PART III.

THE ELECTRIC DRUM.

In part two on the "Electric Trumpet," I promised to explain how to make an electric drum; and this promise I now propose to redeem.

The system on which it works is precisely analogous to that of the electric trumpet, and almost identical with that of the ordinary electric bell, of which I hope to say more in another chapter.

As before, we have a hammer vibrating backwards and forwards in response to pulls from a magnet, which is magnetized and demagnetized by stopping and starting an electric current. In the case of the induction coil, the hammer is only a means whereby the current is broken and started again with great rapidity, and in the case of the trumpet the vibrator is used to make the noise by its vibration, but in this instrument we must have a _bona fide_ hammer, which must be able to beat the drum, and thus cause a stirring and martial sound.

First, then, we will devote our attention to the construction of the magnet. In former chapters (as in the case of the electro-motor for example), I have given you the method of making the magnets out of one solid piece of soft iron, in the form of a horseshoe. This time, however, we will make it of several pieces, for a change; it is far more convenient to make, and looks much neater when finished.

Take a piece of soft iron 1½ inches long by 5/8 inch broad and 1/8 inch thick, and in the middle drill a hole about 3/16 inch in diameter. On each side of this, on a line with it at a distance of about ¼ inch, drill two more holes of the same size. This is to form the back, or, as it is scientifically termed, the yoke of the magnet. To form the poles we require two exactly similar pieces of soft iron bar 1½ inch long and 3/8 inch in diameter. These are to be filed quite smooth at the ends after cutting, and in the middle of one end a hole is to be drilled to admit a screw which will just go through the holes on each side of the center one made in the flat piece of the soft iron. These holes are cut to receive the thread of the screw, but if you can't do this you can simply leave out the end holes for screws, and solder the round and flat pieces of iron together. These are to be soldered or screwed together, so as to form a magnet, the hole in the middle of the flat piece serving to introduce a screw, for the purpose of attaching the magnet to a support. The best plan, if you can do it, is to drill and "tap" this hole to receive a screw which is inserted in a brass support made of a piece of brass 1 1/8 inch, long ½ inch broad, and 1/8 inch thick, bent at right angles at about ½ inch from one end, this shortest end being drilled for two screws to fasten it to the base-board, while the longest end has a hole in the center about 1/8 inch from the end, to admit the screw which fits the hole in the center of the yoke. Having done all this, you will have Fig. 1, which represents the magnet before it is wound.

The soft iron cores have now to be converted into magnets as usual, and here comes in the especial advantages of having screws to fasten the magnet together, as you can take the whole thing to bits, wind the wire on the legs in comfort, and then fasten together again. But if you have soldered the magnet together, you can achieve the same end in a different way by making two small bobbins to hold the wire, the exact size to slip on over the soft iron cores when the wire is wound on them. It is generally considered proper to wind the wire on bobbins, which can be removed from the cores if required. I should think it can seldom be required, but the bobbins are convenient in this case. I may remark parenthetically that bobbins wound and unwound, soft iron cores, and yokes, separately or together, and supports fixed to the yokes or not, can be obtained from any large electrician who sells parts of electric bells, etc.; the magnet can also be got put together complete.

We now have to make bobbins, supposing that we are not going to buy them. The elaborateness of their manufacture will depend entirely on the skill of the maker. Some construct them by sawing off top and bottom of a reel of cotton, and forming a roller of cardboard to fit the magnets, finally joining the ends of the reel to this roller, to make an elongated reel of the right size. Others construct their bobbins entirely of cardboard, the ends being merely two circles of card. Others who are versed in the mysteries of wood-turning, and are lucky enough to possess a lathe with which to do it, make two bobbins of solid wood, drilled to fit the iron cores. For these no instructions are needed, as the dimensions will be as given presently. For those who only want to use the magnet for this special purpose, and do not care about the bobbins being removable, the following is the simplest way to set to work:

Cut two circles of thick cardboard, each 7/8 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 1 1/8 inch broad or 3/8 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.

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, 1 3/8 inch × ½ inch × 1/8 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.

We now want a piece of rather stout brass wire bent into the shape shown in Fig. 4. It must be about four inches long, but its length will be determined by the size of the drum and the length of the magnet when it is all put together. At the end of this wire you must have a wooden knob (not brass, which doesn't produce nearly so much noise). This you will have provided ready for you if you purchase the drum, as they will naturally supply drumsticks with it, and the head of one of these cut off and fastened to the end of the wire, by simply making a hole and sticking it in, will answer the purpose beautifully.

This wire has to be fastened to the soft iron armature, a simple way of doing which is to drill a hole the exact size and insert the end; it can then be soldered in. Or, if you cannot drill a hole, you can simply solder it on. The brass spring has the end bent outwards, as shown in Fig. 4, and is fastened to the soft iron armature by screws, as shown in the figure at S S, or simply soldered on. The point C is the end that is tapered off, and the platinum wire is fixed at that point; the spring should extend about 1¼ inch beyond the armature at the other end. Two holes are drilled in the spring at the points H H, through which screws are passed into the support. This support may be either a piece of iron ½ inch long, ¾ inch broad and ¾ inch thick, or a piece of wood will answer very well, and save drilling holes in the iron. If it is wood it had better be larger, say ¾ inch by ¾ inch by 1¼ inch.