438 Mr. Norman Shaw on Increased Accuracy 



and equation (7) becomes 



*=D(1-«-*)-dJ-=£^-* 



= D - D r^- W , • w 



which is the solution o£ our problem. Referred to the 

 asymptotic limit as origin, this takes the form 



4 * = - D IT^" A " 00 



It is important to note that in the distortion of bifilar 

 suspensions under the conditions of ordinary deflexion 

 measurements we are dealing with the structural alterations 

 that occur at the beginning of what proves to be a group of 

 several types of change. It has been pointed out by Trouton 

 and Kankine, for the case of single wires*, that there is first 

 an immediate effect, which is followed bv an increase with 

 time ; the latter may initially be considerable, but gradually 

 diminishes to a small constant value. They refer to the first 

 as the "primary strain," to the final constant rate as the 

 viscous flow," and to the intermediate effect as the "secon- 

 dary strain/' In single suspensions subjected to large dis- 

 tortions, the primary strain is often great, and it is difficult 

 to examine experimentally because it takes place so rapidly 

 hence it has been with the subsequent behaviour under the 

 secondary strain and viscous flow that most of the previous 

 investigations have dealt. In the case of bifilar suspensions 

 subjected to small distortions, we have the primary strain 

 acting slowly over a much longer interval of time. We 

 shall therefore, not expect our result to hold for large values 

 ot t because our assumptions do not involve any cognizance 

 of these later factors; but it does, however, suitably repre- 

 sent that range of the creep with which we have to deal in 

 the practice and use of bifilar suspensions. When the viscous 

 flow becomes appreciable, we find that the experimental 

 curve lies above the theoretical curve (see fig. 2), and if the 

 suspension be given time enough to recover from the primary 

 strain we find that there is left a considerable permanent 

 distortion These latter effects appear to cause a decrease in 

 the couple approximately inversely proportional to the time 

 and can therefore be represented by an equation of the form' 



x = a + b\ogt HO) 



* Trouton and Ranlnne, Phil. Mag. vol. viii. 1904. p. 538. 



