316 



QUARTZ FIBERS, 



we had here au arm iu a glass case and a mirror to read tlie motion of 

 the arm, it mouUI be easy to observe a movement a hundred or a thou- 

 sand times less than that just produced, and therefore to measure a 

 force a hundred or a thousand times less than that exerted by this feeble 

 magnet. 



Again, if instead of wire as thick as an ordinary i)in I had used the 

 finest wire that can be obtained, it would have oi)i)Osed the movement 

 of the straw with a far less force. It is possible to obtain wire ten times 

 liner than this stubborn material, but wire ten times finer is much n^ore 

 than ten times more easily twisted. It is ten thousand times more easily 

 twisted. This is because the torsion varies as the fourth power of the 

 diameter, so we say 10 x 10 = 100 ; 100 x 100 = 10,000. Therefore with 

 the finest wire, forces 10,000 times feebler still could be observed. 



It is therefore evident how great is the advantage of reducing the 

 size of a torsion wire. Even if it is only halved the torsion is reduced 

 sixteen-fold. To give a better idea of the actual sizes of such win s and 

 fibers as are in use I shall show ui)on the screen a series of photographs 

 taken by Mr. Chapman, on each of which a scale of thousandths of an 

 inch has been printed. 



5 



J-JL 



10 



I I 



Scale of lOOOtlis (if an iucli for !• i^s. 1 ti> 7. The scale of Fiiis. S ;iiul 9 is inucli finer. 



' ^A 





Ei'i ''' I 

 Fio. 



"Ill I'l" 



Fig. 3. 



The first ph()tograj)h (Fig. 1) is an ordinary hair — a sufficiently lamiliar 

 object, and one that is generally spoken of as if it were rather fine. 

 Much fiu'jr than this is the si)ecimen of copper wire now on the screen 



