The Body Magnetic

October 15, 2015

As anyone who's gotten an electrical shock from a switch plate knows, the human body conducts electricity. This also demonstrates that the human body can store electrical charges generated by walking across carpet fabric in low humidity. If you have an ohmmeter at home, you can measure your own electrical resistance, which is the reciprocal of conductance.

For safety reasons, however, it's required that you never do any such measurements across the torso, as from one hand to another. The battery voltages of most of today's ohmmeters are small, but application of any current through the torso can be dangerous. Also, do not puncture your skin to do this measurement, since one known fatality has occurred this way.[1]

I just measured my resistance from the thumb to little finger of my right hand. The value I got, about 20 megohm (20 MΩ), is higher than what would be expected. I needed to use a sensitive ohmmeter, since I obtained no reading on a less expensive, 2 megohm fullscale, meter. Typical values for dry skin are in the 100 kΩ range, with wet skin being in the 1 kΩ range. The contact area was just the sides of the probe tips. Using larger area electrodes, perhaps with brine or a conductive paste, might have given a lower value.

My measurement of my own skin resistance. Note that I did not let the current of the ohmmeter pass through my torso, just from the thumb to the little finger of my right hand. (Wikimedia Commons image, modified using Inkscape.)

Electricity was an exciting thing in the early days after its discovery. There was the idea that electricity was the vital force of living creatures, as presented in modern adaptations of Mary Shelley's Frankenstein (1818). Shelley's original book didn't mention electricity, but the concept was worked into a revised 1831 edition.

The scientific basis for this comes from Luigi Galvani's discovery that electrical excitation caused movement in the legs of a dissected frog (see figure). Such reanimation of a dead organism was, of course, sensational.

Italian physicist, Luigi Galvani (1737-1798), and a diagram of his frog experiment from an 1859 science textbook. (Images, left, right, via Wikimedia Commons.)

Since both electricity and magnetism are easily observed natural phenomena, it was inevitable that someone would postulate the existence of an "animal magnetism." Animal magnetism, proposed by the German physician, Franz Mesmer, wasn't magnetism in the sense that physicists understand the term. It was supposedly an invisible natural force generated by animals. Animal magnetism had supposed medicinal properties, and people were persuaded to believe in its existence from the late 18th century into the early 20th century, along with many other primitive medical theories.

The human body does exhibit real magnetism, albeit at a very low level. Fortunately, modern science now has sensitive SQUID magnetometers that allow detection of very small magnetic fields. SQUIDS are based on the interference of quantum states in Josephson junctions embedded in superconducting materials. Magnetic regions of the brain have been magnetically imaged in a technique called magnetoencephalography.

The simple idea that pairs of electrons could tunnel through a thin insulating barrier secured the 1973 Nobel Prize in Physics for Brian Josephson. (Rendered using Inkscape.)

The X-men villain, Magneto, uses the supposed magnetism of blood hemoglobin as a weapon by drawing it out of the body. Hemoglobin, however, is not ferromagnetic, it's just weakly paramagnetic, so it's barely attracted to even the strongest magnets.

Transformers are devices that use the Faraday law of induction to transmit electrical energy through a magnetic medium by converting an oscillating current to an oscillating magnetic field, conducting it through the magnetic medium, and then converting it back to an oscillating electrical current. A simple transformer, as shown in the figure, does this with two coils and a core of high magnetic permeability. For greater efficiency, the core is typically wrapped into itself to form a closed ring.

A simple transformer with a magnetic core. Although this arrangement works, the core is usually a toroid. (Modified Wikimedia Commons image.)

A team of electrical engineers at the University of California, San Diego, has just shown that the human body can act as the magnetic core of a signal transformer. They demonstrated a communication technique in which data signals can be sent from one bodily appendage to another. A presentation on this research was made at the end of August at the 37th Annual International Conference of the IEEE Engineering in Medicine and Biology Society, Milan, Italy.[2-3]

There were two motivations for this research. The primary motivation was the desire to extend the lifetime of battery-operated transmitters by reducing as much as possible the power required to transmit data between personal monitoring devices. Bluetooth radio, the conventional method, requires considerable power, since the power losses through the body are large at Bluetooth frequencies. The research team has shown that, in theory, 10 million times lower power than Bluetooth would be required in their system.[3]

Jiwoong Park, an electrical engineering Ph.D. student at UCSD and first author of the study, demonstrating a human transformer signaling circuit in which data is transmitted from one arm to the other. (UCSD Jacobs School of Engineering photograph.)

A secondary motivation was data security. Unlike Bluetooth, which has a communications range away from the body, the magnetic communication system has a strong signal at the body that decreases dramatically away from the body.[3] Says Jiwoong Park, an electrical engineering Ph.D. student at UCSD and first author of the study,

"Increased privacy is desirable when you're using your wearable devices to transmit information about your health."[3]

This technique, which utilizes magnetic fields many times smaller than those of MRI imagers and wireless implant devices, should not have an associated health risk.[3] One limitation, however, is the need to have a coil wrapped around a body part. While this is not a problem for smart watches, headbands and belts, patch sensors to detect heart rate, for example, would not work.[3]

References:

1. Resistance is Futile, 1999 Darwin Awards Web Site.
2. Web Site of the 37th Annual International Conference of the IEEE Engineering in Medicine and Biology Society (2015, Milan, Italy).
3. Liezel Labios, "Magnetic Fields Provide a New Way to Communicate Wirelessly," University of California San Diego Press Release, September 1, 2015
4. Web Site of the University of California San Diego Center for Wearable Sensors.