How Two Boys Made Their Own Electrical Apparatus Part 5

APPARATUS 51.

_69. Binding-Post._ Fig. 35. The ends of two or more wires may be quickly joined electrically by placing them between the nuts of a short bolt. By using 3 nuts the bolt will more easily connect a large number of wires.

[Ill.u.s.tration: Fig. 35.]

_Make Additional Notes and Sketches Here._

CHAPTER VI.

PERMANENT MAGNETS.

_70. Permanent Magnets_ may be made in many ways and from many different kinds of steel. The steel used for needles, watch and clock springs, files, cutting tools, etc., is generally of good quality, and it is already hard enough to retain magnetism. (See Retentivity in text-book.)

APPARATUS 52.

_71. Bar Magnet._ A straight magnet is called a bar magnet. Magnetize a sewing-needle. For some experiments a needle-magnet, as we may call it, is better than a large magnet.

APPARATUS 53.

_72. Bar Magnet._ A harness-needle, which is thicker and stronger than a sewing-needle, makes an excellent bar magnet.

APPARATUS 54.

_73. Bar Magnet._ For long slim magnets use a knitting-needle. Some knitting-pins, as they are sometimes called, break off short when bent, but most of them will bend considerably before breaking. These slim magnets are excellent for the study of Consequent Poles. (See text-book.)

APPARATUS 55.

_74. Flexible Bar Magnets._ It is often necessary to have flexible magnets so that they may be bent into different shapes. These may be made from watch or clock springs, as such steel, called spring steel, will straighten out again as soon as the pressure is removed from it.

Corset steels, dress steels, hack-saw blades, etc., make good thin flexible bar magnets.

APPARATUS 56.

_75. Strong Bar Magnets_ may be made from flat files. The handle end may be broken off so that the two ends of the file shall be nearly alike in size. These should be magnetized upon an electro-magnet.

APPARATUS 57.

_76. Compound Bar Magnets_ are made by first magnetizing several thin pieces of steel, and then riveting them together so that their like poles shall be together, and pull together. To make a small compound bar magnet, magnetize several harness-needles, or even sewing-needles, and then bind them into a little bundle with all the N poles at the same end. Melted paraffine dropped in between them will hold them together.

Rubber bands may be used also, or, if but one end is to be experimented with, the points may be stuck into a cork, and the heads used to do the lifting.

APPARATUS 58.

_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.