| 摘要 |
<p>A magnetic casing depth marker which includes a plurality of axially upper and lower magnets dispersed circumferentially about the exterior of the casing. Each magnet has radially inner and outer faces that are magnetized such that the inner face is of one pole and the outer face is of the other. The inner faces of all of the axially upper magnets are of one pole and the inner faces of the axially lower magnets are of the other pole, whereby a strong magnetic field is focused into the interior of the casing.</p> |
