The activated torsion oscillation magnetometer exploits the mechanical resonance of a cantilever beam, driven by the torque exerted on the sample by an AC field applied perpendicularly to the film plane. We describe a model for the cantilever dynamics which leads to the calculation of the cantilever dynamic profile and allows the mechanical sensitivity of the instrument to be expressed in terms of the minimum electronically detectable displacement. We have developed a capacitance detector which is able to detect displacements of the order of 0.1 nm.We show that sensitivities of the order of 0.5×10−11 A m2 can be in principle achieved. We will subsequently describe the main features of the ATOM prototype which we have built and tested, with particular attention to the design solutions which have been adopted in order to reduce the effects of parasitic vibrations due to either acoustic noise or eddy currents. The instrument is mounted in a continuous flow cryostat and can work in the 4.2–300 K temperature range. Finally, we will show that our experimental set up has a second mode of operation named torsion induction magnetometer.