  A puzzle arising from Faraday's law is considered and solved concerning the
question which voltage is induced in an open wire with a time-varying
homogeneous magnetic field. In contrast to closed wires where the voltage is
determined by the time variance of magnetic field and enclosed area, in an open
wire we have to integrate the electric field along the wire. It is found that
the longitudinal electric field contributes with 1/3 and the transverse field
with 2/3 to the induced voltage. In order to find the electric fields the
sources of the magnetic fields are necessary to know. The representation of a
homogeneous and time-varying magnetic field implies unavoidably a certain
symmetry point or symmetry line which depend on the geometry of the source. As
a consequence the induced voltage of an open wire is found to be the area
covered with respect to this symmetry line or point perpendicular to the
magnetic field. This in turn allows to find the symmetry points of a magnetic
field source by measuring the voltage of an open wire placed with different
angles in the magnetic field. We present exactly solvable models for a symmetry
point and for a symmetry line, respectively. The results are applicable to open
circuit problems like corrosion and for astrophysical applications.
