166 A. L. Parsons — New Sclerometer. 



reading is made in divisions on the micrometer screw (E). In 

 the instruments of Seebeck and others this same force is 

 measured by the weights with which the test point is loaded. 

 This measurement by weights has the advantage that the 

 weights are constant, while the bending of the spring (F) is 

 dependent upon the material and dimensions of the same, and 

 with each instrument we get a different factor. In order to 

 reach comparable results it is necessary to reduce the divisions 

 on the micrometer screw to measures of weight This reduc- 

 tion of elevation to weight is called the calibration of the 

 instrument* 



The calibration of the instrument is made in this way. A 

 glass plate is fastened to the object carrier and a scratch is 

 made with the diamond point. The glass is now brought to 

 the position where the test point just touches the beginning of 

 the scratch. This point is determined when the test point just 

 meets its reflection in the glass. The reading on the micro- 

 meter screw (E) is taken, the plate lowered by means of the 

 micrometer screw and a weight placed on the test point holder 

 (H) by which the spring is bent. The plate of glass is now 

 raised until the test point just touches the beginning of the 

 scratch and a second reading is made on the micrometer screw 

 (E). Subtracting the second reading from the first gives the 

 number of divisions on the micrometer screw that corresponds 

 to a given w T eight. 



By lowering the glass plate by means of the micrometer 

 screws not only the plate but the track (6) and the object 

 carrier (O) describe a small arc of a circle about A so that the 

 beginning of the scratch must be brought under the point by 

 means of the transport screw (4).f 



The deflection of the spring for various weights is measured 

 and a table can be made for all. 



The results of the calibration are as follows: 





Weight 

 grm. 



Eeading 



Deflection of 

 needle in 



divisions of 



graduated 



circle 



No. of 



divisions 



equivalent 



to 1 grm. 



Mean No. of 



divisions 



equivalent to 



1 grm. 



irst trial. 







183'0 













5 



170-0 



13 



2-6 







10 



159-0. 



24 



2-4 







20 



134-5 



48-5 



2-425 







50 



60-5 



122-5 



2-45 



2-469 



* Every instrument is furnished with a calibration table so as to give the 

 weight that corresponds to one division on the micrometer screw. 



f A very slight error is thus introduced but in practice this can be neg- 

 lected, as the determinations of hardness are made in the same way and the 

 errors are comparable. 



