class="x-ebookmaker-pageno" title="[Pg 21]"/> each other, and mutually building up each other’s current, until the maximum effect which the machine is capable of giving is produced.

Before winding on the wire, the legs of the magnet between D and E should be covered with a band of silk soaked in melted paraffin wax to increase the insulation. New and soft wire, of the highest conductivity, should be used. Old, rinky, and hard wire will not do.

Fig. 3.—Armature of Dynamo.

S S S S, Grooved cylinder of soft iron. K L, Axle of cylinder. The wire is wound across from end to end on one side, and then from end to end in the same direction on the other side. W W, Ends of wire. A B C D, Grooves in cylinder for thread to hold wires in position. F, Wooden cylinder fixed on axle.

The wire is wound upon the legs of the magnet in such a way that when put together as shown in Fig. 2 the coils are in opposite directions, so that if the magnet were straightened out, or the two portions placed end to end, one coil would be a prolongation of the other. This can be most easily done, in the case of this particular magnet, by winding each leg separately, and the end of the outer coil of wire of one can be joined to the end of the inner coil of wire of the other at D in the cut, the other ends of the coils being left loose as at E and F, these being long enough to go down under the base—say, about 3 inches long to allow for joining up.

The electro-magnet having been wound, may now be placed upright on the base, its two limbs fastened together by a screw at A. The magnet is now to be fastened to the base in the middle of its breadth, and about an inch from one end, by means of two screws at B and C, passing through the base into the legs of the magnet. Before it is fastened on, however, you had better drill two screw holes on each leg at H H H H in the figure, and four corresponding to them on the other side. We shall want eight screws to fit these holes presently.

Fig. 4.—Section of End Armature.

The circles show the position of the coils of wire in the grooves A and B. C D, Ends of soft iron cylinder.

The magnet having been fixed, we now have to construct the armature, which is the next most important part of the machine.

This consists of a soft iron cylinder with an axle passing through its center, as at K L in the illustration (Fig. 3), S S S S being the soft iron cylinder. This cylinder has a deep groove cut from end to end, or is cast in that shape, and round this groove the wire is wound. The wire is number 18, cotton or silk-covered. Begin at the point marked H in the diagram, and wind over and over, from end to end, until that side is full; then cross over to the other side, going from H to R, and wind that side also in the same direction. The ends of the wire are shown at W W, and they must be left about an inch or two inches long, as we shall want to connect them with the commutator presently.

The dimensions of the armature are as follows: Length of axle, 5½ inches; circumference of cylinder, 1 inch; length of cylinder, 2 inches; width of groove, ¾ inch. The axle is composed of a piece of steel rod rather more than ⅛ inch in diameter. The axle must be very truly centered in the armature, and the armature must be accurately mounted, as it has to revolve at a high rate of speed in a very limited space, between the poles of the magnet.

As it is rather difficult to explain the construction of the armature, I give another illustration (Fig. 4) of a section of the armature, which will show how the wire is wound on the groove, and the shape of the grooves themselves.

At one end of the axle is fixed the driving-pulley P, while at the other has to be fixed a small wooden roller F, over which two pieces of sheet brass have been fastened, each reaching nearly half round the surface of the roller, so that two gaps are left between them. This forms part of the commutator; but before we come to that we must consider how the armature is to be fixed between the poles of the magnet.

PART II.