GB2067755A - Magnetic compass - Google Patents

Abstract

A compass assembly which enables corrections for magnetic variation and deviation to be applied, comprises a base (11) marked with a compass rose and a set of three indicator discs (16, 17, 18) mounted on a common spindle (14) and having friction pads (19, 20) interposed between the discs so that the frictional resistance between the first and second disc (16, 17) is greater than that between the second and third discs (17, 18) and the friction between the discs is substantially greater than that between the discs and the base (11). The discs have different diameters and are opaque apart from arcuate peripheral transparent regions through which arcuate scales (23, 24) can be seen enabling the discs to be relatively angularly positioned by reference to radial indicia DEV, VAR marked on the discs, whereby to achieve selected relative angular positions of further indicia T, M, C representing the true, magnetic and compass courses which can be then read against the compass rose marked on the base. <IMAGE>

Description

SPECIFICATION A navigation aid The present invention relates to a navigation aid which will be of assistance to all persons concerned with navigating by use of a compass.

The device is also of particular usefulness in teaching various aspects of navigation.

Magnetic compasses are subject to two main errors, these being called variation and deviation.

Variation of a magnetic compass is the angle between the magnetic North Pole and the true North Pole, and this error is different at different points of the earth's surface and also changes gradually with time. The magnetic deviation, on the other hand, is an error particular to any one compass and is caused by the effect on the compass of magnetic material near to it.

Deviation varies with the heading of the vessel.

The errors can lead to displacement of a compass needle either to the East or to the West of true North, and may not necessarily both be in the same direction. When converting from a true course plotted on a chart, to a compass course in order to determine the heading of a vessel it is necessary to add any westerly errors due to variation and deviation, and to subtract any easterly errors due to these sources. However, when converting from a compass course to a true course it is necessary to subtract westerly errors and add easterly errors.Because both westerly errors and easterly errors are either added or subtracted to the compass course depending on whether the operation in hand is conversion from compass course to true course or vice versa, the problem, particularly for a navigator who does not have a wide experience, can be somewhat confusing and it is very easy to apply to correction in the wrong direction thereby multiplying any errors rather than reducing it.

The instrument of the present invention makes it possible for the two errors of variation and deviation to be set on scales carried by indicator members which can be so moved that either the true course, the magnetic course or the compass course can be read directly from one scale or another on the instrument without there being any possibility of compounding the errors by applying them in the wrong direction.

According to one aspect of the present invention an instrument for use in navigation comprises a base carrying indicia representing the graduations of a compass rose, and a plurality of laminar indicator members which are pivotally mounted on the base and in frictional engagement with one another such that the force required to cause relative movement between two adjacent members is greater than that required to cause relative movement between the base and the laminar indicator member adjacent to it so that the relative positions of the said indicator members remains unchanged when the assembly of indicator members is turned with respect to the base.

In this way it is possible to set the compass errors by turning one indicator member with respect to another, and then turning the whole assembly with respect to the base in order to select individual compass courses without it being necessary again to re-adjust the variation and deviation error settings.

Preferably the laminar indicator members are in the form of discs having opaque and transparent portions. In a preferred embodiment of the invention there are first, second and third indicator discs mounted on a common spindle attached to the base.

The first indicator disc is the one lying adjacent the base, and this disc has an arcuate scale extending symmetrically about a zero index adjacent the periphery thereof. The said first indicator disc is opaque except for a transparent arcuate region at or adjacent the periphery of the disc and diametrically opposite to the said arcuate scale.

The second disc is the one lying adjacent the said first disc on the side thereof remote from the base, and this disc carries an arcuate scale at or adjacent its periphery, which scale extends symmetrically about a zero index. The said second disc is also opaque except for two arcuate transparent zones at or adjacent the periphery of the disc and located diametrically opposite one another. The said arcuate scale of the second disc is peripherally located midway between the said two transparent arcuate zones.

