AN IMPROVED MODE OR METHOD OF CORRECTING THE DEVIATIONS OF THE MARINERS' COMPASS IN IRON SHIPS
Year: 1868

page_text: LETTERS PATENT to John Henry Johnson, of 47, Lincoln's Inn Fields,
in the County of Middlesex, Gentleman, for the Invention of "AN
IMPROVED MODE OR METHOD OF CORRECTING THE DEVIATIONS OF THE
MARINERS' COMPASS IN IRON SHIPS." -A communication from abroad
by Louis Francois Alexandre Arson, of Paris, in the Empire of
France, Engineer.
Sealed the 2nd October 1868, and dated the 21st April 1868.
? PROVISIONAL SPECIFICATION left by the said John Henry Johnson
at the Office of the Commissioners of Patents, with his Petition,
on the 21st April 1868.
I, JOHN HENRY JOHNSON, of 47, Lincoln's Inn Fields, in the County
of Middlesex, Gentleman, do hereby declare the nature of the said
Invention for "AN IMPROVED MODE OR METHOD OF CORRECTING THE DEVIATIONS
OF THE MARINERS' COMPASS IN IRON SHIPS," a communication to me from
abroad by Louis Francois Alexandre Arson, of Paris, in the Empire of
France, Engineer, to be as follows:-
This Invention has reference to a peculiar mode or method of
correcting or counteracting the deviations of the mariners' compass in

page_text: iron ships, and consists in subdividing the hull of the ship into two
or more lengths or sections by interposing at one or more parts trans-
versely across the hull a thickness of copper, brass, or other non-
magnetic metal which shall completely cut off all magnetic connection
between one portion or section of the hull and another. By this
means the mass of iron composing the hull is divided into several
magnetic poles, which by their counter influence upon the needle of
the compass in opposite directions allow the needle to assume that
position which is due simply to terrestrial magnetism. The position
of the compass in the ship and of the interposing bands or belts of
non-magnetic metal in the hull will be dependant on each other, äit
being essentially necessary that the needle be in a state of magnetic
equilibrium as regards any 'local attraction, so that the pole of one
section of the hull shall act in opposition to that of the other section.
In practice it has been found that when the hull is divided into two
divisions, for example, by a non-magnetic band or belt amidships, a
good result will be obtained if the compass be .situate on the line
of such separating band or belt. If, say, two of such bands or belts
be employed dividing the hull into three sections, the compass .should
be situate in the centre of the midships section or length..
'SPECIFICATION in pursuance of the conditions of the Lotters Patent
filed by the said John Henry Johnson in the Great Seal Patent
Office on the 21st October 1868.
TO ALL TO WHOM THESE PRESENTS SHALL COME, I, JOHN
HENRY JOHNSON, of 47, Lincoln's Inn Fields, in the County of Middlesex,
Gentleman, send greeting.
WHEREAS Her most Excellent Majesty Queen Victoria, by Her
Letters Patent, bearing date, the Twenty-first day of April, in the year
of our Lord One thousand eight hundred and sixty-eight; in the
thirty-first year of Her reign, did, for Herself, Her heirs and successors,
give and grant unto me, the said John Henry Johnson, Her special
license that I, the said John Henry Johnson, my executors, adminis-
trators, and assigns, or such others as I, the said John Henry Johnson,
my executors, administrators, or assigns, should at any time agree with,
and no others, from time to time and at all times thereafter during

