  The behavior of a long Josephson tunnel junction drastically depends on the
distribution of the dc bias current. We investigate the case in which the bias
current is fed in the central point of a one-dimensional junction. Such
junction configuration has been recently used to detect the persistent currents
circulating in a superconducting loop. Analytical and numerical results
indicate that the presence of fractional vortices leads to remarkable
differences from the conventional case of uniformly distributed dc bias
current. The theoretical findings are supported by detailed measurements on a
number of $\delta$-biased samples having different electrical and geometrical
parameters.
