Faraday Currents Produced by Changing Magnetic Fields
Biology is a delicate dance of genetics, chemistry and electric currents. These currents are formed by charged particles called ions. The very thoughts in your brain are characterized by patterns of such currents. The movement of your arms and legs are initiated by them. The sensation of touch is carried by ionic currents. Even the balance of chemicals between cell cytoplasm and body fluid is dependent on a difference in ion concentrations across cell walls. So, it is not surprising that artificial electric currents induced in body tissue can affect biology. Such effects have been studied extensively over the past hundred years.
The International Program on Chemical Safety (INCHEM)
is an organization
of scientists that studies chemistry with an eye on public safety. It
funded by the World Health Organization, the United Nations and the
Labor Organization. It's Environmental Health Criteria Report Number 69
a look at magnetic fields. Since changing magnetic fields can induce
currents in living tissue, this report tries to infer health risks by
currents into strength ranges. In the table below, five ranges are
in terms of current density. The unit used is uA/cm2 or
per square centimeter. Ionic current passing through a one centimeter
is described in millionths (1/1,000,000) of an ampere. This is a pretty
unit of measure. Your electric hair dryer runs on 10 million microamps.
Effects of Induced Current on Living Tissue
less than 0.1 uA/cm2
0.1 - 1
1 - 10
|Established effects||Caution advised|
10 - 100
|Significant stimulation||Possible hazards|
more than 100
|Dangerous stimulation||Acute hazards|
When a magnetic field changes, an electric field is generated which may drive an induced current in an electrical conductor. This effect was discovered by Michael Faraday and is known as Faraday's Law. Throughout this website, a standard electromagnetic coil is described with the following attributes: 3 inch inner diameter producing a 100 gauss magnetic field in 1 millisecond. Applying Faraday's Law to a circle inside this coil, allows us to estimate the induced electric field to be 0.15 volts/meter. The analysis is shown in the boxes below. Note that this effect is significant only for electromagnets with sharply changing magnetic fields. Permanent magnets have an unchanging or static magnetic field.
(Average Electric Field)
times (Circumference) =
dB/dt is the
change in magnetic field
E = Electric Field
To calculate induced current density from the electric
field, you have to
know the conductivity of the tissue. Conductivity is a measure of the
with which electricity can flow in a substance and is usually stated in
of Siemens per meter, S/m. You just multiply the strength of the
field by the conductivity of the medium to get current density. As you
expect, different parts of the body have different conductivities which
even exhibit directional variances. So, for various types of tissue,
current density induced by the coil described above is listed in the
Induced Current Density for a 100 gauss/ms 3 Inch Coil
|Blood Plasma||1.4 S/m||21 uA/cm2|
|Skeletal Muscle, long axis||0.53||7.9|
|Brain, grey matter||0.45||6.7|
|Nerve, extracellular fluid||0.33||4.9|
|Brain, white matter||0.15||2.2|
|Bone, longitudinal direction||0.067||1.0|
These current densities range from 1 to 21 uA/cm2. According to the INCHEM table, such currents cause established biological effects or mild stimulation and they are relatively safe with some caution advised. This is a promising window of opportunity for magnetic therapy. What results might be anticipated? In the above current ranges, the following systems have been affected as reported in the medical literature: nerve and muscle stimulation thresholds, cell growth rates, cellular respiration, lipid bi-layer permeability, metabolism of carbohydrates, gene expression, endocrine and hormonal stimulation and immune system response to name a few. It would seem reasonable that pulsed electromagnetic devices could be beneficial under the right circumstances. However, anything in life with the potential for good carries a risk for harm. Common sense and caution are always in your best interest. In most cases, magnetic effects are temporary but, in a developing organism, minor environmental changes can be magnified. So electrical, chemical or magnetic exposure are never recommended during pregnancy.
We live in an electromagnetic society, surrounded by televisions, phones and motors. I'm sure you have wondered what effect these devices might have on your body. In North America, electricity is supplied as 60 hertz sine waves. This means that voltages and currents wiggle like waves on the sea 60 times every second. So, the resulting magnetic fields change and produce Faraday currents in your body if you get near enough. The table below is an interesting list of common household appliances with typical magnetic fields expected at 3 cm from the device (a little over an inch). Note that the can opener at the top of the list operates at only 7% of the rate of our 100 gauss/ms electromagnetic coil, inducing appropriately smaller currents. I don't know about you, but I might be embarrassed to hold a can opener on a sore muscle.
Peak Magnetic Field
Peak Rate of Change
|Can Opener||20 gauss||7.5 gauss/ms|
In hospitals, very strong magnetic fields are generated by magnetic resonance imaging machines, MRI for short. These expensive devices produce precise pictures of organs and tissues in living bodies. They accomplish this by generating a very strong static magnetic field that can supply energy to electrons. Then a much smaller position dependent field is superimposed to create a coordinate system. As the tissue is probed by radio waves, excited electrons emit radiation that is interpreted as pictures. The fields used are commonly 2 tesla in strength with a rate of change of 2 tesla/second or less. In gauss units, this translates to 20,000 gauss static fields with changing fields of 20 gauss/ms. If you don't jiggle your head and aren't wearing a pacemaker, such machines are considered safe. Interestingly enough, in a clinical trial using only the changing portion of an MRI field, patients with a bipolar disorder experienced statistically significant improvement in their depression. The Faraday currents produced by such conditions are in the "established effects" range. Why this should help with depression is anyone's guess.
The authors of the bipolar depression study suggest that the left to right gradient of their MRI field might be responsible. They reason that such a field might generate effects that would impose a mild left to right organization on the brain, somehow countering depression. This explanation seems like a stretch to me. Of course, the brain is divided into left and right hemispheres which operate like brother and sister. But the nerves in each hemisphere are not organized in a left to right fashion. The nerves that code for a particular concept might be physically at any location or angle, depending on the learning history of an individual. The only requirement for organization is that an appropriate group of nerves be interconnected by axons. It seems more reasonable to me that magnetic currents randomly affect cells depending on their orientation to the applied field. Any conceivable magnetic field would then have a mildly disorganizing effect on brain activity. In a broad sense, I think it is reasonable to view depression as the result of very complicated yet very organized thought, maintained by an electrochemical feedback loop. A magnetic field could simply interfere with the organization that perpetuates the depression. Of course, I am only guessing, too. But the same reasoning can explain how a magnetic field relieves tense muscles and chronic pain. If you are a fan of Ocam's Razor, the simplicity of this argument is appealing.