Abstract: Disclosed is an electromagnetic coil system for use during transcranial or transdermal stimulation procedures, in which an electrically conductive coil wraps around a magnetic core at an oblique wrapping angle to provide a more directed focal spot size at a given depth inside of, for example, a patient's body. In certain configurations, the electrically conductive coil may be adjustable with respect to the magnetic core, such that the wrapping angle of the electrically conductive coil may be modified to, in turn, adjust the focal spot size of the induced electric field generated by the system as may desired to concentrate the field at a particularly desired location.