Changes in Torsion of a Wire by Change of Temperature. 161 



(The zigzags in the middle of this diagram arise from the 

 rapid movement setting the needles in vibration.) 



If the change of temperature is pushed in each direction 

 after the limiting elongation is attained, we obtain a cycle of 

 events which can be most simply represented by a parallelo- 

 gram or series of parallelograms, such as the following: — 



Te 



np. 





















80 





















70 



y 















1 



k 



' 



60 







___^ 













Elongations 





3 



4 



5 



6 



7 



8 









g 







Here the vertical lines represent the limits of elongation, 

 the^ one corresponding to rising temperature, the other to 

 falling temperature. On reversal of the direction of change 

 of temperature such lines as the sloping lines are described. 

 The inclination of these is nearly constant, and they may lie 

 at any height between the lines of limiting elongation. 



The above figure is a typical representation of the character 

 of the principal effect; but the lines are never really straight, 

 and there are subsidiary effects, which, -however, in the 

 present case are small compared with that above described. 



The tables and figures at the end of this paper explain 

 themselves. 



There was some difficulty in obtaining that part of the 

 curve which corresponds to the lower horizontal line of the 



Phil. Mag. S. 5. Vol. 24. No. 147. August 1887. M 



