to that of the covering-glass, will be less than the corresponding screw-length 

 by an amount which is just slightly greater than the thickness of the crystal block, 

 slightly greater in order to leave room for the necessary air film between the two 

 reflecting surfaces. 



The compensator is placed on the raised points of the tripod. A masking disc 

 (lens stop), similar to that referred to in connection with the determinations of the 

 tripod expansion, but of smaller aperture, is next laid on the compensator. Its 

 aperture is slightly less than the diameter of the black covering-glass, so that it 

 arrests all light other than that from the covering-glass. The crystal block is then 

 laid with one of its two plane parallel surfaces resting on the centre of the aluminium 

 block within the aperture of the mask, and the black crystal-covering-glass, polished 

 face upwards, is placed on the top of the crystal. On then laying the large cover- 

 wedge over the screws, its lower surface should be found to be separated by the 

 desired air-layer from the polished surface of the black glass. The whole is then 

 transferred to the interference chamber and the adjustment of the images from the 

 reflecting surfaces proceeded with. 



If the method of employing the compensator above the crystal is chosen, the latter 

 is laid first on the three points of the tripod table, and on it is then laid one of the 

 smaller compensators furnished with three points, these latter resting directly on the 

 crystal. No covering-glass is required, the aluminium surface reflecting light 

 adequately. The screw-length should be that calculated for complete compensation 

 by the compensator chosen. 



In effecting the adjustment of the lower reflecting surface by slight variation of 

 the screws, it is preferable to do so in such a manner that any difference of screw- 

 length introduced, from that which is compensated, should be an excess. This can 



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