166 Note of the Radius of Curvature of a Cutting Edge. 



FIG. 3. 



' 



We have 



Whence 



D'F+GE' = 2 a sin 10, nearly; 

 FK+LG= 



KL = 



In the case of the razor on which these measures were made 

 N = 85 % = 3 n z = 2 aje = 8a = 0*00405 in. sin \\Q | and since 

 for soda light \\ O'OOOOllG in. nearly 



KL = 0-0000116 x 88-0-00405 x 0'25 

 = 0-00102-0-00101, nearly. 



Thus K L is not greater than O'OOOOl, and if it is assumed that the 

 actual edge has the curved cross-section, indicated by the dotted line 

 in fig. 3, the radius of curvature cannot be greater than 1/200,000 of 

 an inch. 



A well sharpened razor will cut a hair, when merely pressed against 

 it at about an eighth of an inch, or rather more, from the place 

 where the hair is held. 



Human hair taken from the head has a circular cross-section, and 

 varies in diameter in different individuals from 0'002 to 0*004 in. 



With a hair of 0'0025 in, diameter, fixed at one end and free at the 

 other, it was found that half a grain acting at an eighth of an inch 

 from the fixed end, bent it through an angle of about 30. 



