How to Make Electrical Machines / Containing Full Directions for Making Electrical Machines, Induction Coils, Dynamos, and Many Novel Toys to Be Worked by Electricity

To do this we shall want two supports for the axle. These are made of brass, shaped as in Figs. 5 and 6, 5 being the one at the pulley end of the axle, and 6 that at the other end. They are fastened by screws through the holes P P, into the holes H H H H in the bottom part of the side of the magnet, as previously shown in Fig. 2.

When the armature is fixed in its proper place it will appear as Fig. 7, this being a sectional diagram from above, and the top pieces of the magnet being omitted for simplicity’s sake.

The brass of which the supports are made should be about ⅛ inch thick, and must, of course, be drilled in the center with a hole to admit the axle of the armature. To keep it exactly in the right place while revolving, a piece of circular brass tube, with a bore the size of the hole made to admit the armature, should be soldered to the brass supports in front of the hole; that for the pulley end of the axle should be ½ inch long. One at the other end is not necessary, but looks neater; this may be about ¼ inch long—i. e. as long as the end of the axle projecting beyond the brass support.

This much having been accomplished, we have now to consider the “commutator,” which is a piece of apparatus by which all the currents proceeding from magnet and armature are sent in one direction, and thus, instead of counteracting each other, are made available for experiments.

To make this necessary adjunct to the dynamo, take a circular bar of brass rod about ⅜ inch in diameter and an inch long. Into the middle of this solder a brass screw by drilling a hole and inserting its upper end minus the head. On this screw works a brass nut about ⅜ inch long. At the other end of the rod a hole is drilled for the insertion of another brass screw, long enough to go through the base. Another pillar precisely like this has now to be made, only ½ inch high without the nut. Now cut two pieces of sheet brass 2 inches long and ½ inch broad, sufficiently stout to act as springs and not too stout to be elastic. At one end of each cut a longitudinal hole about ¾ inch long and ⅛ inch broad; that is to say, this slit must be broad enough to slip over the top of the screws above the pillars. At the other ends of the brass springs slits of equal length, but very narrow—only about ¹⁄₂₄ inch wide—may be cut, to make the brass more “springy.” On the under side of this end of one spring and the upper side of the other, two pieces of thin sheet copper are fixed, the same breadth as the springs, and about ½ inch long. These are soldered by one end to the side of the spring, so as to act as springs themselves, their other ends being free.

All this being rather complicated, we must invoke the aid of the engraver once more. Fig. 8 gives you the method of making the pillars—A being the brass rod, B the screw and C the nut, the hole to admit screw to fasten the pillar to the base is made at the end D.

Fig. 9 is the brass spring with slit, A, to slip over the screw of Fig. 8, and the copper spring soldered to one side, at the end, at the point B. Now we slip the brass spring over the screw, the screw coming through the slit, and screw down the nut C. We thus have two springs supported at the ends on pillars at a height of 1 inch and ½ inch from the base respectively. Of course, both the pillars and springs are treated alike, but in the case of the tallest the copper is on the under side, and in the other on the upper side.

Now we go back to the armature, on the axle of which you will remember that I told you to fix a small roller of wood. This is only ¾ inch long and ½ inch in diameter, and is fixed firmly to the axle so as to revolve along with the armature. This roller is soaked in melted paraffin wax for an hour or two before fixing on, or boiled in it for some time, so that it may permeate the wood. The roller can easily be turned (of boxwood, preferably) if you are possessed of a lathe, but if you have none, go to the nearest photographer (or, preferably, a dealer in photographic apparatus), and from him you can buy for 3 cents a roller long enough to cut dozens for dynamos—they are what sensitized paper is sold rolled on.

