« AnteriorContinuar »
THE COMPASS ADJUSTMENTS If a small steel bar, the half of a knitting-needle, were truly balanced and then suspended by a thread or a fibre without twist, it would come to rest horizontally, pointing in no particular direction.
If the same bar were magnetised and suspended as before, it would, if the experiment were tried in London, take up a position in the magnetic meridian with its N. end dipping at an angle of 67° below the horizon; the line through the axis of the needle marks out the line of dip.
A freely-suspended magnet will always try to point directly at the nearest pole, ignoring the curvature of the earth. The dip varies from o degrees at the magnetic equator to 90 degrees at the magnetic poles. In London at the present time (1915) the dip is 67 degrees.
It will be evident that, except on the magnetic equator, there must be a loss of horizontal directive force, which is the only part of use to the navigator. This is shown as follows
A magnet freely suspended at London would come to rest in a position
H indicated by the line NS, which dips below the horizon H 0, equal to the angle N S N=67 degrees.
SN equals the total force 1:0.
A compass is composed of several small magnets attached to a circular
Fig. 1. card, which is mounted on a pivot in the centre of a copper bowl. The plane of the card is below the point of suspension, to prevent dipping and to place the card in a position of stable equilibrium; the horizontal gravitational force of the card and magnets is greater than the dipping force of the magnets, compelling the card to remain horizontal and not dip. The card is constructed and mounted so as to utilise the earth’s horizontal force, which force compels the compass needle to become parallel to the magnetic meridian in any part of the world if under the earth's magnetic influence only.
All magnets have two poles with a neutral zone between.
Cracks or flaws will cause sets of poles called “ consequent poles.” For convenience in discussion the north-seeking end is called the Red end, while the south-seeking end of a magnet is called the Blue end.
(1) Poles or ends of the same name repel one another. (2) Poles of opposite names attract one another.
(3) The attraction between two magnetic points varies inversely as the square of the distance between them.
(4) The attraction between two magnets varies as the cube of the distance between them.
(5) The greatest power to deflect occurs when one magnet is at right angles to the other; the least when they are parallel to each other.
The harder a metal is the more difficult it is to magnetise, but the magnetism is retained with equal tenacity.
The softer a metal is the more easily it is magnetised, but it parts with its magnetism with equal ease once the magnetising influence is altered in position or removed.
The earth is a magnet and is governed by the foregoing laws affecting magnets. There are two poles and a neutral zone called a magnetic equator. The north magnetic pole is in lat. 70° N., long 96° 46' W.; the south pole is in lat. 731° south, and long. 155° (about) E. There are also consequent poles or secondary poles of little power.
About midway between the poles an uneven line traces the magnetic equator. It is not an exact circle. Because the true and magnetic poles are not coincident the true and magnetic meridians cut one another; the angle between them is called the variation. The variation is affected by the before-mentioned secondary poles.
The magnetic poles are not stationary. In the course of many years the magnetic poles alter their positions and bring about an increase or a decrease in the variation. In 1657 there was no variation at London ; by 1815 it had become 24° 27' W.; at the present time (1915) it has again decreased to less than 15° W. and is still decreasing about 6' annually. The dip is also decreasing in a lesser degree.
Because the north-seeking end of a freely suspended magnet is called the red end, the north magnetic pole of the earth must be called the blue end, in obedience to the law “opposite names attract”; and for the same reason the south magnetic pole is called the red pole, as it attracts the south end of the magnet.
When discussing the magnetism of an iron or steel vessel certain technical terms are used; it has been suggested that these terms “could with advantage, be simplified.”
The terms in use and a proposed simplification are placed in parallel columns thus
Explanution of terms on both sides Hard iron.-Permanent or sub-permanent magnetism is supposed to exist in the hard, but more correctly speaking hammered, iron. The hammering seems to capture and enclose the magnetism in the metal worked upon.
Soft iron.—Transient induced magnetism is supposed to exist in soft, or more correctly speaking unhammered, iron. It is non-permanent because such iron is magnetised by the earth without exerted force or concussion. It reaches its maximum disturbing power as the particular piece of iron under discussion becomes parallel with the meridian or with the line of dip, and is at a minimum as it becomes at right angles to the meridian or the line of dip.
All iron and steel structures are magnetised by the earth.
An iron or steel vessel in the course of construction becomes a mass of magnets which disturb the compass in many ways. If the vessel were steered by a gyro compass instead of a magnet there would be no use for this chapter to be written.
The line of dip at any place is the direction a freely suspended magnetised needle will assume.
In the Northern Hemisphere it will point to the north point of the horizon, and downwards to the north magnetic pole of the earth.
The line of dip represents the total force. See Fig. I
A vessel as a whole becomes magnetised as though she were a solid block of iron, the position of the poles depending upon the angle at which the keel cuts the line of dip.
If through the centre of a vessel, while building, the line of dip is drawn from deck to keel, and this line is bisected by a plane at right angles to it, then, in the Northern Hemisphere, the lower side, that is, the side towards the north pole, will be coloured red, and the upper side will be coloured blue. This rule applies to all the iron in the vessel while on the stocks.
