316 



QUARTZ FIBERS, 



we bad here an arm iu a glass case and a mirror to read the motion of 

 the arm, it would 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 pin I had used the 

 finest wire that can be obtained, it would have opposed the movement 

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

 finer than this stubborn material, but wire ten times finer is much m.ore 

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

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

 diameter, so we say 10x10= IQO ; 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 wires and 

 fibers as are in use I shall show upon the screen a series of photographs 

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

 inch has been printed. 





 I 1 



5 

 I I t 



I i 



10 

 [ 



Scale of lOOOtlis of an inch for Figs. 1 to 7. The scale of Fi^js. 8 and 9 is much finer. 





Fig. 1. 



I I 



I'l III 



1 I 



Fig. 3. 



The first photograph (Fig. 1) is an ordinary hair — a suflBciently familiar 

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

 Much fin-^r than this is the specimen of copper wire now on the screen 



