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.
Fig. 11.—Dynamo Complete. Ground Plan.
M, Top of magnet. C C, Coils of wire around magnet. P, Driving-pulley. T, Armature. A B, Pillars of springs which take current from the roller F. S S, Binding-screws. The dotted lines show the position of the wires beneath the base. The wires from the magnet coils go round the pillars A and B, and are joined to the wires from them to the binding-screws S S at the points K K. The wires from the armature coils join the brass-covered roller at F and G.
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.
Fig. 12.—Hand-wheel Arrangement for Working Dynamo.
B, Endless band passing round pulley of dynamo. H, Handle of wheel. S S, Handles attached to binding-screws of dynamo.
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.
Fig. 1.—Shape of Platinum Foil, p, Fastened to Ferrotype Plate, f.
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.
Fig 2.—Section of Simple Electric Trumpet Showing Details of Various Parts.
A, Tooth-powder box. B, Electro-magnet. C, Brass Screw. D, Flange for screw. E, Brass support for screw. F, Ferrotype plate. G, Wire attached to brass strip supporting screw. H, Wire from end of coil attached to platinum foil at P. I, Wire from the other end of coil passing through bottom of box.
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.)
Fig. 3.—Method of Connecting Trumpet to Battery and One Press.
When the button P is pushed in, the spring A touches the metal piece B, and completes the circuit. On removing the finger from P, the spring separates from B, and the current is stopped. P, Press. L, Battery. T, Trumpet.
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.