100 Prof. Richard Threlfall on the 



6. An attempt to discover elastic fatigue by keeping- 

 threads vibrating for many days in succession. 



7. A determination of the Young's modulus of quartz fibres 

 by the method of bending, leading to a calculation of the 

 compressibility of fused quartz. 



I shall describe the experiments and computations made 

 under each of these heads in order. 



Production of Threads, and Conditions of Experiment. 



For the purpose more especially of the experiments on 

 Young's modulus it was necessary to make threads much 

 thicker than those usually employed for purposes of suspen- 

 sion. This was partly due to the fact that in determining 

 both the rigidity and the Young's modulus the diameter of 

 the threads requires to be known with greater accuracy than 

 any of the other data, as it enters the result raised to the 

 fourth power. On the other hand, it is probable that the 

 properties of thick threads are not quite identical with those 

 of thin ones. This is certainly the case with the breaking- 

 strength, but whether the relative difference of strength of 

 thin and thick threads is to be attributed to the difference in 

 thickness per se, or to the consequent difference in annealing, 

 which necessarily accompanies difference of thickness, I cannot 

 say, but I incline to the latter view. In experimenting 

 therefore it becomes a matter of judgment and experience to 

 decide what thickness of threads it is best to work with. I 

 have endeavoured to proceed in such a way as to have the 

 threads neither too thin for tolerably accurate measurement, 

 nor so thick as to be entirely removed in properties from the 

 thin threads which are used in practice. I have in general 

 found that it is impossible, with my apparatus, to be sure of 

 the thickness of threads of about *015 centim. diameter 

 within less than three or four per cent, by a single measure- 

 ment, and have consequently adopted the plan of making 

 many observations of each thickness, with a view to elimi- 

 nating the accidental errors — which, I have satisfied myself, 

 are of the chief importance. I have also experimented in 

 each case on several samples of threads so as to eliminate any 

 possible individual peculiarity of the threads employed. 



I have to thank Mr. Adair for making all the observations 

 of time of vibration, and computing most of the results ; Mr. 

 Pollock for determining the coefficient of expansion of fused 

 quartz and the coefficient of expansion of a brass vibrator, 

 and also for determining the breaking-strength of many 



