390 



Mr. A. Mallock 



[May 27, 



indicate that rigidity- variation depends on the temperature of the 

 melting points very nearly in the same way as does Young's 

 Modulus. (Fig. 7.) 



In the next slides (13 and 14) are shown the percentage variation 

 of rigidity of the various metals between the limits of the tempera- 

 tures employed in 100° 0. to —176°. It will be seen that the 

 increase in rigidity as the temperature falls is not linear, and that 

 there is not sufficient evidence to make even a useful guess at what 

 the rigidity would be at absolute zero. 



In slides (15 and 1G) are shown the variations of viscosity for same 

 selection of metals. 



To explain these diagrams it may be stated that the oscillations 



Fig. 6. 



of viscous substances, where the strain does not exceed the limits of 

 elasticity, naturally decrease at compound interest— that is. the 

 amplitude of each successive oscillation is some constant fraction of 

 the one which preceded it. In symbols, if A„ is the amplitude at 

 time t„ and A t the amplitude at time t, then 



A, =A„e ~r> 



where c is a constant, and this constant is the time in which the 

 amplitude is reduced in the ratio of e to unity. 



The ordinates of the curve shown in the slide are the proportional 

 times in which the viscosity of the metals causes a reduction of e : 1 



