  When a relativistic electron bunch traverses a structure, strong electric
fields are induced in its wake. We present measurements of the electric field
as a function of time as measured at a fixed location in the beam line. For a
12 nC bunch of duration 4.2 ps FWHM, the peak field is measured >0.5 MV/m. Time
resolution of ~5 ps is achieved using electro-optic sampling with a lithium
tantalate (LiTaO_3) crystal and a short-pulse infrared laser synchronized to
the beam. We present measurements under several different experimental
conditions and discuss the influence of mode excitation in the structure.