The said third disc lies adjacent the second disc and is opaque except for a transparent peripheral zone at the same radial distance as the arcuate scale marked on the first and second indicator discs. The third indicator disc has two radial indicia on the transparent region in the form of lines printed or engraved thereon; a first of these represents the magnetic deviation and a second represents the compass course. The said two indicia on the third indicator disc are located perpendicularly with respect to one another.

The second indicator disc has two radial indicia in the form of printed or engraved marks which, respectively, represent magnetic variation and magnetic course. The indicia on the second indicator disc, representing magnetic variation and magnetic course, are located diametrically opposite one another.

The first indicator disc has a radial index mark representing the true course. This index is located diametrically opposite the zero mark of the peripheral arcuate scale on the first disc.

The second and third discs are provided with some means for turning them by engagement with a thumb or finger, such means being in the form of projections, recesses or surface texturing. The first disc has a greater diameter than the second disc and the second disc has a greater diameter than the third disc so that ready identification can be made between them and selected discs can be engaged manually or digitally in order to effect the required relative turning movements.

The frictional inter-engagement between adjacent indicator discs is obtained in the preferred embodiment by means of frictional inserts between the first and second discs and between the second and third discs. The frictional insert between the first and second disc is preferably secured to the second disc and, likewise, the frictional insert between the second and third disc is preferably secured to the third disc. The precise nature of the frictional inserts will, of course, depend on the material from which the discs themselves are made, but it has been found that when utilising plastics material for the discs, a felt ring is particularly suitable as a frictional insert between the second and third discs whilst an element of hard abrasive material can be used as a friction insert between the first and second discs.

The pivotal mounting of the indicator members to the base is preferably achieved by means of a pivot pin having an enlarged head engaging in a counterbore in the base and a shoulder which stand just proud of the surface of the base opposite the counter bore when the head of the pivot pin is fully engaged therein so that the assembly of indicator members is held slightly spaced from the base by contact with the said shoulder. To hold the stack of indicator members onto the shoulder of the pivot pin there may be provided a threaded end to the pivot pin opposite the said enlarged head end and a manually operable screw clamp engaged thereon.

In the preferred embodiment the base is square and the largest indicator disc has a radius the dimension of which lies between one half of the length of the side of the square base and one half of the length of the diagonal thereof.

One embodiment of the present invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a front view of the embodiment; and Figure 2 is a diametrical section through the embodiment shown in figure 1.

Referring now to the drawings, the embodiment shown comprises a base 11 of opaque plastics material bearing a compass rose comprising 360 graduations each representing one degree of arc in the conventional manner.

Mounted on the base 11 by means of a pivot pin 14 is an assembly of indicator discs comprising a first disc 16, a second disc 17, and a third disc 1 8.

Between the first disc 1 6 and the second disc 1 7 there is located a friction element 19, and between the second disc 1 7 and the third disc 1 8 there is located a friction element 20.

The pivot pin 14 has an enlarged head 12 received in a counterbored recess in the base 11.

The shaft of the pin 14 has a shoulder 13, which, when the head 12 of the pivot pin 14 is received in the recess of the base 11 stands just proud of the surface of the base 11 as can be seen in Figure 2. The stack of discs with their interposed friction elements is held together by a nut 1 5 screwed onto a threaded portion of the pivot pin 14.The shoulder 1 3 on the pivot pin 14 holds the first element 1 6 spaced from the base 11 and the nature of the materials, particularly of the friction discs 19, 20 is so chosen that the stack of three discs can be turned about the base 11 by applying the turning force to any one of the discs, the frictional contacts between the discs via the friction elements 1 9, 20, being greater than the friction between the pivot pin 14 or its head 12 and the base 11.

The first disc 1 6 is opaque apart from an arcuate section extending between the angular positions marked A and B in Figure 1 and carries a' radial index marked T at a point angularly midway between the points A and B.

Diametrically opposite the transparent arcuate section A-B of the first member 1 6 there is marked an arcuate scale 23 which extends over 30 degrees of arc to either side of a central zero index which is exactly diametrically opposite the index T marked on the peripheral transparent region A-B. The scale 23 is marked WEST and EAST respectively on either side of the zero index.

