8 Schwartz & Kemp, Physical Properties of Rnbber. 



moving coil mirror galvanometer in the present instance. 

 T}-pical examples of the results obtained by Joule are 

 shown in Figs. 2 and 3, which are useful as indicating the 

 magnitude of the effects referred to. 



In the series of tests which were carried out by the 

 authors, the specimens used consisted of 88% Para 

 rubber, the remaining 12% being mineral matter of small 

 extensibility. The test pieces were 4" square in cross- 

 section and about 6" long. The actual value of the 

 length does not affect the result, since the total quantity 

 of heat generated is directly proportional to the length 

 used. The specimen was suspended inside a glass tube, 

 which was surrounded by a vessel containing crushed ice 

 to keep the temperature constant at o^C. Owing to the 

 very slow rate at which rubber conducts heat, it was found 

 advisable to embed the specimen in a block of ice the 

 night before the test, in order to cool it down to freezing 

 point. In addition to this, the two ends of the containing 

 tube were plugged with cotton wool to prevent the ingress 

 of the outside air. Ten longitudinal incisions were made 

 in the specimen at equal distances apart, in each of which 

 was inserted a thermo-junction made of No. 40 S.VV.G. 

 copper and eureka wires soldered together. These ten 

 thermo-junctions were connected in series and across the 

 terminals of a sensitive moving coil mirror galvanometer, 

 the movement of which was aperiodic, due to the gradual 

 rise of temperature experienced by the thermo-junctions. 

 Weights were attached, by means of a cord, to the lower 

 end of the rubber specimen, and the resulting deflection of 

 the galvanometer noted, the specimen being brought back 

 to a condition of no load between each reading. Frequently 

 a minute or so would elapse before the maximum deflection 

 was registered, so that, except in the cases of the smaller 

 differences of temperature, some heat would be lost by 