The roller having been provided, take a piece of brass tube exactly so large inside that the roller will fit tightly into it, and cut off a piece the same length as the roller, or, if anything a trifle shorter. You have now to cut, with a saw or otherwise, two diagonal lines in this tube lengthwise, so that the tube is thereby divided into two pieces. Having done this the brass is replaced on the roller and fastened by minute screws, or “Prout’s elastic glue,” to each side of it, so that the roller becomes practically one of brass, with two slits in it. The screws must not project above the brass, but must be well sunk into it, so as to leave the surface smooth: and care must be taken that the screws do not touch both pieces of brass by going right through the roller—they must be very short. The object of cutting the slits in a diagonal direction is that the springs when pressing above and below the roller (see Fig. 10) shall not leave one half of the commutator before resting on the other part. If they do so the commutator will “spark” badly, which injures the fittings, and less current is obtained. Both slits are to be equidistant, and both inclined in the same direction. The roller is fixed on the axle in such a position that the middles of the lines of division are exactly in a line with the middle of the groove of the armature. When all this has been accomplished you will obviously have two conducting surfaces, each reaching over half the cylinder, separated by a small distance at top and bottom, the paraffined wood, of course, being a non-conductor of electricity. The brass tube must be made to fit smoothly round the wood, the surface being free from any irregularities, so that the contact with the springs at the sides may be as perfect as possible. Care must be taken that the brass is really separate all down on both sides. It is a good plan to fasten small splinters of paraffined wood in the slits to make sure.

This having been done, the wire from one end of the coil of the armature must be soldered to one of the semi-circumferences (if I may coin a word) of brass on the wooden roller, and the wire from the other end of the coil to the other semi-circumference. This is done at the end or underneath, not at the top, or it will make the surface rough, and we want it to be as smooth as it can possibly be. The wire must be quite tight up to the end soldered on; there must be no loops, or it will catch in something and be torn off when it comes to revolve.

The brass pillars supporting the springs have now to be inserted in the base, at such a distance, one on each side of the roller covered with brass, that the copper springs at the end of the brass ones are exactly one over and one under the brass roller. Of course, if they are put in a line with it, the springs can easily be shifted to the right position by slipping the slits over the screws of the pillars, and screwing down the nuts lightly when they come to the right place. This is very difficult to make intelligible, and I give another illustration of the relative positions of the parts of the commutator which I hope will make all clear. The pillars P P—which were put together as shown in Figs. 8 and 9—are fixed at such distances on opposite sides of the roller R that the springs S S are continually in contact with the brass semi-circumferences, first one and then the other as the armature revolves.

We are now within sight of the end of our task, and to guide off the current that we are going to produce we must screw in two binding-screws at opposite corners of the same end of the base (the end at which the commutator is). The ends of the wire from the magnet are to be brought down through the base and joined to the under part of these binding-screws. Placing the base so that the commutator end of the armature, and not the pulley end, is next to you, the wire from the inner coil of the magnet goes to the binding-screw on your left hand, and that from the outer coil to that on your right hand. The magnet should be wound and placed in such a position that these ends are respectively on the left and right, and then they have only to be joined to the binding-screws in front of them.

But before connecting these wires up, it is necessary to give an initial magnetism to the magnet, which at present has not been magnetized at all! To do this we must make use of another dynamo or a battery and connect the wires coming from the magnet-coil to the terminals of the battery. This having been done, the magnet will attract iron filings or needles, etc., and this shows that it has really become a magnet. Two cells of the chloride battery will be enough to magnetize it as much as it can be magnetized, and enough will remain when the battery is disconnected to start the action when the armature is revolved. Two or three minutes is long enough to connect with the battery.

PART III.

While the current is passing you can try the following experiment, to prove that the wire is wound on all right. If it is not wound as described there will be two north poles or two south poles, instead of one north and one south. Suppose we decide to make the leg on which the wire comes from the outside of the magnet the north pole, the wire from this must be joined to the wire coming from the zinc end of the battery, and the other coming from the inside, between the poles, joined to the wire from the carbon end. Now if, while the current is passing, a magnetized needle is approached to each pole consecutively, and one end of it is attracted and the other repelled in each case, the wire is all right; if both are attracted something is wrong. The needle must have been really magnetized beforehand, or it will deceive you; you can easily test if it is so with an ordinary permanent magnet.

Having magnetized the soft iron in the way described, we now join up the wires to the binding screws, under the base, and, the pulley being fixed on to the axle of the armature opposite to the commutator, the machine is now ready for use. To rotate the armature at a high speed it is necessary to connect the pulley by an endless band with a large, heavy wheel which can be rotated by hand.