Let P. M. represent permanent magnetism, and let N. P. M. represent non-permanent magnetism. It will be found that N. P. M. in horizontal iron varies in two ways: viz., horizontally and vertically. In horizontal iron the N. P. M. reaches its maximum when it is parallel with the magnetic meridian in any magnetic latitude ; this is equally true whether the iron is fore and aft, athwartships, or at any angle between; and it is least when at right angles to the meridian. This applies to both hemispheres, that is, all over the world. Strictly speaking, soft iron is never really free from magnetism, as the earth's magnetic force is always acting on it.
In vertical iron, by which is meant iron not horizontal, the upper ends containing N. P. M. are always coloured the same as the nearest magnetic pole, blue in the Northern Hemisphere, and red in the Southern.
The greatest intensity is reached when the vertical iron is parallel with the line of dip, and least when at right angles to it.
The permanent magnetism retains the same colour always and everywhere. Iron that is magnetised non-permanently varies in intensity and in colour, both depending upon its" present” position with reference to the line of dip or magnetic meridian.
This may be represented as follows
In Fig 2 let W X Y Z represent a rectangular block of hard steel in a fixed position in London. If it is subjected to severe hammering all over it will become permanently magnetised, the colours appearing as at R and B. If the ends are reversed so that the end that was north is placed south no change in the magnetism will take place (see Fig. 2a).
But if a block of soft iron of the same size and shape which had not been hammered lay in the same position as the block of steel the colour towards the north would always be the same, that is, red, no matter how it is turned about.
Herein lies the great difference between a permanently magnetised bar and a nonpermanently magnetised bar (see Figs. 3 and 3a). If for a block of iron or steel we substituted a vessel building on the stocks, the iron or steel in her construction will be magnetised as the block of steel and the
Fig. 2 block of iron are magnetised.
This may be further illustrated. Consider a point at the exact middle of the vessel on the stocks at London. Let the eye follow along the line of dip, which will be north magnetic, and forming an angle of 67° with the horizon ; this line will indicate the direction of the north magnetic pole. Imagine a powerful bull's-eye lamp
Fig. 2a. throwing rays of light (red) upward along the line of dip from the direction of the pole towards the observer ; half the vessel would be illuminated red, the other half would be in the shade (blue). These colours would be divided by a line through the centre of the vessel at right angles to the line of dip. The points of greatest intensity (the poles of the vessel) would lie upon the line of dip, the red pole at a point nearest the magnetic pole, and the blue pole at a point equidistant in the opposite direction. As the vessel was being constructed so would the colours red and blue be developed and permanently fixed in all riveted parts as shown in the block of hard steel (Figs. 2 and 2a), but in the unhammered steel or iron there would be no permanency; the part facing towards the north magnetic pole would always remain the same colour, red, and the other half always blue, as shown in
Fig. 3a. the block of soft iron (Figs. 3 and 3a).
Figs. 4 and 4a show two vessels built at London, one N., parallel with the meridian, the other E., at right angles to the meridian.
The student should draw figures showing vessels built in places of various degrees of dip and at different azimuths.
In a structure like a vessel the iron and steel of which she is built lies in every conceivable direction. She is a great magnet made up of many lesser ones, yet in the midst of these magnetic disturbing forces a compass must be placed which is to point north accurately at all times and in all places.
A "best possible” place should be found somewhere along the middle fore and aft line of a vessel. It should be as far as possible from any vertical iron, especially movable iron, such as ventilators, davits, derrick heads, bulkheads, and the upper parts of iron deck houses which cause disturbances very difficult to handle.
A dynamo should be at least 50 feet away, and all lights should be double wired if passing near a compass, and clipped together in order to neutralise their effect on the compass needle. The selected place should be tested with the vibrating needle for horizontal force and with the dipping needle for vertical force.
The horizontal vibrating needle is a small, strongly magnetised needle. It is flat, pointed at both ends, 3 inches long and half an inch broad, fitted with a cap like the compass card, and works on a pivot of its own for land observations. On board the same pivot must be used when the compass Card and pivot have been removed from the bowl.
The vibrations are observed as follows
With a spare magnet give the horizontal vibrating needle a good deflection, then when the needle is swinging through an arc of about 40 degrees count the vibrations, noting the instant when the north-seeking end has reached the extreme deflection to the right, subsequently noting the instant of every tenth vibration until the needle is nearly at rest.
The proportion of the earth’s horizontal force between any two places is known to be inversely as the square of the number of seconds occupied by the same number of vibrations at each place. If therefore the time of making 10 vibrations on shore is found to be 20 seconds and the time of 10 vibrations on board at the place of the compass is 26 seconds, then the horizontal force at the compass is
or 0:59, the horizontal force on shore being
676 represented by I; or if the vibrations on board had taken 16 seconds instead of 26 the horizontal force on board would be
or 1.5. In 16