Page 39 - Electronic Navigation Cyber Book

The electromagnetic log is based upon the Faraday-Maxwell induction
law; Figure shows the principle of the log.

The induced e.m.f. ‘E’ is given by the following:
E = F x L x V

Where F = the magnetic field

L = the length of the conductor
V = the velocity of the conductor through the magnetic field.

In the EM log a direct current through the windings of a coil, generates a
magnetic field. If the conductors do not move relative to the coil they do
not intersect the magnetic fines of force and no voltage is induced in
them.
In the EM log the ‘F’ and ‘L’ are maintained constants, therefore the
induced e.m.f. is directly proportional to the velocity ‘V’, which is the
velocity of the vessel through the water.

The direction of the voltage E depends on the directions of the lines of
force and the direction of the velocity of the conductor water. According
to the formula the induced voltage is proportional to the velocity V.
Should the velocity have the opposite direction, the direction of the
voltage would change too.

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