Prof.W. Thomson on the Elasticity and Viscosity of Metals. 65 



one end of the wire was attached to a rigid vibrator with sufficient 

 firmness (thorough and smooth soldering I find to be always the best 

 plan when the wire is thick enough) ; and the other to a fixed rigid 

 body, from which the wire hangs, bearing the vibrator at its lower 

 end. I arranged sets of observations to be made for the separate 

 comparisons of the following classes : — 



(a) The same wire with different vibrators of equal weights (to give 

 equal stretching-tractions), but different moments of inertia (to test 

 the relation between viscous resistances against motions with different 

 velocities through the same range and under the same stress). 



(6) The same wire with different vibrators of equal moments of 

 inertia but unequal weights (to test the effect of different longitu- 

 dinal tractions on the viscous resistance to torsion under circum- 

 stances similar in all other respects). 



(c) The same wire and the same vibrator, but different initial 

 ranges in successive experiments (to test an effect unexpectedly dis- 

 covered, by which the subsidence of vibrations from any amplitude 

 takes place at very different rates according to the immediately 

 previous molecular condition, whether of quiescence or of recurring 

 change of shape through a wider range). 



(d) Two equal and similar wires, with equal and similar vibra- 

 tors, one of them kept as continually as possible in a state of vibra- 

 tion, from day to day ; the other kept at rest, except when vibrated 

 in an experiment once a day (to test the effect of continued vibration 

 on the viscosity of a metal) . 



Results. 



(a) Tt was found that the loss of energy in a vibration through 

 one range was greater the greater the velocity (within the limits of 

 the experiments) ; but the difference between the losses at low and 

 high speeds was much less than it would have been had the resist- 

 ance been, as Stokes has proved it to be in fluid friction, approx- 

 imately as the rapidity of the change of shape. The irregulari- 

 ties in the results of the experiments which up to this time I have 

 made, seem to prove that much smaller vibrations (producing less 

 absolute amounts of distortion in the parts of the wires most stressed) 

 must be observed before any simple law of relation between mole- 

 cular friction and velocity can be discovered. 



(b) When the weight was increased, the viscosity was always at 

 first much increased ; but then day after day it gradually diminished 

 and became as small in amount as it had been with the lighter 

 weight. It has not yet been practicable to continue the experiments 

 long enough in any case to find the limit to this variation. 



(c) The vibration subsided in aluminium wires much more rapidly 

 from amplitude 20 to amplitude 10, when the initial amplitude was 

 40, than when it was 20. Thus, with a certain aluminium wire, and 

 vibrator No. 1 (time of vibration one way 1*757 second), in three 

 trials the numbers of vibrations counted were — 



Phil. Mag. S. 4. Vol. 30. No. 200. July 1865. F 



