﻿A NEW COMPARATOR. 



By Alvin J. Cox. 

 (From the Chemical Division, Bureau of Science, Manila, P. I. 



Prof. Dr. Weinstein 1 says that in the construction of a comparator 

 the following things must be. taken into consideration : "(1) The external 

 conditions, under which the comparison of the measures is made, (2) 

 the construction of the measuring apparatus (microscope, micrometer), 

 (3) the properties of the bars investigated." With regard to the first and 

 last points, if the bars compared have the same coefficient of expansion, 

 then it is only necessary to examine them at the same temperature. This 

 is not difficult, for the external conditions can be controlled by the isola- 

 tion of the cathetometer room and the exercise of due care. 



There remains the construction of the apparatus to be considered. 

 The demand for a great many provincial and other secondary standards 

 in connection with the initiation of the new Weights and Measures Law 

 of the Philippine Islands has made indispensable to this Bureau an 

 instrument which could be depended upon to give comparisons with 

 the standard meter bar, accurate within a few numbers in the second 

 decimal of a millimeter. A high grade cathetometer similar to many 2 

 of those especially designed for making eudiometer readings was tried, 

 but found unsatisfactory for our purposes. The one used is described by 

 Gerhardt 3 as — 



A catlietometer with millimeter divisions of one meter length which at 0° 

 is correct to 0.01 millimeter. The vernier permits a reading of 0.1 millimeter. 

 The telescope turns in a plane exactly at right angles to the upright har and the 

 level upon the top has a sensibility of twenty seconds. For the first adjustment 

 of the instrument there is a circular spirit level on the base. The telescope, etc., 

 are perfectly balanced. The micrometer is so constructed that the possibility 

 of an inclination of the telescope with the adjustment of the micrometer screw is 

 excluded. Both the bar and the travel are perfectly straight, so that there is no 

 turning of the telescope by moving up and down. 



1 Weinstein : Deutsche Mechaniker Ztg. (1899) (Berlin), 28. 



2 Terquem, A.: Journ. de Phys. (1883), 12, 496. Miller. F. : Ztschr. f. Instru- 

 mentenkunde (1883), 3, 409. Fues, R.: Ibid. (1886). 6, 153. Wadsworth. 

 F. L. O.: Am. J. Sc. ( 1896) . 1 51 , 41. 



3 Gerhardt, C. : Preisverzeichnis iiber Cheniische Apparate und Geriitschaften 

 (1905), 13th Ed., 119, Bonn. 



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