The current-voltage ( I–V) characteristics of a low-temperature ultrahigh-vacuum scanning tunneling microscope (STM) tip positioned >100 Å from a planar surface have been recorded. We find curvature in the Fowler-Nordheim plots (log 10( I/V 2) versus 1/ V) due to the tip-plane geometry as has been predicted theoretically. In addition, oscillations and sharp breaks in these I–V curves are observed over a wide voltage range, 50–1000 V. These I–V curves are used to characterize the STM tips prior to tunneling.