Picture - Cosmic Polymath

THE COMPASS 113
of gelatine and placing the poles of the horse-shoe magnet in the solution. When the gelatine cools and hardens
a transparent lines of force model is produced.
§26. The Electro- Magnet.
This particular form of magnet consists of a bar of soft iron (frequently bent horse-shoe fashion), placed within
an extensive coil of insulated copper wire through which an electric current is passing. The combination confers
greatly increased strength on the magnet ; the powers being augmented in proportion to the length of wire in the
coil. The electro-magnet is especially useful where interrupted or alternating currents are a desideratum, as it affords
facilities for rapidly inaugurating and breaking the electric circuit. It is employed in the telephone, the telegraph,
in wireless telegraphy, and in the arts generally.
§ 27. Magnets with One Degree of Freedom.
Hitherto the magnets producing the hues of force pictures have been fixed. It is now necessary to consider
cases where the axis joining the poles of the magnet is free to rotate in a horizontal plane about its middle point,
as in the magnetic needle, where one degree of freedom is permitted. Naturally, the axis of a magnet can occupy
an infinitely great number of positions.
In the case of the horse-shoe magnet " one degree of freedom " is given by suspending the magnet by a thread
(previously untwisted) attached to the centre of its curved portion or bend. In the bar magnet (say the magnetic
needle) the same result is obtained by suspending it at its middle portion by a single untAvisted fibre of silk, or setting
it centrally on an agate point, which enables it to swing freely in a horizontal plane. The bar-magnet so poised, and
free to move in a horizontal plane, points, when left to itself, north and south with its poles. When a magnetic needle
is deflected or disturbed it makes smaller and smaller vibrations until it attains its position of rest ; the one
pole pointing to the north, the other to the south. The end of the needle rubbed with the north pole of an
existing magnet becomes a south pole and vice versa.
" The hues of force have, at every point, a determinate direction. The north pole (N) of a fixed magnet always
repels the north pole (n) of a movable magnet along the hues of force proceeding from N. The south pole (S) of a
fixed magnet always attracts the north pole (w) of a movable magnet along the Unes of force proceeding towards S."
Contrariety of direction is conveniently indicated by a plus sign ( + ) for the positive direction, and a minus
sign ( — ) for the opposite or negative direction.
If we inhabit the northern hemisphere of the earth it will be necessary to reckon positively the force exerted
upon a north pole placed in the magnetic field. " In all magnets the hues of force issue from the north pole and
pass through the field to the south pole, where they re-enter the magnet."
The so-called compass -needle is free to move in a horizontal plane, and also to dip vertically at either end. The
horizontal or decUnation movements indicate the position with regard to the poles of the earth ; the dip, the
position with regard to the equator or central region of the earth. The magnetic needle is the essential portion of
the mariner's compass, and enables the mariner to ascertain his position on the earth's surface.
" Generally speaking, the further we go north or south from the equator the more markedly does the magnetic
needle point downwards, the dip increasing with the latitude. In the northern hemisphere it is the north-seeking
pole that points downwards ; in the southern hemisphere it is the south-seeking pole. In the neighbourhood of
the equator the earth is encircled by a line, at each point of which the needle sets itself horizontally,- so that the
angle of dip is zero. This line is called the ' magnetic equator,' and is not coincident with the geographical
equator. There are also points, one in the northern and one in the southern hemisphere, where the direction of
the dipping needle is exactly vertical. From the analogy to artificial magnets, these are called the ' magnetic poles
of the earth.' They do not coincide with the geographical poles. The north magnetic pole is in 70.5° north
latitude and 98.5° west longitude from Greenwich, a point in the North American Archipelago ; the south mag-
netic pole is about 74° south latitude and 148° east longitude. The hne joining these poles does not pass exactly
through the centre of the earth. . . . The
magnetic equator is continuously shifting from east to west, and with it
the whole system of isoclinic lines, the angle of dip throughout Europe gradually decreasing. The total magnetic
force or intensity at any given place is also changing; at some places it is becoming greater, at others it is
becoming less."
§28. The Compass.
" If the declination is known for any given place, we may conversely use the magnetic needle to determine
the direction of the geographical meridian, and hence the ' four points of the compass.' A form of dechnation-
needle convenient for this purpose is the §o-called compass-needle, The needle moves over a circle divided into
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