nr.M.TX FIHKKS AND ITS TK.MPKKATUIK COEFFICIENT. 



417 



In all, the modulus of rigidity of six fibres was determined. These were prepared 

 from different specimens of quartz, in order to see to what extent the modulus could 

 lie taken as being a constant quantity. The first fibre experimented on that to 

 which the above numbers refer was taken from the laboratory stock of fibres, and 

 was probably not so carefully prepared as the others. The modulus for this fibre is, 

 however, in quite good agreement with the other values obtained. These are given 

 in the following table : 



It should be mentioned that fibres 4, 5, and 6 in the above table had been heated 

 several times over a range of temperature from 15 C. to 100 C. in determining 

 the temperature coefficient of the modulus, as described in Part II. of this paper. 

 The effect of this heating is slightly to increase the value of the modulus at the 

 ordinary temperature of the laboratory, but this increase is much too small to account 

 for the fact that the rigidity of these fibres is greater than the other values given in 

 the table. 



The diameter of fibre 2 was found to vary in an irregular manner at different 

 points along its length. The value given as the mean radius of this fibre is therefore 

 not so reliable as in the other cases. This inaccurate knowledge of the radius 

 probably accounts for the value of the modulus of rigidity given by fibre 2 being 

 considerably lower than the other values obtained. The mean of these other 



values, viz., 



n = 3*001 x 10" dynes per sq. centim., 



may be taken as the modulus of rigidity of quartz fibres at 15 C. 



From the numbers in the table given above it is seen that there is no sort of 

 connection between the radius of the fibre and its modulus of rigidity. It follows 

 that, since the same vibrator was used with each fibre, the modulus of rigidity is 

 independent of the longitudinal stress to which the fibre is subjected so long as this 

 is kept within the limits used in these experiments. The stress in each case is given 

 l>elow. 



VOL. cciv. A. 



3 H 



