236 



the frequency of tlie transverse vibrations of the string, if we know 

 its dimensions and the material of which it is made. In table I a 

 few nnmerical data are given regarding various materials which 

 maj be used for making strings. In this table the length is taken 

 as 10 cm and the diameter as one micron (10^^ centimeter). 



TABLE I. 



1 



98100000 



yV/sec. 



Cu 

 Ag 

 Au 

 Al 

 Ft 



0.3100 

 .2356 

 .1724 

 .4408 

 .2448 



3.32' 

 4.25 

 5.80 

 2.27 

 4.08 



It is very difficult to measure directly the vibrations per second 

 in wires of 1 micron diameter. The air resistance in this case is 

 so considerable as to cause the movement to come to a dead stop. 

 We should have to examine such strings in a perfect vacuum. 

 Furthermore we are not able to make wires of 1 micron except in 

 platinum, and perhaps in aluminium and gold. Silver wires of 1 micron 

 are as yet not attainable. But the dimensions used in this table and 

 the figures in the last columns permit us to calculate in a simple 

 way, e.g. with a slide rule, any periodic time when other dimen- 

 sions are given : the vibration time being proportional to the square 

 of the length and inversely proportional to the diameter. 



A wire clamped at the end without any tensile stress will sag 

 under the influence of a load P, uniformly distributed over the 

 total length. The maximum deflection h will be 



P I' 

 h = (2) 



where / represents the axial moment of inertia of a section of the 



wire. As for a round wire / = — , we get 



64 



h — 



PI' 



(3) 



In the string galvanometer the transverse load P is equal ioHil 

 Dynes, if H be the strength of the magnetic field in Gausses, i 