For continuous work, as we cannot always be turning the wheel, a small steam-engine or water-motor must be employed. Worked in this way, the machine I have described can be made to light 2 5 candle-power lamps of 6 volts, and give about 12 volts of current. This is not much, of course, but by enlarging the proportions of the various parts, you can make as large a dynamo as you like; only the power required to work it naturally increases considerably. This machine will do a great deal of the work of a battery—for example it will run an induction coil or an electro motor at full power. By connecting two brass handles to the binding-screws by wires, you will get a powerful shock if you hold them while some one turns the wheel connected with the pulley; in fact, the shock is too powerful, and the person turning the wheel must be prepared to stop when the victim has had enough. If these handles are dipped into a glass of water slightly acidulated with sulphuric acid (to enable the current to pass more freely), and the dynamo briskly turned, you will soon see bubbles rising from the handles—which must, of course, be placed separate from each other—consisting of oxygen and hydrogen gas, into which the water is being decomposed by the force of the current. Water being composed of two quantities of hydrogen gas to every one of oxygen, it follows that double as much hydrogen will come off the handle which evolves it as will come off the other of oxygen, and this you will soon see to be the case; the bubbles on the former being much more numerous than those on the latter.

Now take a 5 candle-power 6-volt electric lamp, and fasten it on to the wires coming from the binding-screws (removing the handles) by the platinum loops at the top. If the dynamo is now briskly turned, you will find that the lamp will light up well, and as long as the wheel is turned and the dynamo is buzzing, so long will the lamp continue to glow. By turning the dynamo by steam or water-motor we have, therefore, a means of producing a continuous light, which will not drop at the end of a few minutes as in the case of a battery. This is the method by which all public buildings, etc., are lighted.

There is said to be always sufficient residual magnetism in the soft iron core (at any rate if constructed of ordinary soft iron, not specially annealed) to act on the armature when revolved, and this, acting on the magnet, increases its magnetism so that they react on each other until the maximum effect of the dynamo is reached. This is the case with the majority of dynamos used for lighting, etc.; but if you are of an experimental turn of mind, and are possessed of a battery as well as the dynamo, you can try the effect of magnetizing the soft iron cores by sending a current from the battery through the coil.

To do this, disconnect the wires from the magnet-coil from the binding-screws, and connect them with the terminals of the battery. The whole current from the dynamo now comes from the armature, and you will find that this current is considerably increased, sparks flying about in all directions when the handles from the binding-screws are approached to each other or rubbed together. The water will now be decomposed much faster, and you will be able to light an additional lamp or two, according to the strength of the battery.

Fig. 11 gives an idea of the positions of the parts of the dynamo when complete; it is not an easy thing to draw, and I can only hope the rough sketch will be intelligible to my readers. The spring A is below the roller of contact breaker, and the spring B above it, the diagonal line on the roller representing the vacancy between the brass pieces covering the wood. The wires from the ends of the magnet-coil go through the base, round the bottoms of the pillars A and B, and join the other wire between the pillars and the binding-screws. The wire from the pole on which the wire comes from outside the magnet is joined to the binding-screw A in the figure. The other wire comes from between the poles, and is joined to the other binding-screw. If you can find out, by means of a galvanometer, which binding-screw is conveying the positive current, the wire from the south pole of the magnet is to be joined to the wire from this, and that from the north pole of the magnet to the wire conveying the negative electricity.

Whenever you join the wires, be sure to scrape off all the insulating material, and twist them firmly together; a little solder is an improvement. Whenever the wires cross the iron work be sure the insulating material is quite sound at that point. It is a good plan to roll paraffined silk round the wires at these places. Cut grooves under the base, in which the wires may lie, or the dynamo will not stand evenly. The dark line in the middle of the top of magnet in Fig. 11 shows where the two parts join. They should be screwed up tightly together.

As a concluding illustration, I give a diagram of my own method of turning my dynamo (Fig. 12). On the leg of an ordinary table T is fixed the heavy iron wheel W, which has a groove cut in its circumference for the reception of an endless band B. These wheels may be obtained for a few shillings from any ironmonger, as they are made for various machines, such as laths, fret-saws, sewing-machines, etc. The wheel is held by an ordinary screw fixed into the leg of the table, and revolves on the screw. The endless band (tape will do) passes over the groove and over the pulley of the dynamo placed on the table above the wheel.

