A unit of fluidity–that is, how runny a fluid is–first defined by Bingham and Thompson in 1928.1 Pronounced ree. From the Greek reo in Greek characters (reo, to flow), the same root that occurs in rheology, the science of fluid flow, a word coined in 1929.

The rhe is sometimes encountered in lists of units of dynamic viscosity, but it is a unit of fluidity, the opposite of viscosity.


The unit of dynamic fluidity in the centimeter-gram-second system of units, the reciprocal of the poise. In terms of base units, it is

A fraction. Ten times meter times second over kilogram

With the coming of SI and the introduction of the pascal, it became easiest to describe a rhe as a reciprocal pascal-second. The rhe is not an SI unit.


Some authorities define the rhe as the reciprocal of the centipoise, instead of the poise. Apparently it was defined in this sense in 1961.


Some sources say the term rhe has been used for a unit of kinematic fluidity, the reciprocal of the centistokes

1. Eugene C. Bingham and Theodore R. Thompson.
The fluidity of mercury.
Journal of the American Chemical Society, volume 50, no. 11, page 2879 (November 1928).

The rhe is defined in footnote 6 as “reciprocal poises,” not centipoises.



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