ON THE MODULUS OF TORSIOXAL RIGIDITY OF MKTAI. U|| 4<> 



100 C. was also observed in the present research, but it is not mentioned by many 

 experimenters. 



STREINTZ* was the first to lay stress on the fact that there is a tendency in 

 metallic wires to accommodate themselves to a definite elastic state. In wires of 

 brass, silver, platinum, copper, iron, and steel examined by him there was a marked 

 diminution in the rate of subsidence produced by keeping the wire for long periods in 

 continual torsional oscillation. Lord KELVIN! had previously stated that the rates 

 of subsidence of the vibrations of several wires tested by his students were much less 

 rapid after they had taen allowed to rest for a week-end than when kept in continual 

 vibration. The present research shows that the whole question depends on the 

 amplitude of the vibration. It is possible that the wires mentioned by Lord KELVIN 

 (which had been kept " as continually as possible in a state of vibration," presumably 

 by impulses administered by students' hands) had been subjected to larger amplitudes 

 of oscillation than were used in the case of the wires which were allowed to rest 

 between the successive observations. In my own experiments on the effect of the 

 amplitude of vibration, in which the greatest amplitude used was about 8 or 9, it is 

 shown that, in general, vibration through a large amplitude very considerably alters 

 both the period and logarithmic decrement of the oscillations at smaller amplitudes. 

 Recently THOMPSON^ has investigated this question and arrived at the conclusion 

 that, when the temperature and amplitude of vibration are constant, both the period 

 and logarithmic decrement are constant quantities, but his experiments do not seem 

 to be capable of sufficient accuracy to establish the point. 



STREINTZ expressed the results of his experiments in the following three laws, the 

 first of which is not strictly borne out by his own results : 



a. The logarithmic decrement is independent of the amplitude of vibration. <>l 



the tension in the wire, of the period of oscillation, of the length and 

 perhaps also of the diameter of the wire. 



b. The logarithmic decrement varies with the chemical and physical nature <>(' 



the wire. It increases with rise of temperature and is less in annealed than 

 in unannealed wires. 



c. The logarithmic decrement tends up to a certain limit to diminish with the 



number of oscillations performed by the wire. 



Perhaps the most exhaustive experiments on the internal viscosity of wires have 

 been made by TOMLINSON. He obtained a decrease of the logarithmic decrement in 

 the case of iron, nickel, and platinum, on raising the temperature from C. to 100 C., 

 the percentage decrease being much the greatest in the case of iron. He states that 



* STREINTZ, 'Sitzb. <1. Wion. Akad,' LXIX., Abth. 2, p. 337, 1874; also LXXX., Ahth. 2, p. 397, 1880. 



t THOMSON, ' Proc. Roy. Soc.,' vol. 14, p. 289, 1865. 



J THOMPSON, ' Phys. Rev.,' VIII., p. 141, 1899. 



TOMLINSON, 'Phil. Trans.,' vol. 177, p. 801, 1886 ; also 'Proc. Roy. Soc.,' vol. 40, p. 343, 1886. 



VOL. CCIV. A. H 



