14 Schwartz & Kemp, Physical Properties of Rubber. 



brass rod ' E ' depends, ending in the upper grip ' F.' The 

 specimen 'G' is suspended vertical ly from this, the lower end 

 being attached to the second grip 'H.' A thin glass rod 'J' 

 extends from the lower grip to a point outside the heater, 

 where it is attached to a cord 'K' passing round the 

 pulleys 'L' and 'M.' The weight 'N' serves to keep the 

 specimen taut without putting an)' further tension on it. 

 In order to exert additional tension, two weights ' PP ' are 

 suspended directly from the lower grip, this method 

 having the advantage of not subjecting the pulley ' L ' to 

 any severe stress. Two weights are here used in order to 

 keep the lower grip in a horizontal position. A galvano= 

 meter mirror 'Q' is fixed on the axle of the pulley 'L,' and 

 this reflects a beam of light on to a scale at a distance of 

 two metres. In order to correct for the expansion of the 

 apparatus itself, a copper strip, of similar dimensions to 

 the test piece, was inserted in place of the rubber specimen. 

 The apparatus was then heated up, and a curve of scale 

 readings at different temperatures was obtained. The 

 correction curve thus obtained was used when measuring 

 the changes in length of specimens. 



It has been frequently stated that rubber has a 

 negative co-efficient of linear expansion, but, from results 

 obtained by means of the above apparatus, it is clear that 

 this co-efficient is positive in the case of vulcanised rubber 

 under no load, if the specimen has never been previously 

 stretched. If, however, the specimen is extended for a 

 short period and allowed to rest for some time under no 

 load, on the application of heat a contraction will be 

 observed, being followed by an expansion. Fig. 7 is illus- 

 trative of these points, the specimens used being 88% 

 vulcanised rubber, the remaining 12% being mineral 

 matter. The specimens used were -4" wide, iV' thick and 

 about 6" long, the length being accurately determined in 



