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We present detailed measurements of ionization of highly excited triplet helium atoms in a static electric field. The atoms were prepared in states with energy F. close to the saddle-point threshold E = -2[F(a.u. )]'~. The electric field Fwas sufficiently strong for the states to be characterized by total spin S and absolute value of the magnetic quantum number ML, . For ML, --0 states the experiments measured ionization properties of adiabatic states. In another case, i Mz~=2, they predominantly measured those of diabatic states. In both cases the ionization rate was found to be a highly nonmonotonic function of the field strength. The observations are analyzed in terms of a theory of the helium density of states in an electric field. A companion paper [D. A. Harmin, Phys. Rev. A 30, 2413] develops in detail the general theory, which uses quantum defects to parametrize the effect of the core interaction. The agreement between measured and calculated ionization curves is good, indicating that the field ionization of a nonhydrogenic atom can now be understood in a detailed, quantitative, and predictive sense.