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Magnetic tweezers are used in single-molecule manipulation experiments to probe systems, such as supercoiled DNA, and mechanisms of molecular motors, such as topoisomerases. A superparamagnetic bead is subjected to an external magnetic field to exert sub-pN to pN forces and torque on a tethered molecule. In this thesis, we compare the calculated forces acting on a bead to experimental forces determined by stretching a single dsDNA molecule and discuss factors which account for the differences between these two curves. The relevant magnetic fields are calculated and experimentally characterised for a rare-earth magnet pair. The induced magnetic moment, alignment and rotation of the superparamagnetic bead are discussed. With knowledge of the field and the moment of the bead, the expected force on a bead is calculated. We present the details of our data analysis along with key corrections, which account for instrumental drift and spherical aberration when using an oil-immersion objective.