It is better to let the pulley of the dynamo project beyond the end of the base, as shown in Fig. 11, in order to be able to connect it with a wheel placed below it, if required.

The best results are produced from the dynamo when the resistance of the interpolar (i. e. the lamp, or whatever it may have to work) is equal to the internal resistance of the machine. It is sometimes required to send a current through a greater resistance than this, and then it becomes necessary to employ what is familiarly termed a “shunt.” If one lamp of high resistance is coupled to the dynamo, the resistance may be too great for the current to get round the magnet in sufficient quantity to give the required electromotive force. Supposing that this is the case, we make a second pathway for it by joining on a piece of iron wire (about ten inches of No. 30) between the two binding-screws, the lamp being connected with the same binding-screws, only further off. The result of this is that the current goes round by the second pathway and excites the magnet more powerfully, and this, in its turn, excites the armature more strongly, and so on, until enough current is produced to light up the lamp. The resistance of the shunt required depends on the resistance of the lamp. If this is low no shunt will be required, if very high the resistance of the shunt must be lowered, or else enough current will not pass to magnetize the soft iron cores, and the dynamo will give no current. The lower the resistance of the shunt required, the less wire we use.

Some Toys Worked by Electricity.

PART I.
THE ELECTRIC TRUMPET.

There are many toys which one meets with in the scientific stores, the making of which for themselves would give great satisfaction to enterprising devotees of the electrical art. They are for the most part easily constructed, and a great deal of amusement can be derived from them. I have my doubts whether the fathers and mothers of the amateur electrician will thank me for introducing the subject of the present article, but they must take comfort in the thought that if it works well it shows real constructive power on the part of the maker.

For the benefit of those whose capability of working in metal is limited, I am first going to describe the making of this remarkable instrument in its simplest form—a form, in fact, so simple that any one can make it and achieve success in a few hours.

First of all we want an old tooth-powder box. These are all made the same size, and consequently it is unnecessary to give dimensions. The top of the tooth-powder box is to be taken, and by means of a fretsaw (this invaluable tool should be in the hands of every boy who likes carpentering; there are many uses to which it can be put quite different for what it is intended for) a circular hole is to be cut out about ⅛ inch less than the inside—that is to say, a rim of about ⅛ inch is to project all around from the rim of the lid.

We now want what is known in photography as a “ferrotype” plate—i. e., a piece of very thin sheet iron. Most dealers in photographic goods will not sell less than four or five dozen of them, and this is too many for us. A photographic friend will let us have one gratis, or a professional photographer may agree to part with one for five or ten cents if he is attacked when in a good temper.

The ferrotype plate having been procured by some means or other, the next thing is to cut from it a circle just small enough to go inside the rim of the top of the tooth-powder box. You can mark out the circle before cutting it by painting the top of the rim of the bottom of the tooth-powder box with ink and pressing it down on the ferrotype plate, when enough ink will come off to guide the scissors, and of course the circle so cut will be the exact size required.

We now have to make the motive power of the machine, for there is plenty of work done in it, though it only makes a noise—no one can “make a noise in the world” without doing plenty of work! And to make this we take a piece of soft iron rod about 1½ inch long and half an inch in diameter, and cut two circles out of cardboard 1¾ inch in diameter. The soft iron rod can be bought from any hardware store, and it ought to be quite soft enough to work at once without doing anything to it; if it is not, it must be heated red-hot in a good fire and left among the coals over-night to get cool very gradually.

Personally I have always found that the ordinary bars of soft iron bought from any hardware man are amply soft enough for any electrical work.

You must get the hardware man to file the ends of your bar flat; if they are not filed you will have to do it yourself, and a fine job it is!

Now we go back to the circles of cardboard. A hole is to be cut in each in the center exactly the size to admit the core of soft iron, then by slipping the circles over the ends we get a reel. Now a hole has to be made exactly in the center of the bottom of the tooth-powder box, and exactly so large that the core of soft iron will fit tightly into it; you can do this again with the fretsaw, the wood of which tooth-powder boxes are made is delightfully easy to cut.

