This paper presents measurements of vertical electric fields obtained using the Tethered Satellite System (TSS) as an electrodynamic double probe. During five hours of TSS-1R deployment from the space shuttle Columbia in February 1996, observations of induced tether potential were gathered over a baseline from 2 to 20 km in the mid and low latitude F-region ionosphere. The corresponding electric fields derived from the potential measurements are found to be consistent with previous satellite and ground-based measurements for similar ionospheric conditions. The data are unique and demonstrate a new capability for measuring the vertical component of the ionospheric electric field. Comparison of the measured electric field at different tether deployment lengths for corresponding local times reveal that the vertical component of the ambient electric field exists on scales of at least 20 km. ¿ 1998 American Geophysical Union