TESTS FOR HARDNESS. 275 



measured. By this means a scale of relative hardness 

 was determined. The work done in causing the point 

 to penetrate the metal was obtained from a falling 

 weight. This method was also employed by Lieut ; -Col. 

 Martel, who has obtained the following values for the 

 relative hardness of a number of metals : 



TABLE OF EELATIVE HARDNESS ACCORDING TO THE 

 MARTEL SCALE. 



Metals. Relative Hardness. 



High carbon steel, hardened 613 



High carbon steel, not hardened 460 



Medium steel, hardened 455 to 300 



Rolled wrought iron 226 



Cast iron 300 to 208 



Bronze, cast in sand 137 



Copper, rolled 156 



Copper, annealed 64 



Tin, cast .' 33 



Lead, cast 9 



A convenient form of test for hardness, also depending 

 upon the indentation of the material, has been devised by 

 Professor Unwin.* In Unwin's test, an indenting tool of 

 hard steel having the form of a knife edge is used, and the 

 indention is produced by a steady load on a short bar of 

 the metal to be tested. Provision is made for the 

 measurement of the amount of the linear indentation 

 produced by a series of loads, and from these observa- 

 tions a constant is determined which is taken as the 

 measure of the hardness of the metal. 



On Fig. 138 is shown the arrangement of the apparatus 

 used by Professor Unwin. This consists, in the first 

 place, of the two blocks marked respectively A and B. Of 

 these, A is a plunger loosely fitting into the cast-iron 

 block B. The whole rests between the compression plates 

 of the testing machine, by which the plunger is forced 

 downwards. At the lower surface of the plunger is a 

 triangular groove forming the seating in which rests 

 the indenting tool C, which consists of a short piece of 

 square steel, hardened and ground. The test-piece is 

 marked C in the figure, and usually consists of a piece of 

 the material 2in. long and fin. square. By means of the 

 vernier and scale shown, the relative movement of the 

 plunger and the base block can be measured to 1,000th of 



* Min. Proc. Inst. Civ. Bug. Vol. CXXIX., p. 334. 