Now comes the adjustment of the reel. You must put the circles on the core, and putting one end of the latter through the hole at the bottom of the box you must push the iron through until the top is exactly flush with the top of the rim of the side of the box. One of your circles will now be much further on the core than the other, and the one at the end that is not pushed through the hole must be adjusted close to the edge, leaving about ¹⁄₁₆ of the core projecting, so that we have now a reel formed at one end of the core, and held in position by the bottom of the box. The more stiffly the core fits the hole the better, and if it has to be hammered into its place, better still, only take care not to split the wood of the bottom of the box.

The circles, being now in their right places, must not be moved again, but the roller has to be wound with wire, for which purpose the core will have to come out of the box temporarily. Before beginning to wind the wire, get some thin paper (French note-paper is best), and wind a piece round and round the core between the circles, fastening it and the circles at its ends to the core by means of a small quantity of mucilage.

We now have to wind the wire on to the roller. The more wire the stronger the magnet will be, but sufficient will be about two ounces. You can get the wire at most hardware stores for fifteen cents an ounce. It is generally cotton-covered, of light green color; medium thickness should be used, not too fine, as this offers too much resistance to the current, and not too coarse, or it will fill the reel too soon.

We begin by making a hole near the core in the circle which is furthest on it, and push one end of the wire through a hole from the inside of the reel. About three inches should be pushed through to allow for future manipulation, and the wire is now to be wound tightly over the paper covering the core in even coils, layer on layer, till the reel is nearly full and we have arrived at within about three inches of the other end of the wire. This is now to be passed through another hole in the same circle as before, which hole will of course be further from the center than the first. The magnet will be much stronger if two or three folds of paper are wrapped round it between each layer of wire.

The coil is now constructed, and can be replaced in the tooth-powder box, passing the ends of the wire through two holes in the side or bottom made to receive them. Before leaving this part of the instrument I may remark that care must be taken that the covering of wire is quite continuous throughout, and has not got rubbed off at any points; if it has, you must wind fine silk over it to cover it up again. Should there be a break anywhere in the wire you must carefully scrape the wire off the two ends and twist the wires firmly together, if possible soldering them together and then wind fine silk over the join.

It is not necessary in this machine to soak the coil in melted paraffin, but might improve the insulation if the cover of the wire is thin. Only if there is a join and you have twisted, not soldered the wires together, you must not soak the coil in wax, or the melted wax gets between the ends of the wires and stops the current (this of course applies to all electro-magnets and should be remembered as a possible cause of failure.)

The core having been pushed through the hole again, up to the circle of cardboard, the ferrotype plate is placed in the top of the box, and the box is shut up. Now the ferrotype plate must be exactly free of the end of the core and that is all. You can test this by tapping it. If it vibrates in and out, it is all right; if the end of the core is too tightly pressed against it, there will be no possibility of moving the center in and out, and the core must be driven further through the hole till it is just free of the ferrotype plate.

Now comes another part of the instrument, viz., the contact-breaker. The following is as good a way of arranging it as any: Take a piece of sheet brass the exact length of the diameter of the top of the tooth-powder box and about half inch wide, and in the middle of it bore a hole which will admit a brass screw—with a milled head preferably. The screw should fit tightly into the hole, so as to screw easily up and down when turned. To the end of the screw, which is cut off flat, is soldered a short piece of platinum wire, inserted in a hole in the end of the screw made to receive it; it can be fastened by any other means, as long as it will screw up and down and is in contact with the brass screw. Adjust the screw so that the platinum point is within a minute distance of the ferrotype plate when the brass support is screwed down at the ends to the side of the box lid, and screw it down with small screws firmly in its position.

Before this is done, however, a thin strip of platinum foil should be soldered to the upper surface of the ferrotype plate, or otherwise fastened to it—elastic glue will answer—this strip terminating in the center, and reaching to the edge of the plate, leaving a short piece over. A very thin strip will be enough, of the shape of P in Fig. 1. Now the ferrotype plate is to be placed in position again (the side of which the platinum foil is fastened being outwards, and the end of the foil going down between the edge of the ferrotype plate and the wood into the inside of the box), and the end of the wire from the coil which was left inside the box is to be securely fastened, either by soldering or otherwise, to the end of the platinum foil which was left loose, so as to be in metallic connection with it. A wire can now be twisted round or soldered to the screw with the platinum point, and the instrument is complete.

