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We have obtained high-res, high S/N ratio CCD echelle spectra of 10 brightred giants in 3 GCs (47Tuc, NGC6752 and NGC6397) roughly spanning the range ofmetallicities of the galactic GC system; they reveal no evidence of star tostar variation of [Fe/H] in these 3 GCs. A large set of high quality literaturedata (EWs from high-res CCD spectra) was re-analyzed in a self-consistent wayto integrate our data and derive new [Fe/H] for more than 160 bright red giantsin 24 GCs. This set was used to define a new [Fe/H] scale for GCs based on highquality, direct spectroscopic data, on updated model atmospheres from the gridof Kurucz (1992) and on a careful fine abundance analysis (using a common setof atomic and atmospheric parameters for all stars). Given the high internalhomogeneity, 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 toour system, metallicities on the widely used Zinn and West's scale are about0.10dex higher for [Fe/H]$>-1$, 0.23dex lower for $-1<[Fe/H]<-1.9$ and 0.11dextoo 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 therange of our calibrating GCs ($-2.24\le[Fe/H]_{ZW}\le-0.51$). A minordisagreement is found at low [Fe/H] between the metallicity scale based onfield and cluster RR Lyrae's (via a new calibration of the $\Delta$S index) andour new GCs metallicities, that could be ascribed to non-linearity in the[Fe/H]$-\Delta$S relationship. The impact of new metallicities on majorastrophysical problems is exemplified through the Oosterhoff effect in theclassical pair M3 and M15.Comment: 18 pages, plain tex, plus 7 .ps figures embedded, uses psfig.tex,  accepted by Astronomy & Astrophysics Suppl.Se

Abundances for globular cluster giants. I. Homogeneous metallicities for 24 clusters