page_text: the term therein expressed, should "and lawfully might make, use,
exercise, and vend, within the United Kingdom of Great Britain and
Ireland,, the Channel Islands, and Isle of Man, an Invention for "AN
IMPROVED MODE OR.METHOD OF CORRECTING THE DEVIATIONS OF THE MARINERS'
COMPASS IN IRON SHIPS," a communication from abroad by Louis
Francois Alexandre Arson, of Paris, in the Empire of France,
Engineer,., upon. the condition (amongst others) that I, the said John
Henry Johnson; my executors or administrators, by an instrument
in writing under my hand and seal, should particularly describe
and ascertain the nature of the said Invention, and in what manner
the same was to be performed, and, cause the same to be filed in
the Great Seal Patent Office within six calendar months next and
immediately after. the date of the said.Letters Patent..
NOW KNOW. YE,. that I, the said John Henry Johnson, do hereby
declare--the nature of the said. Invention,, and in what manner the
same is to be performed; to: be particularly described and ascertained
in and by the following statement, reference being had to the accom-
panying Drawings,, and. to. the letters and figures marked thereon, that
is to say:-
-
The said Invention has reference to a peculiar mode or method of
correcting or counteracting, the deviations of the mariners' compass in
iron ships, and consists in subdividing the hull of the ship into two
or more: lengths: or sections by interposing at one or more parts trans-
versely across the hull ȧ thickness of copper, brass, or other non-
magnetic metal which shall completely cut off all magnetic connection
between one portion or section of the hull and another. By this
means the mass of iron composing the hull is divided into several
magnetic poles,. which by their counter influence upon the needle of
the compass in opposite directions allow the needle to assume that.
position which is, due simply to terrestrial magnetism.. The position.
of the compass in the ship and of the interposing, bands. or belts, of
non-magnetic metal in the hull will be dependent on each other, it
being essentially necessary that the needle be in a state of magnetic
equilibrium as regards any local attraction,, so that the pole of one'
section of the hull shall act in opposition to that of the other section.
In practice it has been found that when the hull is divided into two
sections, for example, by a non-magnetic band or belt amidships, a.
good result will be obtained if the compass be situate on the line

page_text: .
of such separating band or belt. If, say, two of such bands or belts
be employed dividing the hull into three sections, the compass should
be situate in the centre of the midship section or length.
And in order that the said Invention may be fully understood I
shall now proceed more particularly to describe the same, and for that
purpose shall refer to the several Figures on the annexed Sheet of
Drawings, the same letters of reference indicating corresponding parts
in all the corresponding Figures.
Supposing a ship be cut into two equal parts in a plane perpen-
dicular to the length of the hull, as shewn in the diagram, Figure 1
of the Drawings, and that these two halves. be united together, but
insulated the one from the other by a non-magnetic body x, such as
copper for example, the compass being then placed in this vertical
plane, four magnetie poles will have been established in the hull.
In the perpendicular orientations the symetry of things material and
of the magnetic system will still preserve the compass in its normal
state, but in the intermediate positions an inverse derangement of the
preceding will be produced.
Figure 1 drawn on the hypothesis of an orientation of 45°, and
presenting four instantaneous poles in place of two, shews that those
of the centre by reason of their being situate at a less. distance from
the compass-will exert a greater influence than those of the extremities,
and that the direction of their disturbing action will be contrary to
that of the preceding. Now one of the properties of magnetism is. the
perfect insulation as regards the magnetic fluid which is established
between two pieces of soft iron by the interposition of a non-magnetic.
metal, although it may nevertheless be a good conductor of electricity.
Since the correction referred to above exceeds in extent the aim to
be accomplished it suffices to reduce its proportion. This result will
be readily obtained by diminishing the extent of division of the hull.
This reduction will admit of the main portions of the hull, such as
the keel and carlines, remaining intact. The continuity of the hull
alone will have to be interrupted and in a portion only of its extent.
It would certainly be well to be able to determine beforehand the
amount of magnetic interruption required, but on reflection it will be
evident that its absolute determination will be exceedingly different.
It is necessary to observe that the magnetic forces of each half of
the vessel established in a suitable proportion by the extent of the

