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 ohservations made, the rigidity of a cord thus left 
under a constant load seems to pass through a maximum points 
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 ora 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 mgidity when at its. 
3 
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. 
