Fig. 263.—Lines of magnetic force.
Fig. 264.—Magnetization.
The manner of magnetization has already been mentioned, but here we will give further illustrations of the method of magnetization. Four magnets are used, two being placed with their opposite poles apart, and upon them is placed the bar of which a magnet is to be made. Two other magnets separated by a piece of wood are then brought near, and subsequently drawn from the centre to the ends of the bar. This is the separated touch system; the double touch of Mitchell is completed by moving the upper two magnets from end to end backwards and forwards, and finally lifting them away from the centre.
A magnet, then, is a bar of steel endowed with certain properties, such as attracting iron, etc.; and electro-magnetism is the term applied to the production of magnetism by means of electricity, the medium being the electro-magnet. To understand the science it will be necessary to mention Ampère’s theory of magnetism.
It was Œrsted who observed that when a magnet is placed within reach of an electric current and free to move, it sets itself at right angles to the direction of the current; and Ampère defined the law already referred to when treating of the electric current,—viz., “that if a person be imagined as placed in the wire so that the current shall pass through him from feet to head, if he turn his face toward the magnetic needle the north pole will always be deflected to his left-hand side.” When the current is passed above the needle from south to north poles, the deflection is to the west; when from north to south the deflection is east. When the current is below the needle the contrary is the case. Ampère decided that currents circulating in the same direction attracted each other, and when running in opposite ways they repelled each other. He supposed currents to circulate within all magnetic substances, and then—that is, when the body is magnetized—these currents flow in planes parallel to each other, and the material which offers the least resistance to the circulation of these currents becomes the most magnetic.
The earth being supposed to be an immense magnet has currents circulating through it in a direction from east to west; and having the property or power of turning a magnetized bar in a direction similar to that in which the bar would be turned by a magnet, the earth is considered a magnetic mass. This influence is due to what is called “terrestrial magnetism.” If we suspend a bar by a thread it will point in no particular direction, but may be turned towards any side we please. But when once the needle is magnetized it will point north and south; or, as we say (but not correctly), the north pole of the magnet points to the north of the globe. It is really the south pole that points to the north, and the north pole of the magnet points south, as can be proved by suspending the bar over another magnetized bar. So if the earth be considered a magnet our English terms are inconsistent with our theories. Continental writers are more correct.
The line of the magnetic needle’s direction, which differs in different places, is called the magnetic meridian, and the amount of its divergence from the astronomical meridian is termed its declination or variation. When the amount of this variation is known it is allowed for, and the needle can be considered as pointing due north and south.
But the needle does not assume a position perfectly parallel to the horizon. It dips down in different hemispheres. As we approach the north pole the dip or inclination will become greater, and the same effect is observable at the south pole. Again, there are certain places on the earth where the attraction is so evenly balanced that the needle is perfectly horizontal. The line uniting these places is the magnetic equator. This does not coincide with the earth’s equator any more than the magnetic poles coincide with the geographical poles of the earth.
The declination of the needle varies from the meridian of Greenwich at different times. If we travel to the west the variation increases westerly, and is greatest in the Atlantic. It then decreases; the needle points due north in North America. Going still forward the variation becomes easterly, increases, and decreases to nothing in Eastern Russia. Thence the variations are westerly. Columbus discovered the variation of the compass needle in September 1493. In places where the needle is due north and south the lines drawn through them are termed lines of no variation.