CHAPTER XXVIII.

MAGNETISM AND DEVIATION OF THE COMPASS.

ART. 119.-For a full and scientific discussion of the deviation of the compasses in iron ships, the student is referred to the "Admiralty Manual," or to the " Elementary Manual," edited and revised by Captain Creak, R.N. For general students Merrifield's "Magnetism and Deviation of the Compass" is recommended.

The purpose of the following remarks is to enable students to clearly understand the principles involved in the Board of Trade Syllabus of Questions dealing with the practical application of the laws of magnetism to the case of iron ships and their compasses.

Definition.-Magnetism is a property which may be acquired by masses of iron and steel, by virtue of which they attract or repel each other under certain circumstances.

Kinds of Magnets.-There are three kinds of magnets, viz. natural, artificial, and electro-magnets. The natural magnets are the Earth and pieces of magnetic iron ore or "lodestone." Artificial magnets are bars or "needles" of hard-tempered steel, which have been magnetized by rubbing contact with other magnets, or by the influence of an electric current. Electromagnets are elongated or horseshoe-shaped masses of soft iron wound round with copper wire, through which an electric current is passing. The strongest magnets are made in this way; but the magnetic properties disappear when the current

ceases.

Degrees of Magnetism.-Magnetism may exist in three different states or degrees in steel or iron, viz. PERMANENT, SUB-PERMANENT, and TRANSIENT INDUCED.

A bar of hard-tempered steel receives magnetism slowly, but when once magnetized it retains its magnetic strength for an indefinite length of time, and is said to have permanent magnetism.

Iron or steel of a less degree of hardness is more easily magnetized, but is liable to lose a considerable portion of its magnetism with lapse of time, and is therefore said to have sub-permanent magnetism.

Soft or malleable iron becomes instantly magnetized by induction from any magnetic source, but loses it when the

inducing cause is removed. This temporary magnetic condition is known as TRANSIENT INDUCED magnetism.

Saturation. There is a limit to the magnetic strength which

a piece of steel or iron can acquire, and when this limit is reached it is said to be saturated.

Magnetic Poles. Every magnet has two POLES, or points

where the greatest energy is concentrated, distinguished as RED and BLUE poles, because it is common to paint artificial magnets in these colours. The red pole of a permanent magnet is the one which would point north when the magnet is suspended at its centre and free to turn round. This end is also called the N. or marked end. Therefore the end of a compass-needle which points N. is the red pole.

ART. 120. First Law of Magnetism.-It is a well-known law of magnetism that like poles repel each other, and unlike poles attract each other. This can be proved on a compass-needle, for if the red pole of a magnet be brought near to the N. or marked end, it will repel it, whilst the blue pole will attract it. Similarly, the S. end will be repelled by the blue pole, and attracted by the red. It follows from this experiment that the Earth's NORTH magnetic pole is BLUE, because the red pole of the needle is attracted towards it. The needle, if floating on water and free to move, would not, however, travel towards the N. pole, because its length is as nothing compared with the distance from the Earth's pole, its blue end being as strongly repelled as the red end is attracted. It is the same with the action of the south magnetic pole. The only result is, therefore, to give the needle a north and south direction.

If a needle hundreds of miles in length could be supposed to be afloat in the North Atlantic, with its red pole towards north, no doubt it would travel bodily towards the Ñ. pole of the Earth, because its red pole would then be attracted more strongly than the blue would be repelled.

Second Law. It is another law of magnetism that the effect of a magnetic force (such as the pole of a magnet or an induced pole in soft iron) on a small magnet varies nearly in the inverse ratio of the cube of the distance; that is, at double the distance it is one-eighth (2 x 2 x 2 = 8); at treble the distance it is only one twenty-seventh (3 x 3 x 3 = 27), etc. Also

Third Law. A magnet placed end on to a pivoted needle causes twice the deflection that it would do if broadside on at the same distance between the centres of the magnet and needle.

These facts have a very important bearing on the effect of the magnetic forces in a ship on her compasses, and show the advantage of placing the compasses as far as practicable from masses of iron having magnetic properties, especially when end on.

