When you read this sentence, a certain region of your brain is activated. When you smell a rose or feel a pencil in your grip, other regions are activated. One of the best ways to determine which regions are activated is to detect the magnetic field produced by the activation. The apparatus in the photograph can detect the magnetic field set up by a person’s brain so that a map of brain activity can be correlated with what the person does. However, there are no magnetic materials in the brain.
So, how can brain activation produce a magnetic field?
The answer is in this chapter.
One basic observation of physics is that a moving charged particle produces a magnetic field around itself. Thus a current of moving charged particles produces a magnetic field around the current. This feature of electromagnetism, which is the combined study of electric and magnetic effects, came as a surprise to the people who discovered it. Surprise or not, this feature has become enormously important in everyday life because it is the basis of countless electromagnetic devices. For example, a magnetic field produced by an electric current can be found in machines that record or read magnetically encoded tapes and disks such as old-fashioned computer floppies, cassette audio tapes, ...