[en] In the screw-pinch device SP IV a plasma is produced inside a non-circular quartz vacuum vessel which is enclosed by a conducting wall. The experiments performed in this device show that force-free currents surrounding the plasma column permit compression towards a highly elongated equilibrium state that does not exhibit fatal instabilities. It was found that an axisymmetric vertical instability is excited during the formation of the plasma. Since this 'hourglass' instability does not fully develop, it does not terminate the confinement. Apart from the very useful diamagnetic loop, also a vertical and a horizontal magnetic probe, a He-Ne interferometer, a set of pick-up coils, and a streak camera are used for diagnosing the plasma behaviour. Analytic equilibrium calculations support the findings that force-free currents largely increase the attainable compression ratios of an elongated, magnetically closed, geometry. (Auth.)