524 Mr. Phillips on the Slow Stretch in lndiarubher, 



Before the load was put -on, the telescope was placed to 

 view the scale : then the load was put on and the times were 

 taken as the successive divisions on the scale passed the 

 cross-wire of the telescope. 



The following is a fairly typical set of readings for a 

 copper wire : — 



Length of wire = 28*2 cms. 



Diameter of wire = '0390 cm, 

 .*. Cross-section = '001195 cm 2 . 



Weight of clips, &c.= 122 grams. 

 Annealing current = 12 amps, for 5 minutes. 



Load. 



Reading on 



Scale. 



Time. 



_i_| 



log!?. 



122 grams. 



84 











722 „ 





llh 30m 



0s 









542 



31 



13 



1-22 



•09 





543 



31 



23 



1-38 



•14 





544 



31 



36 



16 



•20 





545 



31 



48 



1-8 



•26 





546 



32 



8 



2-13 



•33 





547 



32 



26 



2-43 



•39 





547-5 



32 



36 



26 



•41 





548 



32 



48 



2-8 



•45 





548-5 • 



32 



58 



2-97 



•47 





549 



33 



12 



3-2 



•50 





549-5 



33 



24 



3-4 



•53 





550 



33 



38 



363 



•56 





550-5 



33 



56 



3-93 



•59 





551 



34 



15 



4-25 



•63 





551-5 



34 



34 



4-57 



•66 





552 



34 



52 



4-87 



•69 



t, as in the case of india-rubber, is the time in minutes 

 which has elapsed since the load was put on. If log t is 

 plotted against the scale-reading we again get a straight 

 line, and therefore copper obeys the same law x = a + b log t. 



If x is measured in scale-divisions, b for this case is equal 

 to 16-28, and a = 456'7. >_4^. 7 



In the following table (p. 525) the constancy of x . Zzz_L [ s 



exhibited. lo g* 



To bring b to absolute units we must know the value of 

 the scale-division. This was found by removing the wire, 

 and so placing a micrometer-screw gauge that the leg of the 

 optical lever, which previously rested on the table i, was now 

 resting on the top of the movable jaw of the screw-gauge. 