page_text: division, and which influence in this proportion the origin of the move-
ment to east or to west, become equal when the ship is due east or
due west. Thus therefore the system, the conditions of equilibrium of
which it is requisite to follow, is variable at cach instant in the
intensity of the forces and in their points of application. However,
in considering an orientation of about 45°, which would permit of
attributing to the magnetic forces nearly definite positions and in-
tensities, the relation to be established between these forces, that is to
say, the fraction of the hull which ought to be interrupted, may be
approximately ascertained. A calculation of this kind applied to
ordinary constructions demonstrates that the division of the hull should
not be more than from one-fifth to one-fourth of the section and shews
that the rule is easy of application. In practice it will be sufficient
to determine by the above calculation. the greatest proportion of the
division to be made, then for definite adjustment to limit this inter-
ruption or division to its proper proportion.
j
0
Figure 2 represents a transverse section of an armour-plated ship,
the "division of which is effected by the interposition of plates of
copper c which are rivetted by the aid of angle irons c¹ secured to
the inner and outer skins of the hull. The section, Figure 3, which
follows the line 1-2 in Figure 2 shews the manner of securing these
copper plates. It will be observed that these plates are secured in
the same manner as the other plates C. The substitution of copper
rivets for iron ones, or vice versa, in the joining of the plates of
the hull will enable a proper adjustment to be effected. This may
be effected in any latitude and will stand good in all others. The
solution is in fact independent of the absolute value of the forces
in action; it is based upon their distribution in given proportions,
and these will remain constant since they depend on the relation
of the sections of matter. Other modes of materially realising the
process above referred to may be obtained by arrangements which
will be easily made to agree with the conditions to be fulfilled.
It will be proper to vary the extent of the interruption according
to the external form, size, and uses of the ship. The portion
which appears the most delicate and which will present the greatest
difficulty is the hull. Here are several examples of different arrange-
ments which will answer well in practice. Figures 4 and 5 are
horizontal sections at different heights in Figure 6, which Figure

page_text: represents an internal face view of the parts. Figure 7. is a transverse
vertical section, and Figure 8. is an external face view or side clevation
of a portion of the hull. In all these Figures it is to be remarked
that angle irons care employed in effecting the junction of the
two parts of the ship. A band of copper c separates them, and
copper rivets serve to hold them together. The interposition of
a non-magnetic body is fully realized and yet the uniting of the
whole is effectually and securely accomplished. Figures 9 and 10
represent the use of bars of bronze or hard brass attached by iron
rivets and still producing a break or interruption in the magnetism.
One of these pieces (Figure 5) is simple, the other has a rib c
provided for the purpose of increasing its strength. Copper appears
to be the most convenient non-magnetic metal for making the
divisions or breaks in the hull. It is unfortunately readily oxidised
by contact with sea water, and it should therefore be protected
from its action. But considering the small proportion of the section
of the hull which it is necessary to interrupt, it will be seen that
it is possible to effect this by the arrangements above described, in
which the plates which constitute the sides are bordered by angle
irons which enclose between them a thin band of copper. The rivets,
if desired, may remain of wrought iron,. and the copper band may be
coated on the exterior and interior with a metal less easily oxidised
by the action of sea water. The above renders it scarcely unnecessary
to say that the insulation of the armour plates will be obtained with
the greatest ease and certainty. These plates are separated from the
hull by a thick layer of hard wood 6 (Figure 2), and their attachment
is effected by iron screws the relative section of which is very small.
It will be seen from what has been stated above that the deplacement
of the magnetism of one piece in contact with another only operates
in proportion to the amount of section in contact; the result in the case
of the fastenings for the armour plates need not be taken into special
account. They will moreover enter into the sum of the corrections of
the hull. The only thing which it seems advantageous to effect as
regards the armour plates is their insulation from each other, and the
smallest intervals left between them will suffice for this purpose. From
this it will be evident that the application of armour plates thus
disposed will not alter the equilibrium established in the hull. These
appliances although formed of considerable masses of iron will not
consequently present any difficulties.

page_text: Having now described and particularly ascertained the nature of the
said Invention, and the manner in which the same is or may be used or
carried into effect, I would observe in conclusion that what I consider to
be novel and original, and therefore claim as the Invention secured to me
by the herein-before in part recited Letters Patent is, the system or
mode of correcting or counteracting the deviations of the mariners'
compass in iron ships or vessels by dividing the hull of the ship or
vessel into two distinct magnetic portions.
In witness whereof, I, the said John Henry Johnson, have to
this my Specification set my hand and seal, the Twentieth day
of October One thousand eight hundred and sixty-eight.
J. HENRY JOHNSON. (L.S.)