[SATTERLY] THE PENDULUM 33 



In the experiment quoted the ball made approximately six 



vibrations per second, so that 



t2 = 1/6 sec. 



TU ( V 47:30-5 x 11-04 . , . ^ 



Iheretore, Y = poundals per sq. toot. 



(1/6)2 X .0001152 ^ 



= 1 • 26 X 10^^ poundals per sq. foot 



= 4-0 X 10^ pounds per sq. foot 



If the ball is gripped by the hands and urged up and down with the 

 right frequency it is surprizing what a large resonant vibration can be 

 produced. A change in the frequency of the efifort quickly destroys 

 the motion. 



Calculation. — Having got n and Y deduce Poisson's ratio of the 



Y 



material of the wire. This ratio = — —1 = 1-21 — 1=- 2. 



2n 



Experiment VI. — Determine the frequency of transverse vibration 

 of the wire. 



Twang the wire by the finger in a direction at right angles to its 

 length and determine the frequency of the note by a comparison 

 method with a monochord or by a tonometer. 



If the frequency is N 



tension 

 2 1 ' mass per unit length. 



= - J^I 

 21 ^n^'p 



where p the density of the material of the wire. 



Assume a value of p and calculate N and compare the observed 

 and calculated values. In experiment quoted, N, calculated, is 310. 



Experiment VII. — Determine the frequency of longitudinal 

 vibration of the wire. 



Rub the wire lengthwise between finger and thumb or with a wet 

 rag. The wire gives out a shrill note. Estimate the frequency of 

 this note as nearly as you can (see above). If it is Ni 



N. = ^ 

 21 



where v = the velocity of sound in the wire 

 Y 

 P 



=v- 



