Temperature in Twisting or Untwisting Wires. 255 



(3) A few turns of permanent torsion suffice to produce 

 the maximum alteration of thermal expansibility which can be 

 effected by this means. 



Metal ... 



Copper. 



Iron. 



Aluminium 



Silver. 



Platinum. 



Load on the 

 wire, in 

 kilos. 



) ■ 



1 



2 



1 



1 



Number of 

 complete 

 turns of 



permanent 

 torsion. 



Permanent 



untwist 



produced 



by heating. 



P. 



Temporary 



effect of 



heating. 



+ signifies 



temporary 



twist, T. 



P. 



T. 



P. 



T. 



P. 



T. 



P. 



T. 



10 







24 



3-5- 



800 



74- 



298 



2-5- 





4+ 



20 





31 + 









71- 











30 









6-0- 















40 

















3-0- 







50 



555 



36+ 









74- 







63 



10+ 



100 



690 



41 + 





5-0- 









3-0- 



138 





150 













78- 







273 



12-5+ 



200 



861 





58 















13-0+ 



Small variations of the load hanging on the wire produced 

 no sensible effect on the temporary twist or untwist following 

 on rise of temperature. The case, however, was different 

 when large loads were employed. With platinum, for ex- 

 ample, by increasing the load from 1 to 7 kilos, the temporary 

 twist produced by rise of temperature was increased threefold. 

 With copper also there was a similar effect produced by largely 

 increasing the load. On the contrary, it would seem that 

 with these metals, in which untwist is produced by rise of 

 temperature, increase of load diminishes the untwist ; and if 

 the load be sufficiently large, the wire begins to be twisted 

 temporarily by rise of temperature. This was found to be the 

 case with annealed iron. The wire was twisted permanently 

 with gradually increasing amounts of load on it, and after 

 each twisting was slightly heated by running a burner up 

 and down it. As before, with moderate loads there was tem- 

 porary untwist on heating ; when the load reached 10 kilos, 

 there was neither temporary twist nor untwist ; and finally, 

 when the load was 16 kilos., there was very appreciable tem- 

 porary twist. 



Annealed piano-steel behaved like annealed iron ; but with 



