How Two Boys Made Their Own Electrical Apparatus Part 6

APPARATUS 64.

_85. Compa.s.s._ Fig. 39. With a sharp knife make a cut part way through a flat cork. Into the cut push a short length of magnetized watch-spring.

In the ill.u.s.tration the spring is shown partly removed from the cut.

Float the cork.

APPARATUS 65.

_86. Compa.s.s._ Fig. 40. Stick a pin, P, into a pasteboard, cork, or wooden base, B. Bend a piece of stiff paper double, as shown, and then stick through it, on each side, a magnetized sewing-needle, S N. The north poles of the needles should be at the same end of the paper. Why?

Balance the paper upon the pin-pivot, and see it fly around to the north and south.

[Ill.u.s.tration: Fig. 39.]

[Ill.u.s.tration: Fig. 40.]

[Ill.u.s.tration: Fig. 41.]

APPARATUS 66.

_87. Compa.s.s._ Fig. 41. It is an advantage to have a magnetic needle that is always ready for use. The support is made by driving a pin through the top of a wooden pill-box, which should be about 1-3/4 in. in diameter. This gives plenty of room under and around the needle. If the pin be left too long, it will not be possible to put the bottom and top of the box together when you want to put the compa.s.s away. Cut the pin off (App. 35) at the right length, so that the magnetic needle can be safely put away in the closed pill-box.

88. The "Needle," that is the short bar magnet, may be made of watch-spring. As the spring is already quite hard and brittle, it may be easily broken into desired lengths. It is always better to make 3 or 4 needles at a time, as some will swing more easily than others, and time will be saved in making them. Break off 3 or 4 pieces of thin spring, each about 1-1/2 in. long. Bend them as in Fig. 42. A good dent, not a hole, should be made at the center of each to keep them upon the support or pin-point. A "center punch," not too sharp, is the best tool to use, but a slight dent may be made with a sharp wire nail, provided the watch-spring is first annealed or softened. (See App. 21.) Do not place the spring directly upon iron or steel when making the dent, as these might injure the point of the punch, and the dent would not be deep enough. Fig. 42 shows a good way to make dents in steel springs. Place 2 or 3 layers of copper or lead between the anvil and the spring. A hammer or hatchet will do for the anvil. As the copper will give easily, a good dent may be made by striking the punch or nail with a hammer. If the spring has been annealed before denting it, it should be hardened again (App. 21) before magnetizing it, so that it will retain magnetism well.

(See Residual Magnetism in text-book.)

[Ill.u.s.tration: Fig. 42.]

89. Balancing. After a dent has been made, place the spring upon its support so that the pin-point shall be in the dent. It will, no doubt, need balancing. If one end is but slightly heavier than the other, the spring may be balanced by magnetizing it so that the lighter end shall become a north pole. This will then tend to "dip" and make the needle swing horizontally. If one end is much heavier than the other, it should first be magnetized and then balanced by cutting little pieces from the heavier end with tinners' shears, or by weighting the lighter end with thread, which may be wound around it. The finished compa.s.s-needle should swing very freely, and should finally come to rest in an N and S line after vibrating back and forth several times.

APPARATUS 67.

_90. Gla.s.s-Covered Compa.s.s._ A perspective view of this apparatus is shown in the tangent galvanometer. (See Index.) The outside band, E, is made of thick paper, 1 in. wide, and with such a diameter that it just fits around the gla.s.s. In this model, the gla.s.s from an old alarm-clock was used, it being 4 in. in diameter. Four pasteboard strips were sewed to the inside of the paper band E. They were made 7/8 in. long, so that the gla.s.s, when resting upon them, would be near the top of E.

The needle should be not over 1 in. long, if it is to be used in the galvanometer. A long slender paper pointer should be stuck to the top of the needle. Be careful to have the combined needle and pointer well balanced, so that it will swing freely. A circle graduated into 5-degree s.p.a.ces should be fastened under the needle.

_91. Astatic Needles._ In the magnetic needles so far described, the pointing-power has been quite strong. By pointing-power we mean the tendency to swing around to the N and S. In App. 65 the 2 needle magnets had considerable pointing-power, because they helped each other. For some experiments in electricity a magnetic needle is required which has but little pointing-power; in fact, to detect the presence of very feeble currents by means of the needle, the less the pointing-power the better. Can you think of any way to arrange App. 65 so that it shall have very little pointing-power?

