786 The Bifilar Property of Twisted Strips. 



The following results were obtained for two steel strips. 



1 



dO 

 Dimensions. -^ • 



2 6 3 

 31 ' 



C when 

 T=0. 



n. 



41 -9 X 1-26x0-01 cm. 

 31 -Sx 1-59x0022 



•007 

 •0256 



•008 

 •0214 



25000 

 308500 



8-351 xlO 11 

 8-555 xlO 11 



In the strip 0*01 cm. thick the restoring couple under the 

 greatest tension is 48*5 per cent, above that under least 

 tension : this shows the necessity of allowing for the bifilar 

 effect when obtaining the rigidity of materials by experiments 

 with strips. 



Pealing came to the conclusion that a bifilar action was 

 inadmissible because there was no observable dependence of 

 period upon tension in a steel strip which he used. The 

 dimensions of the strip were 67 X 0*64 X 0*04 cm. Calcula- 

 tion shows that as T varies from to 1000 grams weight per 

 cm. breadth, C should increase by 0*2 per cent. This of 

 course is so small that it escaped observation. The small 

 bifilar effect is masked by the large rigidity effect. 



We may calculate the bifilar effect for the strips of 

 phosphor-bronze used by Pealing, using equation (3) as- 

 suming n— 4*36 X 10 U . 



Dimensions. 



T (dyne 

 per cm.) 



C (dyne 

 cms.). 



Per cent, of C due to 

 bifilar action. 



(1)25x0-022x0-001 cm. 

 (2)25x0-48 x 00043 cm. 



25X981 

 0022 



25x981 

 0-148 



9-594+2-969 



31 per cent. 

 8'5 per cent. 



75 



16630+1413 

 75 



The strips used by Pealing showed an increase of 200 per 

 cent, in some cases; hence the anomalous behaviour is not 

 altogether explained by bifilar action, but is presumably 

 partly due to overstrains, as suggested by Pealing. How- 

 ever, the above strips after being annealed should have 

 exhibited a variation in rigidity due to increase of load, 

 which seems to have escaped observation. 



Campbell has not given the dimensions of the strips used 

 in the suspended system of his galvanometer, but he stated 



