Besides this slow change of declination, taking centuries to effect in any appreciable degree, there is a daily variation or change, which, with a needle delicately adjusted for the purpose, can be detected, in which it appears to follow the sun in his course in the heavens. It has been thought that a slight magnetic influence in the needle can also be traced to the position of the moon; but no exact deductions have yet been drawn on this point. There are also sudden and unmistakable disturbances of the needle co-incident with brilliant displays of the aurora borealis, and with volcanic eruptions; to these the name of magnetic storms is given when they are very violent.

A

977. Dip of the Needle.—In the mariner's compass the needle is so balanced on its pivot as to be horizontal in all parts of the earth; but if the needle is free to turn also on a horizontal axis, it will be horizontal only near the equator, but at other places it will dip down, pointing to the nearest magnetic pole, as here shown at B, D, and E. A needle so arranged is called a dipping needle.

Fig. 280.

N

The degree of dip or inclination, as it is termed, varies for different latitudes; and is also subject to secular, diurnal, and temporary changes, as well as the declination. Like the declination, the inclination is decreasing at the present time in this country, being about 68° near London, whereas in 1723 it was very nearly 75°.

The dip of the needle was first discovered by Robert Norman in 1576; in adjusting a compass-needle he found that, having accurately balanced a needle before magnetization, it would not balance after being magnetized, but required a counterpoise at the south end.

978. Magnetic Charts.-The amount both of inclination and of declination varies with different spots on the earth's surface; and from an immense number of observations made by scientific travellers and nautical men, such as Humboldt, Ross, Parry, and Scoresby, magnetic charts have been drawn up, in which the places of equal

variation gradually fell to 23° in 1842, and at the present date, thirty-four years later, it has fallen to 19° W. It occupied 153 years in reaching its maximum westerly variation, and is now apparently on its way back to the north.

Electro-magnetism.

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declination, or of equal inclination, are marked by lines drawn through them. The chart line passing through all places of no declination is called the agonic line; and, roughly speaking, we may say that lines of equal declination form parallels to this line; but in no case are the lines very regular. Similarly, the chart line passing through all places of no dip or inclination, which may be called the magnetic equator, is called the aclinic line; and lines cí cqual declination, called isogonic lines are, roughly speaking, paral. lels to this. It is worthy of remark that in the case of iron ships, or iron-plated ships, now so common, a large degree of magnetism is often produced by the rivetting and hammering incidental to their construction. This is owing to the inductive action of the earth, and varies, therefore, with the position in which the ship has been built. If it has been built lying north and south, the magnetism induced will be very strong, and will have a very decided influence on the direction of the ship's compasses whenever the ship is sailing out of this line.* Before undertaking voyages with such ships, great care must be used to ascertain exactly the effect of this permanent magnetism imparted to the vessel; and cases have occurred where inattention to the disturbing magnetic effect of an iron cargo, shipped after an inspection of the ship's compasses, has had the most disastrous consequences.

ELECTRO-MAGNETISM.

979. The development of magnetism in soft iron, by a galvanic current circling round it in a coil, is but one of the manifold phenomena which go to make up the distinct branch of electrical science known as electro-magnetism. Currents, in virtue of their magnetic effect, attract and repel currents: they attract and repel the poles of a magnetic needle; and they are themselves also attracted and repelled by magnets. Second only to the original discovery of Galvani and Volta, was the discovery made by Professor Oersted, of Copenhagen, in the year 1819, when he found that the natural direction of a magnetic needle was instantly changed by its being near a voltaic battery in action. Such a needle happened to stand on a table where the wire of a battery lay parallel to it. It had its

* The great iron ship Northumberland was built in a north and south direction, and it was found when completed, after many months, that the vessel had acquired magnetic polarity on a large scale at the stem and stern.

