= Neutral current filament in a boosted frame
{title2=$\lambda'=-\gamma uI/c^2,\quad I'=\gamma I$}
For an ideal infinite filament with zero <line charge> <density> and current $I$ in a laboratory frame, the line-integrated <four-current> is $(0,I)$ in temporal/axial components. An axial boost of <speed> $u$ gives the title <density> and current. The laboratory <magnetic field> $\mu_0I\widehat{\boldsymbol\varphi}/(2\pi r)$ therefore becomes both an <electric field> from $\lambda'$ and a <magnetic field> from $I'$. <Charge neutrality> is frame-dependent for a current-carrying system. A model built from oppositely charged moving and resting populations makes this result explicit; the charge of each fixed material particle set remains invariant.
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