20 Schwartz & Kemp, Physical Properties of Rubber. 



If the relation between load and extension throughout 

 the above cycle of operations be plotted, it will be found 

 that an area is enclosed between the extension curve and 

 the retraction curve which represents the work done in the 



rubber itself 



The authors have employed the mechanical hysteresis 

 of rubber, above referred to, as an index to its quality, and 

 have designed a machine in which the relation between 

 the load and extension can be automatically recorded 

 throughout the test. The machine is shown diagram- 

 matically in Fig. lo. The test piece having been secured 

 in the grips, the fixed grip 'A' is mounted on the pins on 

 the bracket *B,' and the movable grip 'C,' which depends 

 from the specimen 'D,' is connected by the hook 'E' to the 

 cord 'F,' which passes round the floating pulley 'G' to the 

 helical spring 'H.' The load is applied and withdrawn by 

 the up and down traverse of the pulley ' G,' which is 

 effected by means of the cord 'J,' attached to a nut 'K,' 

 which is moved along the guide by the screw ' L', actuated 

 by hand at ' M.' 



The relation between load and extension is charted in 

 the following way : — The grip 'C,'the movement of which 

 represents the extension, is connected to the pencil carrier 

 ' N ' by the thread ' P ' which passes over the pulley ' Q.' The 

 pencil carrier, which moves between the guides 'R,' contains 

 a pulley 'S,' round which the thread 'F' passes to a stop 'T' 

 (which is adjustable as to position), where it is made fast. 

 The movement of the pencil is thereby reduced to one- 

 half of the extension of the specimen. If necessary, this 

 movement may be again reduced by one-half by the in- 

 troduction of a light floating pulley in the thread 'P.' The 

 movements of the pencil carrier are controlled by the 

 counter-weight ' U.' Beneath the pencil carrier, and moving 

 at right angles to it, is a light table ' V,' on which the chart 



