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.
Fig. 5.—Support for Pulley End of Axle.
The dotted lines show position of holes for screws and axle. P P, Holes for screws.
Returning to Fig. 1, we must see that the groove A, which forms half the channel in which the armature is to revolve, is ⅞ inch semi-circle. When the two sides are fixed together as in Fig. 2, the hole between the poles should be about an inch in circumference, and the wire must be wound on the armature so that it easily slips into the cavity G, which must be made quite smooth for it to revolve in. It will be seen from the dimensions given that in diameter the armature is only a little less than the cylindrical space between the poles of the magnet, and in length it is about the same as the width of the magnet. It would be an unfortunate occurrence if the wire was to slip off the armature while revolving at a high speed, and therefore it is necessary to keep it firmly in its place. This is done by filing four small notches in the soft iron of the armature at the points marked A B C D in Fig. 3. Some strong wire or small string is now wound lightly round the armature to hold the coils of wire in their proper place, the notches holding this wire or string from slipping off at the ends of the cylinder.
The armature is now to be fixed in its proper place between the poles of the magnet.