The roller nut CRDM, like the magnetic jack, requires no mechanical or electrical penetrations. It is used in PWRs and fast reactors. Coils are mounted outside the CRDM housing. Inside the housing is a threaded nut that is constrained to a fixed axial position. The magnetic field causes a torque that causes this nut to rotate. Generally the torque is applied in steps so that the nut rotates a fixed angle at each step. The control rod is connected to a threaded shaft that passes through the nut. The rotary motion of the nut causes axial motion of the threaded shaft. Typical rod motions are yj in. per step and the minimum time between steps is ygsec. The motion is probably fast enough to permit one to neglect the move time. A roller nut CRDM with a minimum time between moves of r^sec will permit measure­ment to frequencies up to about 8 Hz if the worth per step is large enough. However, because of the small displacement per step, it may take multiple steps to obtain adequate reactivity even when several rods are moved simultaneously. For example, if a motion of in. were required, the rod would have to take 16 steps and would take 1 sec. This would reduce the useful maximum frequency to 0.5 Hz. The position may be determined by counting the number and timing of move steps.