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Electro-magnetism.

usual direction of north and south, when the battery was not acting, but the moment the current was allowed to pass, the needle was thrown or deflected into a position across the wire, and so remained as long as the current continued. On the current being stopped, as by unclosing or breaking the voltaic circuit, the needle immediately resumed its natural direction. Pursuing the investigation, Oersted found that the movements of the needle were produced as often and as quickly as the acts of closing and breaking the circuit could be repeated. He further ascertained that, near the wire, the changes took place as certainly and rapidly at any distance from the battery, as near to it; a simple fact within which lay concealed the coming prodigy of the electric telegraph, as will be explained some pages hence.

On closer examination we find that the relations between the simple magnetic needle and the voltaic current are not quite so simple as might appear from this single statement of Oersted's experiment. Fig. 281 will serve to make this connection more clear.

If the line, A H F, represent part of the conducting wire-circuit of a voltaic battery, running from south to north, supported by the standards, H and, F and if two magnetic needles, NS, N' S', movable on pivots, are placed near it, one needle being below the wire, the other above it; then the moment that an electric current or wave is allowed to flow along from H to F, as marked by the arrow, both

needles are simultaneously turned aside; but the one below turns its north pole N to the west, as shown by the dotted line, ʼn s, and the other above turns its north pole, N', to the east, as shown by the line, n's. If the wire be bent at F, so that the current flows back below in the direction, G D, the effect of the current in G D, on a needle, N" S", above it, will be to turn it to the same hand as the upper current in F H turns the "cedle, N S, below it. These apparently

contradictory facts are all united under the following simple rule known as Ampere's rule; viz.,

To a person supposed to be swimming along with the current and with his face to the magnetic needle, the north pole of the needle will appear to turn to his left.

980. A remarkable consequence arises from these facts, namely, that if the wire of the voltaic current, A E (fig. 282), over the needle, S N, be bent down at E, and carried back to G, at a short distance below the needle, instead of counteracting the influence of the upper part of the wire, A E, as might be expected from its electric current

being in the contrary direction to that in A E, it doubles the effect. The part of the rotating or wheel current about the upper wire, which acts on the north pole, N, to drive it westward, coincides with that of the upper part of the current about the lower wire, also moving westward, so that the two coincide and assist each other.

From all this it follows that if the wire at G is bent a second time, and carried back above the needle towards E, as shown here, and from thence again is bent round at F, the deflecting force is quad. rupled, and so on, if the turns are farther multiplied. By this device of the coiling the wire around the needle, a very feeble voltaic current at the source is rendered so strong to deflect the needle, as to cause a distinct rattle or clink of it on an ivory pin, placed to limit its motion. And it explains why a voltaic battery in London of moderate strength, having a conducting wire of hundreds of miles in length, suffices to move magnetic needles suspended within such coils of wire, at the most distant as well as at various intermediate stations.

GALVANOMETERS.

981. The Astatic Galvanometer.-One of the first applications of Oersted's discovery was to the construction of a galvanometer

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T

Reflecting Galvanometer.

or current measurer, and one of the most delicate forms of these is what is known as the Astatic-needle Galvanometer. Fig. 283 represents a section of the instrument. It simply consists of an astatic pair of needles, N' S', S N, stiffly connected by twisting some fine copper or brass wire round the needles, and suspended by a fine silk thread—a fibre of unspun silk is preferable-so that the upper needle is outside the coil and visible while the other is inside the coil. The upper needle moves over a graduated circle of paper or pasteboard, so that the angle or degree of deviation of the magnetic needle from its natural position, which is made the zero of the graduation, can be exhibited. The ends of the coil wire are connected with the two binding screws, B, B', into which are inserted the wires from the galvanic cell or battery whose strength it is desired to test.

BA

Fig. 283.

Such an instrument is exceedingly delicate, when the number of turns of wire is very great, and of course different degrees of sensibility in such instruments are desirable according to the intensity and nature of the electric current to be measured or detected, a delicate instrument being useless when the current is at all likely to be strong.

982. The Reflecting Galvanometer of Sir William Thomson is adapted for the detection of still more minute currents than the one we have just described. The exceedingly feeble current which sur

Fig. 284.

vives the passage of the two thousand miles of cable between this country and America, would be quite insensible to any ordinary needle-galvanometer; and, but for the happy execution of the idea