IMPROVEMENTS IN MAGNETIC COMPASSES
Year: 1877

page_text: PROVISIONAL SPECIFICATION left by Alexander Charles Fraser at the Office
of the Commissioners of Patents on the 1st December 1877.
ALEXANDER CHARLES FRASER, of 449, Strand, in the County of Middlesex,
Manager to Messieurs Elliot of the same Place, Opticians. "IMPROVEMENTS IN
MAGNETIC COMPASSES."
This Invention relates to improvements in compasses.
In the manufacture of compasses for ship use, also for those of a portable
character, termed prismatic compasses, the bowl or casing is of metal with flanges,
leadings, or projections on the exterior, and with other flanges upon the inside
below, and under which the edge of the card finds a lodgment in order to give a
neat appearance to the article. The card is generally formed of an enamelled plate
or disc of insulated metal, or of talc, on which the printed and divided sheet is
pasted, and with the magnetic needle upon its under face, by which its magnetic
readings can be observed when looking from above.
In the case of prismatic compasses, the card is formed of a metal ring with a
cross bar, by which the card is suspended on a "centre" that it may turn freely
thereon, and find its own direction of polarity when released from a catch plate,
which is caused under the pressure of a knob to press against its edge, and thereby
hold it firmly when desired.
The top face of the ring is divided, or has a degree scale marked upon it, to
enable the readings to be taken when desired by a hair line in a hinged flap piece
which can be arranged vertically, so that the shadow of the hair line from the sun
light may be cast across the face of the scale, the reading off operation being made
by a small lens fitted on the edge of the casing for the purpose. These prismatic
compasses, although of a convenient size for carrying are of unnecessary weight; they
are also expensive by reason of the engraved scale upon the face. The cards require
extra care in centreing, and the "point" has to sustain a weight which diminishes
the sensitiveness of the card to turn thereon.
The above objections to compasses generally are great drawbacks to their
universal adoption, aided to the cards sensitiveness to enlarged outside magnetic
Price 2d1

page_text: influences, which often set the needlé several degrees away from the normal or true
magnetic point.
By my Invention the foregoing objections and inconveniences are obviated, the
magnetic power or polarity of the card or needle is insulated from outside
surrounding influences, consequently a more correct reading can be taken than
under existing circumstances.
The compass bowl or casing is of considerably less weight, as is also the card.
The division lines and figures on the card are more defined and more easily read,
even without a lens or magnifying glass than with one of present construction.
The shadow of the hair line is more bold and distinct, and the article, especially
as a prismatic compass, is superior in every sense.
For the purposes of my Invention, I make my compass bowl or casing of black
hard rubber moulded into shape with a bulb in the centre of the bottom piece, into
which the point or centre stem is embedded; on this I mount a card of printed
paper (tinted) pasted upon an aluminium disc or plate, whose outer edge projects
beyond the edge of the paper to take the pressure of the spring plate (shaped to
correspond), when the card has to be temporarilly fixed for the reading to be takou,
I form a lip rim around the top edge of the casing, in or on to which I cement or
screw a lid or top, this having a V shaped aporture in it, and provided with a glass
through which the reading can be taken, and across which the shadow of the hair
line from the sun's light can be seen very distinctly.
The adaptation of hard black rubber as an insulator, and moulded to shape in
conjunction with an aluminium base to a compass card having printed divisions on
tinted paper, from which magnetic readings can be taken, said casing having a
glass in its lid as described, forms a new article of commerce of great value.