OK THE VISCOSITY OB INTERNAL FRICTION 



Professor W. Thomson has shewn that something corresponding to internal 

 friction take* place in the torsional vibrations of wires, but that it is much 

 increased if the wire has been previously subjected to large vibrations. I have 

 also found that, after heating a steel wire to a temperature below 120, ita 

 elasticity was permanently diminished and its internal friction increased. 



The viscosity of fluids has been investigated by passing them through 

 capillary tubes t, by swinging pendulums in them*, and by the torsional vibra- 

 tions of an immersed disk, and of a sphere filled with the fluid ||. 



The method of transpiration through tubes is very convenient, especially 

 for comparative measurements, and in the hands of Graham and Poiseuille it 

 has given good results, but the measurement of the diameter of the tube is 

 difficult, and on account of the smallness of the bore we cannot be certain that 

 the action between the molecules of the gas and those of the substance of the 

 tubes does not affect the result. The pendulum method is capable of great 

 iiccuracy, and I believe that experiments are in progress by which its merits 

 us a means of determining the properties of the resisting medium will be tested. 

 The method of swinging a disk in the fluid is simple and direct. The chief 

 difficulty is the determination of the motion of the fluid near the edge of the 

 disk, which introduces very serious mathematical difficulties into the calculation 

 of the result. The method with the sphere is free from the mathematical 

 difficulty, but the weight of a properly constructed spherical shell makes it un- 

 suitable for experiments on gases. 



In the experiments on the viscosity of air and other gases which I propose 

 to describe, I have employed the method of the torsional vibrations of disks, 

 but instead of placing them in an open space, I have placed them each between 

 two parallel fixed disks at a small but easily measurable distance, in which 

 case, when the period of vibration is long, the mathematical difficulties of deter- 

 mining the motion of the fluid are greatly reduced. I have also used three 



* Proceedings of the Royal Society, May 18, 1865. 



t Liquids : Poiaeuille, Mem. de Savants Etrangert, 1846. Gases : Graham, Philosophical Transactions, 

 1846 and 1849. 



t Bly, Phil Trans. 1832 ; Bessel, Berlin Acad. 1826 ; Dubuat, Principes <t HydrauHque, 1786. All 

 the** are discussed in Professor Stokes's paper " On the Eflect of the Internal Friction of Fluids on the 

 Motion of Pendulums," Cambridge Phil Trans, vol. IX. pt. 2 (1850). 



S Coulomb, Mem. de FlnttittU national, in. p. 246; O. E. Meyer, Pogg. Ann. cxm. (1861) p. 55, and 

 CrMi Journal, Bd. 69. 



y HelmholU and Pietrowski, SUzungtberichte dr k. L Akad. April, 1860. 



