We have measured the stability boundary for steady electroconvection in thin, freely suspended smectic liquid crystal films. As the voltage across the film, or the film length, is varied, convective vortices are created or destroyed at the ends of the film to keep the pattern wave number within a stable range. The measured stability boundary lies substantially inside the Eckhaus boundary for this system. Our results are consistent with a mechanism for boundary-induced wavelength selection proposed by Cross et al.
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