284 ON THE RIGIDITY 



It appears from these results that increase of tension in a 

 cord which has been under tension for some time immediately 

 decreases the rigidity, but that if the cord is left under the 

 tension the rigidity increases again. According to the longest 

 series of observations made, the rigidity of a cord thus left 

 under a constant load seerns to pass through a maximum point* 

 but that is perhaps doubtful. 



Since the immediate effect of increase of tension is to 

 decrease the rigidity, it might be expected that the twisting of 

 the cord in the determination of its rigidity would increase the 

 rigidity, and that consequently the greater the angle the cord is 

 twisted through the smaller will be the value of the rigidity 

 found. This expectation is borne out by the results as given 

 in the tables. It was also noticed when taking several obser- 

 vations of the time of oscillation in order to get a mean value, 

 that the first values were always the smallest, a fact which 

 seems to indicate that the rigidity decreases with strain. 



From the preceding it is easily seen that the procedure 

 followed above was not such as could give a simple relation 

 between the observed rigidity and the tension. For since the 

 rigidity of a cord under tension varies with time, the experiments 

 must always be made, if they are to give a definite result, when 

 the rigidity is at a minimum or a maximum. Now the minimum 

 value of the rigidity of a cord under tension appears from the 

 last table to be immediately after the tension is applied. If, 

 however, we decide to determine the rigidity when at its 

 minimum, not only must the rigidity be found immediately 

 after the tension is applied, but the rigidity of the cord due to 

 its previous tension must not have had time to change from the 

 minimum value. Hence the procedure should be as follows : 

 Load the cord and find the rigidity immediately, then increase 

 the load and find the rigidity immediately, and so on, the whole 

 series of experiments being carried out in the shortest time 

 possible. 



With a new cord of the same kind as before, this pro- 

 cedure was followed, and the results of Table V obtained. 



