On a recent afternoon at Columbia University's renowned College of Physicians and Surgeons, Dr. Holly Lisanby, assistant professor of clinical psychiatry, demonstrates the technique. The paddle is connected to a power source and a computer. When electrical current surges through the coil, it generates a magnetic field. Lisanby holds the paddle directly above the left temple of an imaginary person seated in an old dentist's chair in the center of the room. A flashing window on the laptop displays the disconcerting words "Armed Mode." Lisanby hits a key on her laptop. There's a loud rat-a-tat, like the sound of a staple gun, and then the device falls silent. "Most people describe the sensation as a kind of tapping on the head, like a woodpecker," says Lisanby. The demonstration over, the imaginary patient is presumably on his or her way to feeling better.

If transcranial magnetic stimulation, or TMS, emerges with good results from several studies being conducted by Lisanby and others around the country, the little paddle Lisanby wields could represent a breakthrough in the treatment of depression and other intractable conditions. TMS could be "the neuropsychiatric tool of the 21st century," one physician says. But as even the most ardent proponents of TMS admit, that's a big if.

Electromagnetism has been harnessed in everything from kitchen appliances to the space shuttle. It has also been employed -- unsuccessfully -- at different periods in medicine's colorful and at times dark past. In 18th century, pre-revolutionary France, the craze for all things scientific led to the discovery of so-called animal magnetism and launched the fad of mesmerism, a kind of hypnotherapy. Invalids of every description flocked to healers with mesmeric tubs or linked hands with each other to form human electrical circuits. In the late 19th century, medical fascination with invisible energies resurfaced in experiments with magnetotherapy. Jean-Martin Charcot, the eminent nerve doctor, staged public demonstrations in which he claimed to move hysterical symptoms across the surface of the patient's body with the aid of a magnetic coil. However bizarre, these experiments did have the virtue of fueling interest in hypnosis, and led indirectly to Sigmund Freud's discovery of the talking cure.

In the 1930s, electricity was harnessed in a brutal form in shock treatment, which induced seizures in patients. Electroconvulsive or electroshock therapy, as it was known, became a staple of the psychiatric arsenal for decades before falling out of favor -- only to be revived in recent decades, in a safer form, to treat depression. In the meantime, popular interest in magnetism seems once again to have peaked, albeit in contradictory ways -- ranging from self-proclaimed magnetic healers to technology critics who warn of the ill effects of the force fields emanating from cellphones and electric power lines.

It is against this backdrop of dubious science and medical dead ends that researchers have staked their claim for TMS. Despite the treatment's unorthodox appearance, its proponents are not exactly tilting at windmills. TMS has become one of the hottest fields in psychiatric medicine, part of the revolutionary change in the field brought about by new insights into the functioning of the brain.