Since the work of Galvani and Volta in the 1790s it has been known that neuromuscular tissue can be externally stimulated by externally applied electric currents. Stimulating the peripheral nervous system by electricity is done by applying electrodes to the skin and sending current through the electrodes and thereby through the tissue. The current sent through the tissue will activate the nerves thereby activating the muscles in the area affected. As those who have tried this will know, this is an unpleasant and painful experience.
In 1831 Faraday made a discovery that forms the basis of magnetic stimulation. This discovery told us that a time varying magnetic field can be used to induce an electric current. Thus by using magnetic stimulation it is possible to activate the peripheral muscles without the pain experienced by electrical stimulation and no electrodes have to be positioned.
If a constant current is sent through a coil, a magnetic field is generated. But Faraday's law tells us that a constant magnetic field is not able to induce a current in tissue and is therefore unable to generate a movement of the muscle. If the current sent through the coil is varied in time, the magnetic field generated by the coil will also vary in time. The keyword here is varying and only a varying field is able to induce an electric field.
If the coil is placed over e.g. the left arm and a short current pulse is sent through the coil, an electric field will be induced in the left arm and if the change of the magnetic field is sufficiently high, the fingers of the left hand will twitch. This happens because the electric field will change the electrical potential over the nerve cell wall and if this change is large enough, an action potential in the nerve will be generated. If the nerve is a motor nerve, a muscle fibre in the arm will be activated. If enough muscle fibres are activated, the fingers on the left arm will twitch.
So if the induced electric field and thereby the induced current is of sufficient amplitude and duration, neuromuscular tissue will be stimulated in the same way as with conventional electrical stimulation. Adding that magnetic stimulation is independent of clothing and bone/tissue structure makes the technique of magnetic stimulation superior to electrical stimulation in both research and in the clinic.
Stimulation of excitable tissues with a time-varying magnetic field is highly attractive because the technique can be applied noninvasively and is virtually painless. This electrodeless technique is ideally suited to stimulating superficial tissues such as the motornerves (known as Functional Magnetic Stimulation - FMS) and the cerebral cortex (known as Transcranial Magnetic Stimulation - TMS - or repetetive Transcranial Magnetic Stimulation - rTMS).