The Electro-Magnet.--This should be of good round iron, and bent into a horse-shoe shape (Fig. 62). The part _a b_ must be quite straight, and not damaged by the forging; the bend should be as flat as possible, so as to make the magnet as short as may be (to save space). When made, the magnet is put into a clear fire, and when red hot, taken out and laid in the ashes to slowly cool; care must be taken not to burn it. Lastly, 2 small holes are drilled in the centre of the ends at _c_, about 1/16 in. deep; drive a piece of brass wire tightly into the holes, and allow the wire to project sufficiently to allow a piece of thin paper between the iron and the table when the iron is standing upon it; this is to prevent the armature adhering to the magnet from residuary magnetism, which always exists more or less. The measurements are-- No. 1 size iron ¼ in., _d_ to _e_ ⅝ in., _a_ to _b_ 1¼ in. No. 2 ” 5/16 in., ” ¾ in., ” 1⅜ in. No. 3 ” 7/16 in., ” ¾ in., ” 1½ in. The Bobbins or Coils.--These are made by bending thin sheet copper round the part _a b_ of the magnet; the edges at _a_ (Fig. 63) must not quite meet. The thickness of this copper must be such that 4 pieces just equal in thickness the edge of a new threepenny-piece (this is rather an original gauge, but then all can get at the thickness this way). The hole in the brass end _b_ must be just large enough to push on firmly over the copper when on the iron; they must then be set true, and soldered on. The brass for the ends may be about as thick as a sixpence; a 1/16 in. hole must be drilled at _c_, close to the copper. The other measurements are as follows:-- No. 1, diameter ⅜ in., length over all 1⅛ in. No. 2, ” ¾ in., ” 1¼ in. No. 3, ” 1 in., ” 1⅜ in. The brass ends should be neatly turned true and lacquered. [Illustration: 63. Bobbin. 64. Winding Bobbin.]