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 1/8 inch less than the inside--that is to say, a rim of about 1/8 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.