Fig. 1.—Sectional Diagram of One Side of Magnet.
The dotted lines show position of coils of wire. A, One side of hole for armature.
The greatest difficulty of the whole business has now already to be confronted—viz., the manufacture of the magnet. This is almost invariably cast in two pieces, and for those who cannot make the castings there is no help for it but to have recourse to the ironmonger, or, better still, a practical electrician. The following instructions will then assist you to put the castings together:
Supposing this difficulty to have been overcome, and two pieces of soft iron to have been cast in the form of Fig. 1, both exactly the same size and shape; the next thing to do is to convert it into an electro-magnet by winding seven layers of No. 16 cotton covered wire over each leg, at the part shown by the dotted lines in the illustration.
The size of the legs of the magnet is as follows:—Total length from B to C, 4⅛ inches; thickness of top piece from B to D, ½ inch; length of top piece from B to D (half total length of top of magnet), ¾ inch; breadth of side of magnet all the way down, 1¾ inch; height from E to C, 1½ inch; thickness of the part between D and E, round which the wire is wound, ⅜ inch. When I say “breadth” in this description, I mean what you can’t see in the sectional drawing, because it recedes from you; when I say “thickness,” I mean what is shown in the drawing. It is necessary to explain this, as the terms are rather confusing. The ends of the sides between D and E are rounded to admit of the wire being more evenly wound on them.
Fig. 2.—Magnet Put Together.
A B C, Screws. D, Junction of two wires. E F, Ends of coils. H H H H, Holes for screws at end. The dotted lines show position of wire, and screws fastening magnet together and to base.
It is not essential to use a permanent magnet in this machine, as a certain amount of “residual” magnetism remains in the iron when once excited; and the coils of wire on the armature being acted on by the armature, which is slightly magnetized by this residual magnetism in the magnet, have a reactionary effect, and excite the armature, which excites the magnet afresh; and thus the magnet and its coils, and the armature and its coils, go on acting on each other, and mutually building up each other’s current, until the maximum effect which the machine is capable of giving is produced.