  Data from the first three years of RXTE observations have been systematically
analyzed to yield a set of 300 day, 2-10 keV light curves with similarly
uniform, ~5 day sampling, for a total of nine Seyfert 1 galaxies. This is the
first X-ray variability survey to consistently probe time scales longer than a
few days in a large number of AGN. Comparison with ASCA data covering a similar
band but much shorter (~1 day) time scales shows that all the AGN are more
strongly variable on long time scales than on short time scales. This increase
is greatest for the highest-luminosity sources. The well-known anticorrelation
between source luminosity and variability amplitude is both stronger and
shallower in power-law slope when measured on long time scales. This is
consistent with a picture in which the X-ray variability of Seyfert 1s can be
can be described by a single, universal fluctuation power density shape for
which the cutoff moves to longer time scales for higher luminosity sources. All
of the Seyfert 1s exhibit stronger variability in the relatively soft 2-4 keV
band than in the harder 7-10 keV band. This effect is much too pronounced to be
explained by simple models based on either the dilution of the power-law
continuum by the Compton reflection component or on the hard X-rays being
produced in a static, pair-dominated, plane-parallel Comptonizing corona.
