A uniform magnetic field is compressed in a circuit containing two semi‐infinite slabs of constant conductivity &sgr;. The separationlof the slabs decreases uniformly according tol=l0−vt. An equation involving the magnetic Reynolds numberRm=&sgr;&mgr;0l0vis derived for the magnetic field in the gap. Expressions are given for the magnetic and electric fields everywhere up to the instant of collision. These expressions are evaluated for several values ofRmand it is found that the ratio of Joule energy density to magnetic energy density at the surfaces of the slabs is fairly close to unity throughout the motion. ForRm»1 it is shown that the maximum magnification of the initial field is approximatelyRm/8.