PHYSICS

flow of current, which in turn produces a force tending to oppose
the movement of the conductor. Work will then have to be done
to move the conductor:
F.d,i =J( Lx jj) ·di. (18.8)
The power input is P = dAjdt, and therefore
P =J (L x jj). V, (18.9)
where V is the velocity of the conductor. But the power input is
given by P = JU, where U is the potential difference across the
ends of the conductor; therefore
- - -
U = (L xB)·V. (18.10)
When L, iJ, and V are mutually perpendicular, as in the
case of the electromagnetic flow meter, the last equation
simplifies to
U = LBV. (18.11)
This equation can be applied directly to measurement of the
blood flow.
Fig. 18.5. Electromagnetic flowmeter:
1 - signal voltage; 2 ­
electromagnet; 3 - readout
system; 4 - blood vessel; S ­
cross-sectional area of the vessel;
V - velocity of the blood
Fig. 18.6. Principles of threedimensional
magnetocardiography
through the mapping
flow; U - potential difference;
B - the magnetic field; / _
of the magnetic field around
distance between the poles
the thorax of patient
of magnet
i f
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Chapter 19. BIOMAGNETISM
19.1. MAGNETIC FIELDS IN LIVING ORGANISMS
The word magnetography originates from Greek words magnetos
meaning magnetic field and graphi meaning to write on. Neural
tissue produces electrical potentials within the body and these
potentials give rise to electrical currents in tissue. These currents
will give rise to magnetic fields. Biomagnetic fields are produced
also by the currents which circulate in biological membranes,
blood vessels, around the heart (magnetocardiography), brain (magnetoencephalography),
muscles (magnetomiography) , eye (magnetooculography).
d5
Magnetocardiography provide
the mapping of the magnetic
field around the thorax as a research
tool. The magnetic field is
a vector quantity and has therefore
three components at each
V location in space. The method of
magnetocardiography is explained
in fig. 18.6. Each of the three
components are measured symmetrically
(i.e., on both sides of
the source). Such a system requires
six magnetometers or six consecutive
measurements with one
magnetometer.
The electromagnetic flow meter for blood flow measurement
consists of an electromagnet to generate a magnetic field and two
electrodes to sense the flow signal. They are encapsulated in an
inert hard plastic in a form which permits them to fit around the
blood vessel of interest (fig. 18.5). In this way the system can be
used to measure the potential difference versus the blood flow.
136
Chapter 20. MAGNETIBIOLOGY
• ... • M oQ'ii!'!S!' r
20.1. MAGNETIC ORIENTATION IN ANIMALS
The total intensity of the geomagnetic field is highest near the
magnetic poles, with more than 60000 nT; it decreases to values
of about 26000 nT at the magnetic equator. The magnetic orientation
means that animals use information from the magnetic field
to direct and control their behavior.
137