Handbook of Magnetic Compass Adjustment - Maritime Safety ...

CHAPTER XII
USE OF THE HORIZONTAL FORCE INSTRUMENT
1201. Occasionally it will be necessary to determine the actual strength of the magnetic field at some compass location. This
problem may arise for one of the following reasons:
(1) It may be desired to determine accurately the horizontal shielding factor, lambda (λ), for:
(a) A complete mathematical analysis.
(b) Accurate Flinders bar adjustment.
(c) Accurate heeling adjustment.
(d) Calculations on a dockside magnetic adjustment.
(e) Determining the best compass location on board ship.
(2) It may be desired to make a dockside magnetic adjustment, and hence determine the existing directive force at the
magnetic compass both for its magnitude and direction.
Lambda, λ, is the horizontal shielding factor or ratio of the reduced earth's directive force, H', on the compass to the
horizontal earth's field, H, as:
H'
λ = H
From this, it is apparent that λ may easily be determined for a compass location by making a measurement of the reduced
earth's directive force, H'. On a corrected compass, this value H' may be measured with the ship on any heading, since this
reduced earth's directive force is the only force acting on the compass. If the compass is not corrected for the ship's
magnetism and the deviations are large, H' is determined from the several resultant directive forces observed with equally
spaced headings of the ship, as indicated later. Lambda, λ, should be determined for every compass location on every ship.
1202. The actual measurement of such magnetic fields may be made by use of a suitable magnetometer, or by the use of a
horizontal force instrument. The magnetometer method is a direct reading method, which needs no calculation. The force
instrument is by far the simpler form of equipment, hence the force instrument method is discussed below.
The horizontal force instrument is simply a magnetized needle pivoted in a horizontal plane, much the same as a compass.
It will settle in some position that will indicate the direction of the resultant magnetic field. The method used to determine the
strength of this resultant field is by comparing it with a known field. If the force needle is started swinging, it will be damped
down with a certain period of oscillation dependent upon the strength of the magnetic field. The stronger the magnetic field
the shorter the period of time for each cycle of swing; in fact, the ratio is such that the squares of the period of vibration are
inversely proportional to the strengths of the magnetic fields, as:
H' T 2
H = T' 2
In the above formula, let H represent the strength of the earth's horizontal field in gauss and T represent the time in seconds
for 10 cycles of needle vibration in that earth's field. Should it be desired to find the strength of an unknown magnetic field,
H', a comparative measurement of time in seconds, T', for 10 cycles of vibration of the same needle in the unknown field will
enable calculation of H'.
Since λ is the ratio of two magnetic field strengths, it may be found directly by the inverse ratio of the squares of the
periods of vibration for the same horizontal force instrument in the two different magnetic fields by the same formula,
without bothering about the values of H and H'.
H' T 2
λ = = H T'
2
The above may be used on one heading of the ship if the compass deviations are less than 4°
To obtain the value of λ more precisely, and where deviations of the compass exceeds 4°, the following equation should be
used:
T 2 cos d n cos d e cos d s cos d λ = w
4
2 Tn
+ 2
T
+ 2
e T
+ 2
s T w
where:
T is the time period for the field H.
T n , is the time period for the resultant field with ship on a north heading, etc.
cos d n is the cos of the deviation on the north heading, etc.
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