In Michel Gondry's new madcap comedy, Be Kind Rewind, Jerry (Jack Black) is electrocuted while trying to sabotage a power plant, which causes him to become magnetized. The next day he browses through the VHS tapes at a down-and-out rental shop, demagnetizing each cassette and thus erasing the footage. Can a man become a magnet?
Not permanently. Only select materials, like iron, cobalt, and nickel, can become magnetized in the presence of a magnetic field and remain so in the absence of that field. These are called ferromagnets. The quantity of such material in the human body is tiny, so there's simply no chance that you'll wake up one morning, reach for the fridge, and end up sticking to it. If you were wearing ferromagnets—a steel belt, steel buttons, a steel-rimmed hat—and were unlucky enough to get struck by lightning, your accoutrements might become permanently magnetized, but not your body.
The human body can generate very weak or transient magnetic fields, however. Neurons communicate with one another via electrical impulses, which means there's a subtle, fluctuating electromagnetic field associated with brain activity. (Scientists can detect this signal using magnetoencephalography). Likewise, when someone is being shocked, struck by lightning, or electrocuted, he temporarily acts like an electromagnet. Once the current stops flowing, the field disappears.
Could a person demagnetize a tape while being electrocuted? The strength of a magnetic field is proportional to electrical current and inversely proportional to distance (the separation between the current and the videocassette, in this instance). It takes at least 1,700 gauss to erase a TDK brand XP Super Pro VHS, so at a distance of 1 centimeter, you'd need a current of 8,500 amperes. By way of contrast, less than one amp could send you into cardiac arrest and cause internal organ damage (though lethal levels depend on numerous subtleties, like how wet your skin is and the duration of exposure). Not even a trip to the electric chair would generate a magnetic field strong enough to erase a tape; the chair delivers somewhere on the order of five amps for a minute and a half (regulations vary by state).
There is one way to personally demagnetize a tape and not die in the process, although the Explainer strongly advises against it. A typical lightning bolt conveys tens of thousands of amps (and thus generates a strong magnetic field), but when a person gets struck, only a small fraction of that energy penetrates the skin. The rest passes over the surface of the body. So, if you were to get zapped by lightning while holding a VHS and survive (as 80 percent to 90 percent of strike victims do), you'd end up with a blank tape.
Gondry is hardly the first to wonder about the potential effects of magnetization on the human body. In the late 18th century, Austrian physician Franz Friedrich Anton Mesmer postulated that magnetic fluids in nature could cure physical and mental ailments. He called his technique "animal magnetism" and is said to have cured a young female pianist of blindness by channeling invisible magnetic fluids. His opponents claimed the girl only imagined she could see and that Mesmer had seduced her.
Got a question about today's news? Ask the Explainer.
Explainer thanks Adam Cohen of Harvard University, Martha Harbison of Popular Science, Lawrence Krauss of Case Western Reserve University, Jim Livingston of MIT, Don Pickrell of the R.B. Annis Company, and Mark Reed of Yale University. Explainer also thanks reader Ann Bartkowski for asking the question.