548 



Mr. a V. Boys 



[June 14, 



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 move- 

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

 ten times finer that this stubborn material, but wire ten times finer 

 is much more 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 = lOU ; 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 advantaoje of reducing the 

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



10 

 J 



Scale of lOOOths of an inch for Figs. 1 to 7. The scale of Figs. 8 and 9 is 

 much finer. 



Fig. 1. 



Fig. 2. 



Fig. 3. 



reduced sixteen-fold. To give a better idea of the actual sizes of 

 such wires and fibres 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. 



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