It has taken some space to describe, but I made my own in about half an hour. Fig. 2 gives a general view of the parts put together.

The lid of the box should be tightly fastened down by four small screws, two of which may be those which fasten on the brass strip holding the screw.

Now to consider its action. The wire I in Fig. 2 is connected to one wire of the battery, and the wire G to the other. The current then starts from the battery, round the coil B, converting the core into a magnet, and up the wire H to the platinum foil P, along the platinum foil, which was fastened to the upper side of the ferrotype plate F, to the platinum wire which tips the screw C. It then goes up the screw C, along the brass piece E, which is fastened to the box by screws, as shown in the figure, to the wire G, and so back to the battery by the other wire.

The screw C must be therefore screwed down till the platinum wire at its tip is just in contact with the foil on the ferrotype plate. Now of course when the current goes round the coil, and thus converts the soft iron into an electro-magnet, the latter instantly attracts the ferrotype plate which is immediately above it. But the latter moving its center near the core, the platinum foil which is attached to it is thereby moved out of contact with the wire on the screw C, and the current is instantly stopped. Thereupon the attraction of the magnet ceases, and the ferrotype plate flies back to its former position and so joins the platinum wire and foil, and starts the current again, and the former process is repeated. The ferrotype plate therefore vibrates with tremendous rapidity between the core and the platinum screw. Now the vibrating armature of an ordinary coil makes quite a hum when hard at work, but of course a large plate such as this makes a much louder noise, consequently you will hear a ferocious buzzing like an army of millions of bees let loose from a hive, and on screwing the screw C up or down till you get to the correct point you will get a shrill note very like a penny whistle. If screwed up the vibrations are slower, and a deeper note is produced; if screwed down the vibrations are more rapid and a higher note is sounded. Therefore you can amuse yourself by screwing it rapidly up and down, or adjusting it by pressing the brass piece with your finger, and a little practice will enable you to bring out a sort of tune produced by electricity!

When you have become tired of jingling out your tune you can fix the electric trumpet up in a permanent position, adjusting the wires from the battery so as to pass through an ordinary “press” which may be in another room. The trumpet will then begin buzzing or hooting whenever the button of the press is pushed in, and stop when the pressure is released. In this way of course the trumpet will act as a “call” instead of a bell, and as the double wire can be easily hidden under the carpet and in dark corners, and painted to match whatever wood-work it crosses, you can arrange it from an up-stairs room to a down-stairs one or vice versa with very little trouble. I give an illustration of the method of connecting the battery and trumpet with one switch or “press,” to show how to arrange the series. (See Fig. 3.)

The trumpet made in the very simple way I have described will not produce a very loud noise, but quite loud enough, if properly put together, to attract a person’s attention who was in the room when it went off. The sound can be rendered louder by fixing a cardboard funnel or “cornucopia” to the front of the tooth-powder box to make a kind of horn.

PART II.

The trumpets sold in the shops, as a rule, make a very loud noise indeed—in fact, a little of it goes a very long way with most people. The increased sound is probably due to the body of the trumpet being composed of brass, which, vibrating in unison with the ferrotype plate, increases the sound. Wood will therefore not give so loud a sound, and if you can construct the case of metal you should certainly do so. The vibrations of the plate, and therefore the sound, may also be increased by using a horseshoe magnet, the two poles attracting the plate more strongly. In the bought trumpets the case is shaped like a horn, in which the magnet is placed, the platinum contact-breaker being behind (where it is in the one I have described, supposing there was no bottom to the box and the magnet was supported by a bar across from side to side, the cornucopia being placed on that side of the box, instead of the other, with the magnet inside it). I think it is unnecessary to describe their construction further, as the principle and details of construction of the simple one I have described will apply to any, and any method of structure may be adopted which suits the mind of the maker.

