The wire used in making helices for the magnets, and for connecting the telegraphic stations, is made of copper of the best quality, and annealed. It is covered with cotton thread, so as to conceal every part of the metallic surface, not so much to prevent corrosion or waste from the action of the atmosphere, as to prevent a metallic contact of one wire with another, when placed near each other. After the wire is covered, it is then saturated with shellac, and then, again, with a composition of asphaltum, beeswax, resin and linseed oil. It is now in a condition to be extended upon the poles. That portion of the wire of which the helices are made is only saturated with shellac. THE ELECTRO MAGNET. The electro magnet is the basis upon which the whole invention rests in its present construction; without it, it would entirely fail. As it is of so much importance, a detailed account will be given of the construction of the electro magnet, as used for telegraphic purposes. A bar of soft iron, of the purest and best quality, is taken and made into the form presented in figure 5, which consists of four parts, viz. A F and A F are the two legs or prongs of the magnet,[2] of a rounded form, and bent at the top, approaching each other towards the centre, where the ends of each prong, without touching, turn up, and present flat, smooth and clean surfaces, level with each other at F F. The other end of these prongs or legs is turned smaller than the body, on the end of which is a screw and nut, C C. These ends pass through a plate of iron, B, of the same quality, at I and I, until they rest upon the plate at the shoulder produced by turning them smaller. They are then both permanently secured to the plate, B, by the nuts, C C, and the whole becomes as one piece. This arrangement is made for the purpose of putting on the coils or taking them off with facility. The form most common for electro magnets is that of the horse-shoe; and is simply a bar of iron bent in that form. E represents a small flat plate of soft iron, sufficiently large to cover the faces of the two prongs, F and F, presenting on its under side a surface clean and smooth, and parallel with the faces, F and F.