Читать книгу Harper's Electricity Book for Boys - Joseph H. Adams - Страница 23

As has been said, the induction-coil is one of the mysterious phenomena of electrical science. While its practical value is known and recognized in all branches of voltaic electricity for use in transforming currents, its actual workings have never been clearly explained.

The construction of a small induction-coil was explained in the description of a shocker or medical battery. For bigger equipments, wireless telegraphy and other uses, a large induction-coil will be necessary, and the following illustrations and descriptions should enable the young electrician to construct an apparatus that will be both simple and efficient in its working.

For the tube (in which to wind the primary coils) obtain a piece of red fibre-tubing, one inch inside diameter and not more than one-eighth of an inch in thickness. The length should be ten inches. If fibre cannot be had use a paste-board tube.

From white-wood, half an inch in thickness, saw two blocks four inches square and in the centre of each cut a hole so that the tube will pass through it and fit snugly. Some shellac and a few slim brass escutcheon pins will hold the blocks in place, as shown at Fig. 15. The wood blocks and fibre or paper tube should be treated to several successive coats of shellac to give them a good finish and prevent the absorption of moisture. Four binding-posts, with wood screw-ends, are to be made fast at the top edges of the end-blocks, as shown at Fig. 15. Holes bored in the blocks near the foot of the binding-posts will admit the ends of the coil-wires so that contact can be made. The ends of the conductor-wires should then be placed in the holes in the binding-posts and held in place with the thumb-screws.

The primary coil is made by winding four layers of No. 20 insulated copper wire on the tube and between the end-blocks, as shown at Fig. 16. Each layer must be wound evenly, and the strands should lie close to each other. When the first layer is on give it a coat of shellac; then wrap a piece of thin paper about it and give that also a coat of shellac. When the second layer is on repeat the operation of shellacking and paper-coating, and continue with the third layer. When the fourth layer is on give the coil a double wrap of paper and two or three coats of shellac to thoroughly insulate it and keep out all moisture. The winding may be done by hand, but it is much easier to do it on a winder or reel, which can be operated to revolve the core, the wire unwinding from its original spool as it is wound on the tube.

A convenient winder may be made on a base-board which can be clamped to a table or bench. The board is twelve inches long, eight or ten inches wide, and seven-eighths of an inch thick. Two uprights, three inches wide, ten inches long, and three-quarters of an inch thick, are screwed and glued to the ends of the base-board. A notch is cut in the top of the end-boards, into which the spindle or shaft can rest; and at the top of the end-pieces two small plates of wood or metal are screwed down to hold the spindle in place when the tube and ends are being revolved. A small hole, bored in each upright end two inches above the top of the base-board, will admit a rod on which a spool of wire can revolve, as shown at Fig. 17.

Two plugs of wood, shaped like corks, are made to fit in the ends of the fibre-tube. A hole is bored through each one so that a wire or rod spindle will pass through them and fit tightly. One end of the rod is bent and provided with a small wooden handle, by means of which the core may be revolved.

This winding-rack makes it easy to handle the core-tube while putting on the layers of wire, and it holds the tube securely while the wraps of paper and shellac are applied.

The secondary coil is laid over the primary, and should be of Nos. 30 to 36 insulated copper wire. The finer the wire the higher the resistance and the longer the spark, but nothing heavier than No. 30 should be used.

Begin by making one end of the wire fast to a binding-post; then turn the core-tube with one hand, holding the wire in the other. Take care not to bind the wire nor stretch it, but wind it on smoothly and evenly, like the coils of thread on a new spool of cotton or silk. Be very careful to avoid kinks, breaks, or uninsulated places in the wire. Should the wire become broken, give the coil a coat of shellac to bind the wound strands; then make a fine twisted point and cover it with the silk or cotton covering, with a coat of shellac to hold it in place, and proceed with the winding. Between each layer of wire place a thin sheet of paper and coat it with paraffine, or shellac, to make a perfect insulation; then proceed with the next layer.

With a battery and small bell test the wire layers occasionally to see that everything is all right, and that there are no breaks or short circuits. This is very necessary to avoid making mistakes, and, considering the time and care spent in winding the coils, it would be a great disappointment if the coil were defective.

