  We show that the NMR Knight shift anomaly exhibited by a large number of
heavy electron materials can be understood in terms of the different hyperfine
couplings of probe nuclei to localized spins and to conduction electrons. The
onset of the anomaly is at a temperature T*, below which an itinerant component
of the magnetic susceptibility develops. This second component characterizes
the polarization of the conduction electrons by the local moments and is a
signature of the emerging heavy electron state. The heavy electron component
grows as log T below T*, and scales universally for all measured Ce, Yb and U
based materials. Our results suggest that T* is not related to the single ion
Kondo temperature, T_K, but rather represents a correlated Kondo temperature
that provides a measure of the strength of the intersite coupling between the
local moments. Our analysis strongly supports the two-fluid description of
heavy electron materials developed by Nakatsuji, Pines and Fisk.
