192 ELEMENTARY CHEMICAL MICROSCOPY 



standard load is 3000 kilograms, for soft materials 500 kilo- 

 grams. The number used to express the " Brinell Hardness " 

 is the ratio of the applied load to the area of the indentation 

 produced. To calculate the area of indentation we may measure 

 either the depth of the indentation or its diameter. 



Micrometric microscopes of short range were formerly 

 employed for these measurements, but in America the practice is 

 generally to measure the depth of the indentation with some type 

 of depth gauge. All measurements are expressed in millimeters. 



Since in many cases the forcing of the steel ball into the test 

 piece causes the edges of the impression to become slightly 

 raised above the surface of the surrounding metal, great care 

 must always be taken in focusing the microscope. If the depth 

 is to be determined by means of the fine adjustment, focus the 

 instrument upon the very center of the spherical depression, 

 then move the test piece until an area of the true surface is 

 brought into the field of view. Read the graduated circle of 

 the fine adjustment and carefully focus up with the final adjust- 

 ment until the surface is sharply defined, read the graduated 

 circle again. The difference in the readings will give the depth 

 of the indentations in terms of the fine adjustment graduations. 

 Knowing the value of one division of this scale, the depth 

 expressed in millimeters may be calculated. Make several 

 determinations, always focusing up, as directed above for the 

 calibration of the fine adjustment. Make all measurements 

 as near as possible to the circumference of the indentation yet 

 scrupulously avoiding the ridge of metal right at the brim. 



The diameter can rarely be measured with an eyepiece microm- 

 eter in an ordinary compound microscope since it is of too great 

 a magnitude for even very low powers. Recourse should be 

 had to graduated mechanical stages. In such cases the accuracy 

 of the measurements will not be greater than one- tenth of a 

 millimeter. Always measure several diameters. 



If h be the depth of the indentation: The Brinell Hardness 

 may be calculated from the formula: 



or B.n. = r 



10. h h 