The second indicator disc 17 is opaque apart from two peripheral arcuate transparent regions, one extending in approximate coincidence with the transparent arcuate region A, B of the first disc 16, and being indicated by the angular end positions C and D in Figure 1 and the other located diametrically opposite the first, and extending between the angular positions marked E and F in Figure 1. The first transparent region C, D of the second disc 1 7 has a central radial index marked M and the transparent region E, F has a central radial index marked VARIATION, this index being intended to be read over the scale 23 on the first disc, whereas the indices T and M of the first and second disc respectively both indicate angular positions on the compass rose which can be seen through both overlapping transprent portions A, B, C, D.

Centred on a zero perpendicular to a diameter joining the radial indicia M and VARIATION is a scale 24 which also extends to 30 degrees on either side of the centre zero point. The two sides of the scale 24 are marked WEST and EAST respectively like the scale 23.

Finally, the third indicator disc 1 8 is opaque apart from a peripheral transparent border which extends entirely around the disc. This border has two radial indicia which are mutually perpendicular, a first marked C and a second marked DEVIATION. This latter is located over and intended to be read on the scale 24 on the second disc. The positions of the radial indicia T, M, C, VARIATION and DEVIATION are such that when the index VARIATION is set to the zero point on the scale 23 of the first disc and the index DEVIATION is set to the zero point of the scale 24 of the second disc, the indicia T, M and C are coincident on their respective discs.

As can be seen from the drawings, the first indicator disc 1 6 has a diameter slightly greater than the length of the side of the base 11, which is square, and the diameters of the second disc 1 7 and third disc 1 8 are each, respectively, slightly smaller than that of the preceding disc. The second disc 1 7 carries a small stud 22 adjacent its periphery opposite the zero mark on the scale 24 and the third disc 18 carries a stud 21 diametrically opposite the DEVIATION index.The friction pad 19 interposed between the first disc 1 6 and the second disc 1 7 generates a slightly greater frictional resistance to movement between these two discs than that generated by the friction pad 20 interposed between the second disc 17 and the third disc 1 8.The precise degree of frictional contact is determined by the tightening of the knurled nut 1 5 the dimensions of which are such that it can readily be manipulated by hand to adjust the friction as required.

In use of the device the largest of the indicator discs, that is the first disc 16, which overlaps the base 11, is held by the thumb of one hand against the base 11 of the instrument. The thumb of the other hand is then used to turn the second disc 1 7 (the third disc 1 8 being carried with this movement) until the radial index line marked VARIATION is aligned with the required variation on either the west or east sector of the scale 23 marked on the first disc 1 7. This required variation is determined by reference to tables or by computation.The deviation is then adjusted by turning the third disc 1 8 with respect to the second disc 17, the relative friction between the first and second disc, being greater than that between the second and third disc ensuring that the second disc 1 7 does not move during this operation. Again, the required setting of the deviation is determined by reference to tables or by computation. The indicia marked T, M and C now represent the true magnetic and compass courses with both the variation and deviation corrections automatically made. The stack of three indicator discs can then be turned to any position around the compass rose to set the course from a chart, or to determine the true heading from a magnetic compass reading.In the drawings the instrument has been illustrated in a setting for 10 degrees west VARIATION and 5 degrees west DEVIATION. The true course is shown as being set to 350 degrees so that, from the instrument, it can be determined that the magnetic course is 0000, while the indicated compass course is 005 degrees.The instrument can also be used in reverse for finding the deviation of the compass on any particular heading. To do this the index M is aligned with the magnetic heading and the index C is aligned with the compass reading. The deviation on that heading can then be read off directly on the DEVIATION scale.

By using the instrument described above corrections for variation and deviation are automatically applied in the correct sense without any possibility of multiplying the error by applying such corrections in the wrong sense as can easily be done without a thorough understanding and familiarity with navigational procedures.

Claims (25)

1. An instrument for use in navigation comprising a base carrying indicia representing the graduations of a compass rose and a plurality of laminar indicator members which are pivotally mounted on the base in frictional engagement with one another such that the forces required to cause relative movement between two adjacent members is greater than that required to cause relative movement between the base and the adjacent laminar indicator member so that the relative positions of the said indicator members remains unchanged when the assembly of indicator members is turned with respect to the base.

