A A, Soft iron cores. B B B B, Ends of bobbins, on which wire is wound as shown. C C, Yoke of magnet fastened to support and held up by D, bottom of brass support, with holes for screws. E, Joined ends of wires. F F, Exterior ends of coils. The arrows show relative directions in which the two coils are wound.
Cut two circles of thick cardboard, each ⅞ inch in diameter, and in the center cut a hole the exact size to slip over the soft iron core. Now wrap several thicknesses of thin tissue paper—or preferably French note paper or tracing paper—over the magnet, between the circles of cardboard, cutting the strip about 1⅛ inch broad or ⅜ inch less than the length of the cores. Now you can fasten the two circles of cardboard at the ends of the tracing paper, and keep them in their proper places on the magnet by means of mucilage—beat the soft iron before applying, and it will then adhere firmly to it. In this way, of course, you form a roller, on which we now have to wind the wire. If you have soldered the magnet’s parts together, you must have movable bobbins, as it would be simply impossible to wind the wire evenly on the cores when fixed in position, as the edges of the bobbins will be so close together that it is not possible to wind the wire on between them without the coils becoming displaced.
The method of winding the wire is simple enough. No. 24 wire is a good size to use; it can be cotton-covered or, preferably, silk-covered, as in the latter case the insulation is better. Begin by making a hole near the roller in the circle of cardboard that is next to the end where the hole for the screw has been made. Pass about three inches of wire through the hole and then wind it evenly on over the tracing paper from end to end and back again. You ought to have five or six layers of it; an ounce, or an ounce and a half, of wire will probably be enough. When it is all on, make another hole in the disc and pass out the wire. This is only to hold it safe while you wind the other bobbin. When that is finished you can put the magnet together, and ends of the two wires have now to be joined together. The two ends that are joined together must be those which come from the wire that is wound from the right to the left over one core and left to right over the other, that is to say, taking the wire when joined as one, it must be so wound on both limbs of the magnet that if they were bent into one straight bar it would all be wound in the same direction.
Fig. 3.—Shape of Spring for Armature.
P, Platinum foil. A A, Holes for screws to armature. H H, Holes for screws to support.
With a composite magnet, however, there is no earthly difficulty in getting it right, for you have only to connect the battery to two wires and join the other two, and if they don’t make the magnet work, join up one to the battery instead of one of those joined, and connect the other two wires; whichever gives the best result stick to. You must get all the silk or cotton off the wire, where you join them, and twist them over and over tightly together; if you can solder them, so much the better. Pull the wire tight and wind it on the reels until the place where it is joined is pulled tightly and not left in a loop, which would look untidy. Fig. 2 gives an idea of the magnet completed, and I have endeavored by means of the arrows to show how the wire is wound, they are supposed to give the direction of the top layer of wire in each case; of course either may be wound from the inside, so you must also consider that in this picture the outside coils are joined. The magnet having been thus constructed, we must now turn our attention to the vibrating hammer which is to beat the drum. To make this we want another piece of soft iron of about the same size as that forming the yoke of the magnet, say, 1⅜ inch × ½ inch × ⅛ inch. We shall then require a piece of brass spring about three inches long and half an inch broad. This is made of very thin springy brass, so as to make a spring which will move the armature quickly. One end of the spring should be tapered off as shown in Fig. 3, and at the point P in the figure a small piece of platinum foil (the real thing, not tin-foil, which I am sure is often sold in cheap apparatus instead of it,) should be fastened, by solder if possible.
Fig. 4.—Drum Hammer Put Together.
A, A bar of soft iron. B, Brass spring of contact breaker. C, Portion of brass spring bent outwards, the platinum foil is soldered on at the point C. S S, Screws holding spring to soft iron. Holes are made at the points H H to fasten spring to support. D, Bent wire fastened to soft iron armature. E, Wooden head of drumstick.