AUTOGRAPHIC APPARATUS. 223 



produce an apparatus capable of drawing accurate and purely 

 autographic diagrams of tension tests. This piece of appara- 

 tus is represented diagrammatical ly on Fig. 106. It consists 

 essentially of two parts, namely, the strain recording portion 

 and that for recording the stress. The diagram is drawn 

 upon a sheet of smoked glass at A. This glass slides in 

 a frame B, which is attached to the middle of the " spring- 

 piece " P. This frame also carries the centre about which 

 the radial arm, C, rotates. On the end, D, of this arm 

 rotates a frame consisting of an arm and two pieces 

 forming arcs of circles whose radii are in the ratio of 1 to 2. 

 Around the arc having the smaller radius passes a 

 wire, or its equivalent in the form of a strip of steel. The 

 other end of this is attached to the further end of the 

 specimen. To the arc of larger radius is fastened a thread 

 which itself is attached to the sliding frame carrying the 

 sheet of smoked glass upon which the diagram is to be 

 drawn. The mode of operation of this part of the apparatus 

 is as follows : The test specimen, S, stretches under the load, 

 and the two points at which are respectively linked to the 

 arm C and E, and the free end of the wire F, move with 

 respect to one another. The result is that the wire is 

 pulled, and the radial arm rotated through a small arc, 

 and, consequently, the thread which is fastened to the slide 

 is moved through twice the distance represented by the 

 stretch of the specimen. This constitutes the movement 

 necessary to record the strain, the movement of the sheet 

 of glass being proportional to, but double, the extension 

 of the bar. 



In order to obtain the other co-ordinate of the diagram, 

 a special piece of apparatus is interposed between the 

 shackles of the machine and the specimen. To this has 

 been given the name of "spring-piece." By making use 

 of this, the load on the specimen at any moment is 

 recorded in a manner which is entirely independent 

 of the lever of the machine and its poise weight. 

 This spring piece is simply a test piece of larger 

 size than the one which is being tested, its size being such 

 that the maximum load on the specimen does not stretch 

 it beyond its elastic limit. As the spring piece, P, and the 

 specimen, S, are placed " in series,' any load which comes 

 upon the specimen has to be transmitted to it through the 

 spring-piece, which itself sustains precisely the same load. 

 In consequence of the spring piece at all times carrying 

 stresses which bear a fixed ratio to the stresses on the 



