Disclosed is an apparatus for application of a magnetic field to a sample, in particular an elongated sample. At least in places, the apparatus has magnetic bodies which are arranged such that they can rotate with respect to one another and have, and leave open, a common access opening for the sample. In particular, this allows the apparatus to be applied to an elongated sample whose ends are not accessible. As a result of the bodies being arranged such that they can rotate with respect to one another, the magnetic fields of the bodies can be adjusted with respect to one another before the apparatus is opened and closed, so as to minimize the magnetic field in the common access opening. In consequence, only small magnetic interaction forces need be overcome during opening and closing.