Harper's Electricity Book for Boys

by Joseph H. Adams

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.