On Silk v. Wire Suspensions in Galvanometers. 47 



The first method was almost identical with that introduced in 

 this laboratory thirty -five years ago by Sir W. Thomson, and 

 now commonly adopted for the determination of the rigidity 

 of metallic wires. It consisted in suspending from a fixed 

 support, by means of a measured length of the fibre, a thin 

 circular rim of non-magnetic material and of easily calculated 

 moment of inertia, and observing the period of the torsional 

 vibrations. From this the torsional rigidity of the fibre can 

 be readily calculated by a well-known formula. The second 

 method consisted in suspending a small mirror, to which was 

 rigidly fixed a small magnetic needle of known magnetic 

 moment by means of a measured length of the fibre, and 

 observing the deflection of the mirror produced by twisting 

 the top of the fibre through a measured angle. This gives a 

 ready means of calculating the rigidity of the fibre in terms 

 of the magnetic moment of the suspended needle, and the 

 strength of the magnetic field in which it is suspended. 



The fibres were of Japanese floss- silk, which had been 

 thoroughly washed in hot water to remove the gum which is 

 always found in considerable quantity on cocoon fibres. The 

 fibres were in all cases single fibres ; and it will be seen, both 

 from the direct measurements by the microscope and from the 

 rigidity, that they vary considerably in thickness. Even a 

 rough estimate of the rigidity per square centimetre section 

 of the substance is impossible, as the fibre is not even approxi- 

 mately circular in section, and its diameter not nearly regular 

 along its length. The results of the experiments are given in 

 the following Table, the headings of the different columns 

 being sufficiently explanatory of the numbers. 



Vibrator Method. 



o . 

 o S 



3 







srator, 

 ds. 



gidity 

 nits of 

 1 cen- 

 fibre. 



V s 



© So 



03 ■>-< 



Eadius of g 



of vibrato 



centiinet 



*3 § 

 Hog 



^.5 



s » 



** o 



«4- Q 



O <B 



.2.2 

 © 



Torsional ri 

 in O.G.S. u 

 a length of 

 tim. of the 



0-0274 



0-20 



8-60 



0-0008 



29 



0-00096 



0114 



0-29 



860 



0-0010 



16 



0-00132 



