  A field in a homogeneous medium can be amplified or enhanced by inserting
closely located perfectly conducting inclusions into the medium. In this paper
precise quantitative estimates for such enhancement are derived when the given
field is the one excited by an emitter of a dipole type and inclusions are
spheres of the same radii in three dimensions. Derived estimates reveal the
difference, as well as the similarity, between enhancement of the field excited
by the emitter and that of the smooth back-ground field. In particular, an
estimate shows that when the enhancement occurs, the factor of enhancement is
$(\sqrt{\epsilon}|\log \epsilon|)^{-1}$, which is different from that for the
smooth background field, which is known to be $(\epsilon|\log \epsilon|)^{-1}$
($\epsilon$ is the distance between two inclusions).
