  A novel device that can be used as a tunable support-free phase plate for
transmission electron microscopy of weakly scattering specimens is described.
The device relies on the generation of a controlled phase shift by the magnetic
field of a segment of current-carrying wire that is oriented parallel or
antiparallel to the electron beam. The validity of the concept is established
using both experimental electron holographic measurements and a theoretical
model based on Ampere's law. Computer simulations are used to illustrate the
resulting contrast enhancement for studies of biological cells and
macromolecules.