APPARATUS 68.

_92. Astatic Needle._ Fig. 43. Turn one of the needle magnets of App. 65 end for end, so that the N pole of one shall be at the same end of the paper as the S pole of the other. You can see that by this arrangement one needle pulls against the other. The magnetic field still remains about the little magnets, otherwise this combination would be of no value in the construction of galvanometers. The more nearly equal the magnets are in strength, the less the pointing-power of the combination.

[Ill.u.s.tration: Fig. 43.]

[Ill.u.s.tration: Fig. 44.]

APPARATUS 69.

_93. Astatic Needle._ Fig. 44. Magnetize two sewing-needles as equally as possible, by rubbing them over the pole of a magnet an equal number of times. Remove the covering from a piece of fine copper wire, say No.

30, and use the bare wire to wind about the needles, as shown. Be sure to place the poles of the little magnets as in the Fig. This combination may be supported by a fine thread. It is used for Astatic Detectors.

(See Index.)

CHAPTER VIII.

YOKES AND ARMATURES.

_94. Yokes_ are used to fasten two straight electro-magnets together to form a horseshoe electro-magnet. The reasons for using them should be understood. Soft iron should be used for yokes and armatures, as this is the best conductor of lines of magnetic force. Sheet-tin is made of thin iron, which is coated with tin. (Try a magnet upon a tin can.) This soft iron is very easily handled, bent, and punched, and is very useful for many purposes. The tin from old tomato cans, cracker boxes, etc., is just as good as any. The method of making your yokes will depend entirely upon the tools at your command. Several ways are given. Y, Fig.

47, shows the position of the yoke.

APPARATUS 70.

_95. Yoke._ For the experimental magnets (App. 89) a fairly large yoke is required in order to have the magnets far enough apart. If you have only a nail punch (App. 26) with which to make holes in tin, you will be obliged to punch but one thickness at a time. (See method of punching sheet-metal, App. 26.) Cut 5 or 6 pieces of the tin, 3-1/4 1 in. With a center punch (tools) or sharp-pointed nail make small dents (2 in.

apart) in each piece to mark the places where the holes are to be punched. Punch 5/16 in. holes in each piece. If you do this carefully, the holes in the different pieces will match, and the bolts can be pushed or screwed into these. When s.c.r.e.w.i.n.g in the bolt magnets turn them by their heads; do not pinch the coils, as this loosens the wire.

If you have a good punch, it is better to make the yoke as in App. 27, instead of using separate pieces of tin.

APPARATUS 71.

_96. Yoke._ Fig. 45 and 46. Cut a strip of tin 6 in. long by 3-1/4 in.

wide. Bend one end of it so that it will lap over 3/4 in. (Fig. 46); hammer it down gently, then bend this over and over until the whole tin is used. The final result will be a flat roll, 3-1/4 by about 1 in. This should be hammered flat.

[Ill.u.s.tration: Fig. 45.]

[Ill.u.s.tration: Fig. 46.]

If you have the tools it is easy to drill two 5/16 holes in this strip.

They should be 2 in. apart; that is, 2 in. from the center of one to that of the other. Start the holes with a center punch.

_97._ If you have no way of drilling the holes, they must be punched.

(See App. 27.) This will make the strip bulge out on the underside around the holes. This bur, or most of it, should be filed off. (See App. 79 for method of filing thin pieces of metal.) The resulting yoke may be held firmly to the magnets by the use of 2 extra nuts, as in Fig.

67. Remember that the magnets must be held firmly in the yoke.

APPARATUS 72.

_98. Yoke._ The best way of making this, of course, is to cut a piece of bar-iron the right size. For 5/16 bolts the strip of wrought iron should be about 3/4 in. wide and 3/16 or 1/4 in. thick. Any blacksmith can make this and punch or drill the holes. If taps and dies (tools) are at hand, the hole may be drilled and tapped to fit the thread on the bolt.

It is very easy to make good looking apparatus if you have, and can use, a whole machine shop full of tools. The lengths of yokes will depend upon the special uses to be made of them.