PROCEEDINGS. 75 



accuracy desired. These extensions are measured between two 

 small holes in the test piece which in metal specimens are usually 

 10 inches apart. Two steel points fit into these holes, one of 

 which is fixed and the other moveable by the stretching of the 

 specimen. The whole apparatus is slung on the test piece and is 

 totally unaffected by its motions as a whole, and of course by any 

 change of form in the frame of the machine. The instrument as 

 you see requires delicate handling, but it possesses the advantage 

 of being easily calibrated and checked from time to time on 

 vernier callipers, such as the ones exhibited. There are many 

 methods of measuring minute elongations, but the present 

 apparatus commends itself from its simplicity. 



I am also indebted to Prof. Kennedy for the beautiful piece of 

 apparatus which I am able to show you this evening which is 

 called an Autographic Stress-strain apparatus as it draws a 

 diagram which shows clearly the strain produced by stresses which 

 vary from to that required to break the bar. The form of the 

 diagram for a piece of mild steel is somewhat as sketched upon 

 the board, but which is more correctly represented by the 

 photographs lying upon the table. It will be observed, that the 

 extension produced by a given load is represented as an abscissa, 

 while the load itself is represented as a curved ordinate. The 

 diagram represents the behaviour of the specimen during the test, 

 in the first place the application of the load from zero to 14-44 

 tons per square inch produced an extension which was proportional 

 to the stress producing it. This point is therefore termed the limit 

 of elasticity as between and 14*44 tons the material is perfectly 

 elastic. But for stresses exceeding this amount a considerable 

 change takes place in the diagram, the extensions are propor- 

 tionately greater, and at 16*65 tons per square inch there is a drop 

 in the curve showing a sudden yield in the specimen. The further 

 application of load causes the material to draw out until at 25*38 

 tons per square inch the specimen fractures. 



The curve not only indicates the limit of elasticity, yield point, 

 breaking load, and the amount of extension which occurs at these 

 points, but it is seen by inspection that the local extension which 

 occurs at the breaking point is measured by drawing ordinates at 

 the commencement and termination of the curve drawn during 

 the time the specimen is undergoing local extension. Again the 

 area of the diagram represents the gross mechanical value of the 

 material as it represents the work done in breaking the bar, which 

 of course depends upon its breaking strength and ductility. Like 

 the apparatus for measuring small extensions it is obvious that it 

 should be handled carefully. 



The principle of the apparatus is as follows : the test piece is 

 placed in the machine with a stronger bar, which is called a spring 



