Harper's Electricity Book for Boys

by Joseph H. Adams

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.