Friday, October 15, 2010

Michael Faraday, Biological Physicist?

Last week in this blog I discussed the greatest physicist of all time, Isaac Newton. However, if we narrow consideration to only experimental physicists, I would argue that the greatest is Michael Faraday (with apologies to Ernest Rutherford, who is a close second). In Section 8.6 of the 4th edition of Intermediate Physics for Medicine and Biology, Russ Hobbie and I discuss Faraday’s greatest discovery: electromagnetic induction.
In 1831 Faraday discovered that a changing magnetic field causes an electric current to flow in a circuit. It does not matter whether the magnetic field is from a permanent magnet moving with respect to the circuit or from the changing current in another circuit. The results of many experiments can be summarized in the Faraday induction law.
I have always admired the 19th century Victorian physicists, such as Faraday, Maxwell and Kelvin. Michael Faraday, in particular, is a hero of mine (it is good to have heroes; they help you stay inspired when the mundane chores of life distract you). I had the pleasure of quoting from Faraday’s Experimental Researches in Electricity in an editorial I wrote in 2005 for the journal Heart Rhythm:  “Michael Faraday and Painless Defibrillation.” I tried to get a picture of Faraday included as part of the editorial, but alas the journal editor removed it. The article described a heart defibrillator having a design that included a type of Faraday cage.
Michael Faraday, arguably the greatest experimental physicist who ever lived, first demonstrated the shielding effect of a hollow conductor in 1836 by building a 12 ft × 12 ft × 12 ft cubic chamber out of metal. We would now call it a “Faraday cage.”

“I went into the cube and lived in it, and using lighted candles, electrometers, and all other tests of electrical states, I could not find the least influence upon them, or indication of anything particular given by them, though all the time the outside of the cube was powerfully charged, and large sparks and brushes were darting off from every part of its outer surface.” [Faraday M. Experimental Researches in Electricity. Paragraph 1174. Reprinted in: Hutchins RM, editor. Great Books of the Western World, Volume 45. Encyclopedia Britannica, Chicago, 1952.]

Faraday cages are used to shield sensitive electronic equipment. The metal skin of an airplane, acting as a Faraday cage, protects passengers from injury by lightning. Researchers perform electrophysiology experiments inside a Faraday cage to prevent external noise from contaminating the data. A rather spectacular example of shielding can be seen in the Boston Museum of Science, where a van de Graaff generator of over one million volts produces a dramatic display of lightning, while the operator stands nearby—safe inside a Faraday cage.
Why this little physics lesson? In this issue of Heart Rhythm, Jayam et al. [Jayam V, Zviman M, Jayanti V, Roguin A, Halperin H, Berger RD. “Internal Defibrillation with Minimal Skeletal Muscle Activation: A New Paradigm Toward Painless Defibrillation,” Heart Rhythm, Volume 2, Pages 1108–1113, 2005] describe a new electrode system for internal defibrillation that eliminates the skeletal muscle activation and pain associated with a shock. The central feature of their design is a Faraday cage: a conducting sock fitted over the epicardial surface of the heart…
In Section 8.7, Russ and I describe what may be the most important biomedical application of Faraday’s work: magnetic stimulation.
Since a changing magnetic field generates an induced electric field, it is possible to stimulate nerve or muscle cells without using electrodes. The advantage is that for a given induced current deep within the brain, the currents in the scalp that are induced by the magnetic field are far less than the currents that would be required for electrical stimulation. Therefore transcranial magnetic stimulation (TMS) is relatively painless. Magnetic stimulation can be used to diagnose central nervous system diseases that slow the conduction velocity in motor nerves without changing the conduction velocity in sensory nerves [Hallett and Cohen (1989)]. It could be used to monitor motor nerves during spinal cord surgery, and to map motor brain function. Because TMS is noninvasive and nearly painless, it can be used to study learning and plasticity (changes in brain organization over time). Recently, researchers have suggested that repetitive TMS might be useful for treating depression and other mood disorders.
I worked on magnetic stimulation for many years while at the National Institutes of Health in the 1990s. It was a pleasure to explore an application of Faraday induction; it is my kind of biological physics.

Faraday’s name can be found in a few other places in our book. It first appears in Chapter 3, when the Faraday constant is defined: F = 96,485 Coulombs per mole. It also appears in an abbreviated form in the unit of capacitance: a farad (F).

I suppose by now the reader realizes that I like Mike. But is he a biological physicist? Doubters might want to look at another physics blog: Faraday apparently did studies on the electrodynamics of electric fish. So, yes, I claim him as a biological physicist, and the question mark in the title of this blog post is unnecessary.

No comments:

Post a Comment