The trumpet having been made I will now give you a plan of fitting it up which adds enormously to the effect. We want to hide the trumpet so that no one shall know where it is. My own plan of doing this is as follows: I have made a wooden erection, of which I give a drawing which will explain itself. It consists of a back with a shelf at the bottom and a kind of canopy at the top. It can be made almost any size, small or big, to suit the occupant of the shelf. My own measurements are about as follows: From the top A to the bottom B, the length of back piece, including bracket, 1 foot 3 inches. Breadth of back 5¼ inches. Side of canopy (D), breadth 4½ inches, height 3½ inches, breadth of front (C to D) 5¼ inches; height of course the same as sides. The top piece will then be about 5¼ inches by 4½ inches. The shelf at the bottom is about the same size as the top of the canopy, and is supported by a bracket of rather thick wood, which you can carve as elaborately as you like.

Now take the electric trumpet, whether made at home or purchased, and fasten it to the under side of the canopy (this is best done before the sides are put on), and fasten a double wire behind the back (cutting a groove for it to go in) up to the back of the canopy, where it goes through and divides, one wire being fastened to one terminal of the trumpet and the other wire to the other. The double wire goes right down the back and emerges at B. Obviously if you now join your press and battery on to the double wire, when you squeeze the press the trumpet will squeak. But here we are going to practice a little innocent deception, and to that end we go to a toy shop and purchase a small and pretty doll of the male sex, and if you can get one (or dress one up) attired as a soldier or trumpeter, by all means do so. The doll is now to be fixed on to the bracket by means of a long wire—say a hairpin bent out straight, one end being pushed into the wood, the other passing up one trouser leg of the doll and into its body; the wire is thus completely hidden and is much better than glue, as it admits of the doll being placed in a natural attitude, and being removed if required. In one of his hands you must make him hold a small trumpet (this is a very expensive item; it will cost two cents) with the mouthpiece to his mouth, as represented in the picture.

The whole thing is now fastened to the wall in a convenient place, by driving nails through the back, and the double wire is completely hidden by passing it behind furniture, books, etc., down to the floor. There is great scope for ingenuity on the part of the worker in hiding the wire, and no definite instructions can possibly be given. In my own case I have no back piece below the shelf the support being against the wall. The wire descends behind the support (to B in the picture), and below that I have hung a “date calendar” over it, it makes a turn to the right and goes down behind a chiffonier covered with books to the floor. Under these circumstances no human being could possibly tell that there was a wire at all, and there being no back piece under the bracket (so that the paper of the room can be seen), nothing but the support touching the calendar, it does not look as if any wires could possibly be hidden anywhere.

Now, if you press the button, of course the trumpet squeaks, but the doll being just underneath it, and the trumpet being in the dark under the canopy, no one thinks it is a separate instrument, but of course every one jumps to the conclusion that it is the doll blowing! Hide the battery in a corner in a black box, the wires coming through the side next the wall, and the press in a dark corner, or on the floor under a table so that you can put your foot on it while your hands are free, writing, etc.

You can of course now tell the doll to blow, at the same moment putting your foot on the press, when the trumpet blows accordingly. Of course this is mysterious to the last degree to the uninitiated friend to whom you are displaying the doll, as you may be any distance off from the doll with your hands free, speaking to him across the room.

The wooden erection to hold the doll can be painted any color; preferable the back should be black, as it shows off the doll. In front of the canopy you can paint a monogram or heraldic device. If the doll is one of those extremely pretty little specimens which can be procured at any good toy shop for about twenty-five cents, dressed as base ball players, soldiers, etc, (what our grandmothers would have thought of them in their young days it is difficult to imagine) it will really be quite an ornament to the room, independently of its electrical qualities.

This chapter has outgrown the space I meant to occupy, and I must wait for the next to tell you how to make the doll work from various parts of the room as you walk about and talk to him, and how to make the battery. The best battery to use is to Leclanche. You can use three or four cells of No. 2 size according to length of wire through which the current has to pass.

In my next chapter I will try and explain how to make an electric drum, so that you can have a kind of drum and fife band.

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 ⅝ inch broad and ⅛ 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 ⅜ 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 1⅛ inch long, ½ inch broad, and ⅛ 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 ⅛ 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.