Page 40 - Electronic Navigation Cyber Book

The electromagnetic log is based upon the Faraday-Maxwell induction
law;
A direct current through the windings of a coil, generates a magnetic field.

Four conductors (ab, bc, cd and da) are arranged in the form of a loop
around the coil.

If the conductors do not move relative to the coil they do not intersect the
magnetic lines of force and no voltage is induced in them.

Alternating current through the coil

Instead of a direct current, suppose that we send an alternating current
through the coil. Then the induced voltage that we will have would be also
an, alternating voltage with amplitude that is proportional to the velocity,
V.

For the electromagnetic log an alternating voltage is preferred to a direct
voltage.
The speed out put from an EM log depends upon the water flow by way
of the sensors. Thus siting of the probe is critical. This is so since if too
close to the hull then due to the non-linearity of the hull form the speed

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