Chapter 8

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 x 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 screwing 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.

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.

APPARATUS 73.

_99. Yoke._ Fig. 47. The yoke, Y, is a part of a carriage. This can be bought at a blacksmith's. The holes are already in, but it may require some filing before the nuts of the bolt magnets will fit down firmly.

APPARATUS 74.

_100. Tin Armatures_ may be made by bending together 5 or 6 thicknesses of tin. Different forms of tin armatures are shown under telegraph sounders; these should have a hole punched at the center; through this is put a screw. The length of the armature will depend upon the distance the magnets are placed apart; they should be about 3/4 in. wide.

APPARATUS 75.

_101. Nail Armatures._ Fig. 48. A nail, N, placed through a piece of wood, A, will serve as a very simple armature. To make it a little heavier, if necessary, a piece of annealed iron wire, W, may be wound around N. Care should be taken to have the two parts fairly alike in size and weight.

APPARATUS 76.

_102. Wire Armatures._ Fig. 49. Annealed iron wires make good armatures. The short lengths of wire should be straightened (See App. 28) before binding them into a bundle. They may be held together with thread or paraffine, until they are in place, as, for example, in a wooden piece, A, Fig. 49. The bundle of wires should fit snugly into the hole made through A, and the wires should be bound together at each end with wire.

APPARATUS 77.

_103. Trembling Armature._ Fig. 50. Armatures to be used upon electric bells, automatic current interrupters, buzzers, etc., may be called trembling armatures. They may be made entirely of sheet-tin. The part, F, which gives it the spring, should be about 5/8 in. wide. Its length will depend upon the particular apparatus to be made. It is made of 2 thicknesses of thin tin. See Fig. 50 for dimensions. The part N projects beyond L. This may be used to tap against a regulating screw, or to fasten a hammer on for an electric bell. The part, L, should have about 4 layers of tin on each side of F, and it should pinch F tightly.

APPARATUS 78.

_104. Trembling Armature._ Fig. 51. When very rapid motions are desired in a trembling armature, App. 77 will be a little heavy. A light and quick-acting armature can be made of sheet-tin. The exact dimensions will depend upon the use to be made of it, but you will find the following a guide. Cut the part, B, E, out of thin tin. The covers and bottoms of tin cans are thinner than their bodies. The narrow part, B, should be about 1/4 in. wide and 2 in. long for a small apparatus, while E may be 3/4 in. square. Through E is a screw, which holds it firmly to a wooden piece, D, about 3/4 in. square. The part, E, can be made longer than its width, so that two screws can be used; this will keep A from jarring up or down.

APPARATUS 79.

_105. To File Thin Metal Strips._ Fig. 52. When sheet-metal is punched by the methods usually employed by boys, a bulge or bur is made on the underside around the hole. If this bur be hammered to flatten it, the hole is distorted and made smaller. It is better to file the bur down, at least part way. It is not convenient to file a piece of thin metal when it is held in a vise. It is better to use either a metal or a wooden clamp, as shown in Fig. 52; then the filing can be quickly and easily done. Y is the yoke to be filed. It is well to place a piece of metal, I, between the table and the end of the screw.

APPARATUS 80.

_106. Clamp._ Fig. 53. If you have no clamp to hold metal strips while filing them, you can put a screw, S, through one hole to hold the strip down fairly tight. Drive a nail, N, behind the strip. This will keep it from turning while you file the free end.

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_Electro-Magnetic Armatures._ A description of this form of armature is given in the chapter on electric motors. (See Index.)