ART. 121. Transient Induced Magnetism. When either pole of a magnet is brought near to one end of an iron rod, it develops two magnetic poles in the iron, the near end being of the opposite name to that of the pole applied. The process may be repeated through a series of rods end on, so that the N. point of a compass-needle would be deflected by the last one in the same manner as by the magnet itself.

The same fact may be illustrated by the familiar experiment of causing a number of soft iron bars or nails to hang on to the N Magnet S N

SN

FIG. 181.

SN

end of a strong magnet; but whether in contact or not, the effect is the same.

The above are examples of magnetic induction and the acquiring of transient induced magnetism.

Lines of Force.-The mysterious action of one magnet on

another at a distance, and the magnetization of iron with or without contact, are explained by the existence of lines of force, which issue from the red pole and return to the blue pole. These lines of force pass without interruption through all intervening non-magnetic substances, and it is the passing of lines of force into iron which gives it magnetic properties. Their existence and direction can be shown by experiment (Fig. 182). If a sheet of white paper be laid on a magnet and iron filings sprinkled on the paper, the filings will arrange themselves in lines corresponding to the direction of the lines of force from one pole of the magnet to the other.

FIG. 183.

FIG. 182.

If a suspended magnetic needle were enclosed in a thick iron shell, the lines of force would follow round through the iron by preference to reach the blue pole, and the consequence would be that the needle would lose its directive power. This is the reason why compass-bowls are made of copper, and not of iron. Another reason

is that poles would be induced in an iron bowl, which would oppose the Earth's directive action on the needle. Therefore a ship's compass which is closely surrounded by iron would have a weakened directive power.

The Earth's Magnetism.-The Earth itself acts as a magnet, having a blue pole towards the north and a red pole towards the south. Its action on a compass needle is merely to give a directive tendency, as before explained, but it is also a powerful inducing source. No iron can escape from its influence. Hence fireirons, knife-blades, needles, etc., are nearly always magnets with two fixed poles, as may be tested on a compass-needle,

whilst bars or rods of soft iron have also induced poles from the same cause, which are not, however, fixed. A fairly strong magnet can be made by holding a bar of hardened steel in a N.-and-S. direction and tapping it with a hammer; the_vibration quickens the process of magnetization by the Earth's induction.

A Ship's Induced Magnetism.-Even a ship becomes a huge magnet from the Earth's induction in the process of building, whilst the separate parts, such as beams, stanchions, davits, rudder and stern-posts, etc., become so many temporary magnets, liable to act separately and collectively in disturbing the compasses.

ART. 122. Variation. The magnetic poles of the Earth do not coincide with the true geographical poles, the N. or blue pole being in about lat. 70° N., long. 90° W., and the S. or red pole was found by Lieut. Shackleton to be in lat. 72° S., long. 154° E. (from which it is seen they are not exactly opposite). This is the cause of the variation of the compass, which is the difference in direction between true N. and magnetic N. The amount of variation is stated on ordinary charts, and for the entire globe on special variation charts. The variation is not constant at any place, but changes at a known rate (in the United Kingdom now about 8' or 9' annually).

Previous to the year 1657 the variation was E. in this country. In that year there was no variation at London, the compass pointing true N. Then it became westerly, and attained its maximum W. variation in 1816. Since then it has constantly decreased, and will again become 0 in 1977; then, becoming E. again, it will reach its maximum in the year 2290. The explanation of this progressive change of

Mer of Greenwiek

Greenwich

variation is that the N. magnetic pole describes a complete circle round the true N. pole in a period of about 640 years. Twice in the course of this period the compass would point true N. at Greenwich, i.e. when the magnetic pole is on the meridian of Greenwich, and on the opposite meridian (Fig. 184). The annual change would evidently be greatest when the magnetic pole is at or near to the points 0 and 01 in the figure, and slowest when near the points W and E, when the direction of the needle is a tangent to the small circle. At present the magnetic poles are coming towards the meridian of Greenwich. The same remarks apply to other places. A line in the direction magnetic N. and S. at any place is called the magnetic meridian; hence variation may be defined as the

FIG. 184.