Improvements in Apparatus for Deflecting the Compass to Find the Magnetic Ship's Forces, the Deviation, and other Compass Errors.
Year: 1895

page_text: Date of Application, 1st July, 1895-Accepted, 7th Mar., 1896
COMPLETE SPECIFICATION.
Improvements in Apparatus for Deflecting the Compass to Find
the Magnetic Ship's Forces, the Deviation, and other Compass
Errors.
I, JOHN MAURICE, Civil Engineer and Compass-Expert, a citizen of the
United States, residing at 1283 South California Avenue Chicago, in the County
of Cook and State of Illinois, do hereby declare the nature of this invention and
in what manner the same is to be performed, to be particularly described and
ascertained in and by the following statement :-
This invention is an instrument for deflecting the compass, and which call
Compass-Deflector, for the purpose
1. Of finding the horizontal directive magnetic force ou board ship as the vessel
is headed in different directions, and from the forces thus found, ascertaining by
the application of the formulæ of the Admiralty-Manual and those presented
herein-after the magnetic co-efficients and the semicircular and quadrantal
deviation.
2. Of finding the correctness of any compass as to its make, and whether it is in
a proper position or not on board ship.
Heretofore deflecting magnets have been used with an apparatus placed in the
center of the glass cover ou top of the compass bowl, thus partly obscuring the
card from view, rendering it difficult to read off deflections; the deflecting magnets
being applied within the radius of the card, deflections are more or less liable to
error, on account of too close proximity of the magnets to the needles, a small
difference in the space between magnets and needles, causing a great difference in
the deflections.
The object of my invention is to keep the card free from all obstructions to sight,
and increase the accuracy of the deflections by placing deflecting magnets at a
distance from the compass center, not less than the radius of the card.
I attain these objects by the instrument illustrated in the accompanying drawing,
in which
Fig. 1. is a top view, and Fig. 2. a vertical section.
a is a circular ring with two uprights diametrically located, each uprigh
bearing a slide d with a receptacle e for magnets or counterbalancing weights.
Ring a slides on the glass cover of the compass bowl, and is kept in position or
guided by the inner rim of the bowl, which it fits so as to admit of being moved
freely in a circle. The ring a is strengthened by another ring of greater
diameter, and by braces between both rings. A fine wire or thread k fastened
to ring a, diametrically at right angles to the uprights, serves as pointer to keep
the deflecting force at any desired angle with the compass meridian. The
uprights 6 are graduated for the purpose of reading off the position of the slides
with the magnets. Each slide is adjustable by means of a screw and the
magnet is held in the receptacle of the slide by means of a screw i. To keep the
magnets from rusting they are soldered up in brass tubes having projections at one
end fitting recesses of the receptacles to ensure their being exactly inserted in the
same position. The magnets are of round steel of equal length, from six to
eight inches loug, and from to of an inch in diameter. As a counter balancing
weight for a single magnet is used in the opposite slide a brass tube filled with lead.
All parts of the instrument are of brass or of some other non magnetic material.

page_text: The construction and use of the instrument is based on the principle, that the
moment of the directive force equals the moment of the deflecting force. If
R represents the directive force, F the deflecting force, & the deflection, r the
distance of the poles of the needle from the compass center, sin is the lever
of R, and rm the lever of F, m being a function of the angle which the direction
of the deflecting force forms with the compass meridian, depen ling on the arrange-
ment of the needles in the compass. The moments of the two forces are Rr sin
and Frm respectively. Therefore, when the compass is at rest
The product Fm may be called the moment and be designated by M
If the directive force equals R, and the deflection 1 M remaini
R, sin ₁M consequently,
from which follows, that the directive forces are inversely as the sines of deflection,
and that any directive force may be taken as unit to express another directive
force; and further, that when the moment of the deflecting force is known, the
amount of any directive force equals moment divided by the sine of deflection.
Different moments are obtained by altering the position of the deflecting force,
either by raising or lowering the deflecting force, or, by changing its starboard
angle, that is the angle which the direction of the deflecting force forms with the
north-south line of the compass card; or, by both operations combined.
The relation of moments for different positions of a deflecting force or magnet
holds good for any deflecting force or magnet in combination with the same
compass, provided the magnets be of equal length. For the same position in
regard to the compass card, the deflecting forces are as the sines of the deflection.
Therefore, if moments have been ascertained once for certain positions of a
deflecting force, they may be used to find the moments of any other deflecting
force, if the relation between the two forces is known.
In general, the best position of the deflecting force is at the east- or west-point
of the compass card, because it is then farthest removed from the poles of the
needles, and errors of observation are at a minimum. This position is always
preferable to any other for compasses with needles arranged excentrically. But
for compasses of the single needle type the position of the deflecting force may be
varied by changing its starboard angle up to 45 degrees either way from east or
west. Going any farther would more or less affect the reliability of moments,
errors of observation increasing with the starboard angle decreasing.
As the directive forces are inversely, as the sines of the deflection, the effect of
an error in the deflection increases rapidly with small angles of deflection; and as
the remaining directive force of a compass after a deflection of 90 degrees is zero
and the compass entirely paralized, deflections should be limited to from 20 to
70 degrees.
On board ship the directive force is different for different courses. If y be the
compass course, R the directive force for that course, H the horizontal force of
the earth, the mean directive force on board ship, and the deviation, we have
according to the Admiralty Manual 3d edition page 113

