68 MEASUREMENT OF COUPLES. 



The smallest diameter of the wire is defined as above by the 

 equation 



which gives 



With a unifilar suspension the wire should have twice the section, 

 that is, a diameter equal to d,J^; if the length is the same, the 

 moment of torsion is 



y 



We deduce from these expressions the ratio of the coefficients 



C' 



This ratio is equal to unity if 



TT T' 8T' 



T 

 making T' = , we get 



a i 



The numbers in (711) show that for most ordinary metals the 

 value of the ratio L / is near 10; it follows from this that for the 



same metal the sensitiveness of the bifilar becomes equal to that of the 

 unifilar when the two wires are removed five times their diameter. 

 A very different result would be arrived at with non-metallic threads, 

 such as cocoon fibres. 



720. Notwithstanding the advantages of the bifilar as regards 

 changes of temperature, this mode of suspension seems to have been 



