6 .FLUIDITY AND PLASTICITY
Methods of Measurement.—Almost numberless instruments
have been devised for the measurement of viscosity, but ttie
greater part of these are suitable for giving relative values only_
There are, however, several quite distinct methods which are
susceptible of mathematical treatment so that absolute viscosities
may be obtained. The possible methods for measuring viscosity
may be classified under three heads as follows:
1. The measurement of the resistance offered to a moving
body (usually a solid) in contact with the viscous fluid.
2. The measurement of the rate of flow of a viscous fluid.
3. Methods in which neither the flow nor the resistance to
flow are measured.
1. The various methods for measuring viscosity while maintaining the
fluid in a nearly fixed position, together with the names of investigators
who have developed the method are as follows:
(a) A horizontal disk supported at its middle point by a wire and oscil-
lating around the wire as an axis. Coulomb (1801), Moritz (1847), Stokes
(1850), Meyer (1865), Maxwell (1866), Grotrian (1876), Oberbeck (1880),
Th. Schmidt (1882), Stables and Wilson (1883), Fawsitt (1908).
(b) A sphere rilled with liquid and oscillating around its vertical axis.
Helmholtz and Piotrowski (1868), Ladenburg (1908).
(c) A cylinder filled with liquid and oscillating around its vertical axis.
Miitzel (1891).
(d) Concentric cylinders. The outside one is rotated at constant velocity
and the torque, exerted upon the inner coaxial cylinder which is immersed
in the viscous fluid, is measured. Stokes (1845), de St. Venant (1847),
Boussinesq (1877), Couette (1888), Mallock (1888), Perry (1893).
(e) An oscillating solid sphere immersed in the viscous substance and
supported by bifilar suspension was used by Konig (1885).
(/) A body moving freely under the action of gravity, e.g., falling sphere
of platinum, mercury, or water, a f ailing body of other shape than a sphere,
a rising bubble of air. Stokes (1845), Pisati (1877), Schottner (1879), de
Keen (1889), 0. Jones (1894), Duff (1896), J. Thomson (1898), Tarnmann
(1898), Schaum (1899), Allen (1900), Ladenburg (1906), Valenta (1906),
Arndt (1907).
2. The methods for measuring the rate of flow of a viscous fluid:
(a) Efflux through horizontal tubes of small diameter. Gerstner (1798),
Girard (1816), Poiseuille (1842), G. Wiedemann (1856), Rellstab (1868),
Sprung (1876), Rosencranz (1877), Grotrian (1877), Prfbram and Handl
(1878), Slotte (1881), Stephan (1882), Foussereau (1885), Couette (1890),
Bruckner (1891), Thorpe and Rodger (1893), Hosking (1900), Bingham and
White (1912).
(6) Efflux through a vertical tube of small diameter. Stephan (1882),