I PHYSICAL PROPERTIES AND STATES OF MATTER 9 



to that shown in Fig. 4. Notice the position at which the 

 weight comes to rest ; then twist the weight through a certain 

 angle, and let it go. The 

 weight untwists back to the 

 starting point and beyond it, 

 and then spins in the way it 

 was twisted, and goes on oscil- 

 lating in this manner until it 

 comes to rest. Observe how 

 long the weight takes to make 

 ten or fifteen complete spins. 

 Repeat the observation with 

 wires of different length and 

 diameter, and made of dif- 

 ferent metals. The time of 

 an oscillation depends upon 

 the tendency to untwist, so 



that the experiment shows FIG. 4. Elasticity of Torsion. 



that this depends, among 

 other things, upon the length, diameter, and nature of a 



Solids possess Tenacity, Ductility and Hardness. The 

 force required to tear asunder the particles of a body varies with 

 different materials, in other words, some substances are more 

 tenacious than others. 



1. Tenacity is measured by ascertaining what weight is necessary 

 to break solids when in the form of wires. 



EXPT. 13. Suspend a balance-pan from the lower end of a 

 thin copper wire attached to a beam. Add weights to the pan 

 until the wire breaks. The force required to break the wire 

 is the joint weight of the balance-pan and the weights in it. 

 Repeat the experiment with wires of the same diameter but 

 made of different material. 



In making the measurement of tenacity, the area of the cross 

 section of the wire must first be carefully estimated. By the 

 cross section is meant the area of the end of the wire when it is 

 carefully filed to be at right angles to the length. This cross 

 section is estimated by measuring the diameter of the wire and 

 calculating. * 



1 Area of cross suction = x 



