Apparatus and method for harvesting energy from the environment and/or other external sources and converting it to useful electrical energy. The harvester does not contain a permanent magnet or other local field source but instead relies on the earth's magnetic field of another source of a magnetic field that is external to the sensing device. One advantage of these new harvesters is that they can be made smaller and lighter than energy harvesters that contain a magnet and/or an inertial mass. A small implantable stimulator(s) includes at least one passive magnetostrictive/electro-active (PME) magnetic-field sensor for delivering electrical stimulation to surrounding tissue. The PME is charged utilizing a changing magnetic field from an external alternating magnetic field source at a frequency particular to the PME. The small stimulator provides means of stimulating a nerve, tissue or internal organ with direct electrical current, such as relatively low-level direct current for temporary or as needed therapy. The field source may be a hand-held device or a small antenna affixed to the wearer's skin, clothing or accessories. The stimulator may be configured to be small enough to be implanted through a surgical needle. Open- and closed-loop systems are disclosed with measurement of current flow and therapy through PME sensor function.