2. An instrument as claimed in Claim 1, in which the laminar indicator members are in the form of discs having opaque and transparent portions.

3. An instrument as claimed in Claim 1 or Claim 2, in which there are first, second and third indicator discs mounted on a common spindle attached to the base.

4. An instrument as claimed in Claim 3, in which the first indicator disc lies adjacent the base and has an arcuate scale extending symmetrically about a zero index adjacent the periphery thereof.

5. An instrument as claimed in Claim 4, in which the said first indicator disc is opaque except for a transparent arcuate region at or adjacent the periphery of the disc diametrically opposite the said arcuate scale.

6. An instrument as claimed in any of Claims 3, 4 or 5, in which the second disc lies adjacent the said first disc and carries an arcuate scale at or adjacent its periphery which scale extends symmetrically about a zero index.

7. An instrument as claimed in Claim 6, in which the said second disc is opaque except for two arcuate transparent zones at or adjacent the periphery of the disc and located diametrically opposite one another.

8. An instrument as claimed in Claim 6 and Claim 7, in which the said arcuate scale is peripherally located midway between the said two arcuate transparent zones.

9. An instrument as claimed in any of Claims 3 to 8, in which the said third disc is opaque except for a transparent peripheral zone at the same radial distance as the arcuate scales marked on the first and second indicator discs.

10. An instrument as claimed in Claim 9, in which the third indicator disc has two radial indicia on the transparent region, a first to represent the magnetic deviation and a second to represent the compass course.

1 An instrument as claimed in Claim 10, in which the said two indicia on the third indicator disc are located perpendicularly with respect to one another.

12. An instrument as claimed in any of claims 3 to 11, in which the second indicator disc has two radial indicia to represent magnetic variation and magnetic course.

13. An instrument as claimed in Claim 12, in which the indicia on the second indicator disc representing magnetic variations and magnetic course are located diametrically opposite one another.

14. An instrument as claimed in any of Claims 3 to 13, in which the said first indicator disc has a radial index to represent the true course.

1 5. An instrument as claimed in Claim 14, in which the index representing true course is located diametrically opposite the zero mark of the peripheral arcuate scale on the said first disc.

1 6. An instrument as claimed in any of Claims 3 to 15, in which the said second and third discs have projections, recesses or surface texturing to serve as a thumbgrip for manipulation of the discs.

17. An instrument as claimed in any of Claims 3 to 1 6, in which the said first disc has a greater diameter than the said second disc and the second disc has a greater diameter than the said third disc.

18. An instrument as claimed in any preceding Claims, in which the frictional inter engagement between adjacent indicator discs is obtained by means of inserts between the said second and third disc.

19. An instrument as claimed in Claim 18, in which the frictional insert between the first and second disc is secured to the second disc and the frictional insert between the second and third disc is secured to the third disc.

20. An instrument as claimed in Claim 1 8 or Claim 19, in which the insert between the second and third discs is a felt ring.

21. An instrument as claimed in any of claims 18 to 20, in which the friction insert between the first and second disc is an element of hard abrasive material.

22. An instrument as claimed in any preceding claim, in which the indicator members are pivoted to the base by a pivot pin having an enlarged head engaging in a counterbore in the base and a shoulder which stands just proud of the surface of the base opposite the counterbore when the head of the pivot pin is fully engaged therein, so that the assembly of indicator members is held slightly spaced from the base by contact with said shoulder

23. An instrument as claimed in claim 22, in which the pivot pin has a threaded end opposite the said enlarged head and a manually operable screw clamp is engaged thereon to hold the stack of indicator members to the shoulder of the pivot pin.

24. An instrument as claimed in any preceding Claim in which the base is square and the largest indicator disc has a radius lying between one half of the length of the side of the square base and one half of the length of a diagonal thereof.

25. An instrument for use in navigation substantially as hereinbefore described with reference to the accompanying drawings.