In the 1970s, it became clear that the clocks participating in TAI were ticking at different rates due to gravitational time dilation, and the combined TAI scale, therefore, corresponded to an average of the altitudes of the various clocks.
ɪnðə nˈaɪntiːnhˈʌndɹɪd sˈɛvənti z, ɪt bɪkˌeɪm klˈɪɹ ðætðə klˈɑːks pɑːɹtˈɪsᵻpˌeɪɾɪŋ ɪn tˌiːˌeɪˈaɪ wɜː tˈɪkɪŋ æt dˈɪfɹənt ɹˈeɪts dˈuː tə ɡɹˌævɪtˈeɪʃənəl tˈaɪm daɪlˈeɪʃən, ænd ðə kəmbˈaɪnd tˌiːˌeɪˈaɪ skˈeɪl, ðˈɛɹfoːɹ, kˌɔːɹɪspˈɑːndᵻd tʊ ɐn ˈævɹɪdʒ ʌvðɪ ˈæltɪtˌuːdz ʌvðə vˈɛɹiəs klˈɑːks.