This paper reviews some of the conditions for electron MHD,
and shows how this
leads to anomalous resistivity, which plays an important role in the behaviour
of
plasma opening switches (POS are switches, based on the modification of
the global
resistance of a plasma flow, used to shorten the characteristic time of
pulse power
machines and increase their performances). Anomalous resistivity is build
up from
a transfer of momentum between the electrons and ion acoustic waves, which
in
turn modify the momentum of ions. One of the main consequences of EMHD
is the
diffusion of B in the plasma. A rough model envisions this
diffusion as a propagation
of nonlinear waves that may compete with convective transport.