  We have obtained high-res, high S/N ratio CCD echelle spectra of 10 bright
red giants in 3 GCs (47Tuc, NGC6752 and NGC6397) roughly spanning the range of
metallicities of the galactic GC system; they reveal no evidence of star to
star variation of [Fe/H] in these 3 GCs. A large set of high quality literature
data (EWs from high-res CCD spectra) was re-analyzed in a self-consistent way
to integrate our data and derive new [Fe/H] for more than 160 bright red giants
in 24 GCs. This set was used to define a new [Fe/H] scale for GCs based on high
quality, direct spectroscopic data, on updated model atmospheres from the grid
of Kurucz (1992) and on a careful fine abundance analysis (using a common set
of atomic and atmospheric parameters for all stars). Given the high internal
homogeneity, our new scale supersedes the discrepancies of previous attempts.
The internal uncertainty in [Fe/H] is very small: 0.06 dex (24 GCs) on average,
that can be interpreted as the mean precision of cluster ranking. Compared to
our system, metallicities on the widely used Zinn and West's scale are about
0.10dex higher for [Fe/H]$>-1$, 0.23dex lower for $-1<[Fe/H]<-1.9$ and 0.11dex
too high for [Fe/H]$<-1.9$: the non-linearity is significant at 3$\sigma$
level. A quadratic transformation corrects older values to the new scale in the
range of our calibrating GCs ($-2.24\le[Fe/H]_{ZW}\le-0.51$). A minor
disagreement is found at low [Fe/H] between the metallicity scale based on
field and cluster RR Lyrae's (via a new calibration of the $\Delta$S index) and
our new GCs metallicities, that could be ascribed to non-linearity in the
[Fe/H]$-\Delta$S relationship. The impact of new metallicities on major
astrophysical problems is exemplified through the Oosterhoff effect in the
classical pair M3 and M15.
