Part 5 (2/2)

_77. Small Horseshoe Magnets_ may be made from needles or from other pieces of steel used for bar magnets. They should be annealed (App. 21) at their centers at least, so that you can bend them into the desired shape. In the case of bright needles, like harness-needles, the part annealed will become blackened. If you heat the center only, and the ends remain bright for about 1/2 inch, you will not need to harden the needle again. It is an advantage to have the center of the magnet a little soft, as it is not then liable to break. The ends alone may be hardened by holding the bent portion away from the candle or gas flame, while heating the ends. The bent steel should be magnetized by drawing its ends across the poles of a horseshoe magnet.

APPARATUS 59.

_78. Flexible Horseshoe Magnets_ may be made of thin spring steel. The distance between the poles can be regulated at will by bending the steel more or less. The poles may be held at any desired distance apart by thread or wire, which should be wound around the legs of the magnet a little above the poles. This will keep the steel from straightening out.

APPARATUS 60.

_79. Horseshoe Magnet._ Fig. 36 and 37. Magnetize two harness-needles, and stick them into a cork so that the poles shall be arranged as shown.

The distance between the poles can be regulated to suit. This forms a very simple and efficient magnet, with the advantages of a real horseshoe magnet.

[Ill.u.s.tration: Fig. 36.]

[Ill.u.s.tration: Fig. 37.]

APPARATUS 61.

_80. Armatures._ All home-made magnets should be provided with armatures, or keepers. These are made of soft iron on the regular magnets, and tend to keep the magnet strong. (See text-book.) For the bar magnets described, a piece of sheet-tin, upon which to lay them, is all that is needed for an armature. The lines of force will pa.s.s through this. For the horseshoe magnets described, strips of tin, soft iron wires, or even a wire nail placed across the poles will greatly aid in keeping in the strength. The little magnets should not be dropped or jarred. (Study the theory of magnetism in text-book.)

CHAPTER VII.

MAGNETIC NEEDLES AND COMPa.s.sES.

_81. Magnetic Needles and Compa.s.ses_ consist chiefly of a short bar-magnet. When used to tell the directions, north, east, etc., the apparatus is generally called a compa.s.s. When we speak of the ”needle,”

we really mean the compa.s.s-needle. The little magnet may be almost any piece of magnetized steel, provided it is arranged so that it can easily swing around. There are several ways of supporting the compa.s.s-needle.

It may rest upon a pivot, it may be hung from a fine thread, or it may be floated upon water with the aid of a cork, etc.

82. Uses. We all know that compa.s.ses are used to point to the north and south, and we speak of the ”points of the compa.s.s.” This, of course, is the most important use of the compa.s.s, and it has been known for centuries. In the laboratory it is used to show or detect the presence of currents of electricity, and, in connection with coils of wire, it may show the relative strengths of two currents, etc. When used for such purposes it generally has special forms and sizes. (See Galvanometers and Detectors.)

APPARATUS 62.

_83. Compa.s.s._ An oily sewing-needle will float upon the surface of water, when it is carefully let down to the water. A little b.u.t.ter may be rubbed upon the previously-magnetized needle to make it float better.

[Ill.u.s.tration: Fig. 38.]

APPARATUS 63.

_84. Compa.s.s._ Fig. 38 shows a magnetized sewing-needle floated upon a cork. The needle may be permanently fastened to the cork with a few drops of melted paraffine.

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