page_text: four cardinal compass points. Distinguishing these forces and also the deviations
pertaining thereto by the indices N. S. E. & W. and putting
-
=
On board ship the course of the vessel must not be altered during the observations.
On shore the compass has to be carried to a place free from all foreign magnetic
influences.
The instrument being mounted on the compass bowl with a magnet in one of the
receptacles of the sides, and a counterbalancing weight in the other receptacle to
keep the glass cover of the compass bowl always level, the instrument is turned
around so as to keep the wire or pointer always over the north-south line of the
compass card.
If the deflection is over 70 degrees it can be lessened by raising the
slide with the magnet; if the deflection is less than 20 degrees it is increased by
substituting a more powerful magnet. When the scale reading and the deflection
is noted, the moment is expressed by R sin ₁, R being the directive force and
the deflection. By raising or lowering, as the case may be, to the next mark of the
scale the slide containing the magnet and noting the deflection d, a second moment

page_text: for any course the vessel is headed on, only one deflection is necessary in the
manner explained herein before. If the deflection be ẞ for a certain position of the
deflecting magnet or magnets for which the standard moment is M, the directive
M
force R₁ =
sin ẞ
that is, the directive force on any course equals standard moment
divided by the sine of deflection.
By using more than one moment or deflection for each course, the accuracy of
the results may be increased at will by taking the arithmetical mean of the directive
forces so found. It will be remembered, that the directive forces so found are
expressed in units of the directive force used as standard.
c. TO TEST THE CORRECTNESS OF ANY COMPASS AS TO ITS MAKE, &C.
This operation is best performed on shore in a place free from all foreign magnetic
influences. With the instrument mounted on the compass bowl and a single magnet
in one of the receptacles counterbalanced as herein mentioned before, a deflection
is produced of from 40 to 50 degrees by keeping the wire or pointer successively
over each of the four quadrantal points of the compass card. If the deflections thus
found are the same in amount for all four quadrantal points, the compass is correct,
otherwise it is defective in one or more respects. The magnetic axis may not
coincide with the north-south line of the card; the poles of the needles may be of
different force for north and south, or their position may not be symmetrical in
regard to the compass center, and so on.

page_text: When the compass is correct as to its make, the same operation as before repeated
on board ship for any course the vessel is headed on, will also give equal deflections
for the four quadrantal points, if the compass is in a proper position; but, if the
deflections differ from one another, the compass is influenced by magnetic agents in
too close proximity to it, and its position has to be altered if these agents can not
be removed.
Having now particularly described and ascertained the nature of my said inven-
tion, and in what manner the same is to be performed, I declare that what I
claim is:
1. The instrument for deflecting the compass, consisting of a circular ring having
two uprights arranged diametrically at right angles to the plane of the ring, with
adjustable slides having receptacles for holding magnets or counterbalancing
weights, substantially as described.
2. In an instrument for deflecting the compass, the combination of a circular ring
having two uprights arranged diametrically with a graduated scale for each upright,
substantially as described.
3. In an instrument for deflecting the compass, the combination of a circular ring
having two uprights arranged diametrically, with a wire or thread serving as
pointer, diametrically at right angles to said uprights, substantially as described.
4. In an instrument for deflecting the compass, the combination of two uprights
and adjustable slides with a magnet or counterbalancing weight for each slide, sub-
stantially as described.
5. In au instrument for deflecting the compass, the combination of a circular ring
strengthened by a ring of greater diameter and braces between the two rings
supporting two uprights arranged diametrically, substantially as described.
Dated this 15th day of June 1895.
JOHN MAURICE.