About one pound and a half of wire should constitute the secondary coil, and, if possible, it is best to have it in one continuous strand, without splices.

Over the last coil, after the winding is completed, several thicknesses of paper should be laid and well coated with shellac between each wrap. This is a protector to insure the fine wire strands from damage. To improve the appearance of the coil a wrap of thin black or colored leather may be glued fast, with the seam or point at the under side.

The ends of the wires forming the primary coil should be made fast to the binding-posts at one end, while those of the secondary coil should be attached to the posts at the other end.

For the core, obtain some soft iron wire, about No. 18, and cut a number of lengths. Straighten these short wires and fill the tube with them, packing it closely, so that the wires will remain in place under a mutual pressure. It is better to make a core of a number of rods or wires rather than to have it of one solid piece of soft iron.

Now, from hard-wood, cut a base three-quarters of an inch thick, five or six inches wide, and twelve inches long. Attach the coil to the base by means of screws passed up through the board and into the lower edges of the end-blocks. The wood is to be stained and given several successive coats of shellac.

Now connect the wires of a battery to the binding-posts in contact with the primary coil, and attach two separate wires to the secondary coil binding-posts. Bring these ends near to each other, and a spark will leap across from one end to the other, its size or “fatness” depending on the strength of the battery. The completed apparatus is shown at Fig. 18.

In producing a long spark a condenser is an important factor; it is used in series with an induction-coil. There are several forms of condensers, but perhaps the simplest and most efficient is the Fizeau condenser, which is made up of layers of tin-foil with paraffined paper as separators.

From a florist’s supply-house purchase one hundred and fifty sheets of tin-foil seven by nine inches, or sheets that will cut to that size without waste; also ten or twelve extra sheets for strips. At a paper supply-house obtain some clear, thin, tough paper about the thickness of good writing-paper. Be careful to reject any sheets that are perforated or have any fine holes in them. The sheets should be eight by ten inches, or half an inch larger all around than those of the tin-foil. The paper must be thoroughly soaked in hot paraffine to make it moisture-proof and a perfect non-conductor. This is done by placing about two hundred sheets on the bottom of a clean tin tray, or photographic developing-dish of porcelain. Don’t use glass or rubber. After placing some lumps of paraffine on the paper, put the tray in an oven so as to dissolve the paraffine and thoroughly soak the paper.

Open the oven door and, with a pin, raise up the sheets one at a time, and draw them out of the liquid paraffine. As soon as it comes in contact with cool air the paraffine solidifies and the sheet of paper becomes stiffened. Select each sheet with care, so that those employed for the condenser are free from holes or imperfect places.

From pine or white-wood, a quarter of an inch in thickness, cut two boards, eight by ten inches, and give them several good coats of shellac.

To build up the condenser, lay one board on a table and on it place two sheets of paraffined paper. On this lay a sheet of tin-foil, arranging it so that half an inch of paper will be visible around the margin. From the odd sheets of tin-foil cut some strips, one inch in width and three inches long. Place one of these strips at the left end of the first sheet of foil, as shown at Fig. 19. Over this lay a sheet of the paraffined paper, then another sheet of the foil. Now on this second sheet of foil lay the short strip to the right end, and so proceed until all the foil and paper is in place, arranging each alternate short strip at the opposite end. Care must be taken to observe this order if the condenser is to be of any use.

When the last piece of foil is laid on, with its short strip above it, add two or three thicknesses of paper, and then the other board. With four screw-clamps, one at each corner, press together the mass of foil, paper, and boards as closely as possible, then bind the boards about with adhesive tape, or stout twine, and release the clamps. Attach all the projecting ends of foil at one side by means of a binding-post, and those at the other end with another binding-post. The complete condenser will then appear as shown in Fig. 20.

When in operation one wire leading from the secondary coil should be connected with a binding-post of the condenser, so that it is in series.

The object of the condenser is to increase the efficiency of induction, and it should be made in proportion to the size of the induction-coil with which it is to be employed.