R. A. R. Bennett

“ There is no reason whatever why a boy who is accustomed to metal working should not succeed in making the small machine described if he first masters the principles of its construction.
The advantage of a dynamo, I may here remark, is that by its means we are able to produce a current of voltaic electricity at any moment by turning a wheel without bothering with acids or carbons, or zincs, or any other of the various articles necessitated by the use of a battery. ”

“ But for experiments in which a strong continuous current is required, the dynamo comes to the front, as there is no “polarization” to detract from its value, as in the case of the battery. One does not always want to be messing with chemicals in setting up a battery, when one only requires the current for a short time, and the dynamo is always ready, and merely turning the handle produces the required current in a moment. Besides this, viewed merely in the light of a magneto-electric machine, it will give a considerable shock to any one who holds two handles fixed to its terminals. ”

“ 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 ”

“ 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 current can be regulated by means of a “regulating tube,” that is simply a brass tube which is made to slip over the core between it and the primary coil; the farther the tube is pushed over the core, the less powerful the shock. The dimensions of the coil being the same, a little ingenuity will enable any one to affix a regulating-tube. I will only say that instead of winding the coil direct on the core a tube of brown paper is formed a little larger than the core, and on this the wire is wound. ”

“ 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. ”

“ A wire is now fastened to the end of each tube, the other ends being inserted in the binding screws connected with the thin wire of the secondary coil, which are at the opposite corners of the base to those which are joined to the ends of the primary coil. When the coil is buzzing, if these handles are tightly held, a powerful shock will be felt, in fact, a weak battery only should be used with the coil of the dimensions given, or it may be impossible to release the handles, and this is too strong to be pleasant. ”

“ When cold you may remove the binding wire, and the wax will be found to hold all the pieces together in a solid lump. The two pieces of wood must now be fixed one at each end of the core (the holes being the same size as the bore of the brass tube, the core should fit into them quite tight) , one of them (the front) being pushed a little distance over the core, so as to leave about 1⁄4 of an inch of the core projecting from it, the other one only being pushed on sufficiently far to make the end of the coil flush with the wood (Fig. 3) . ”

“ The end of the contact breaker is now soldered to this brass strip in such a way that the piece of soft iron at the other end is exactly opposite the core and about 1⁄16 inch distant from it. The screw of the paper fastener must now be tipped with platinum by cutting off the end and drilling a fine hole in it, in which hole a small piece of platinum wire can be soldered. The amount of wire and foil required, although very minute, will cost you about twenty-five cents, platinum being a very expensive substance. It can be bought from a chemist or electrician. ”

“ 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. ”

“ If the needle can be soldered to a metal stand, or the cork covered with tinfoil, the wire is not needed. On rubbing the paper and placing it on the tray, the electricity passes up the wire into the needle, thence into the wire cross, and escapes by the bent portions of the wires, each of which should be filed to a point. In escaping it electrifies the surrounding air, and this, according to the law that “like electricities repel each other,” has a reacting force on the wire arms. Accordingly the windmill begins to turn, and may attain a tolerable rate of speed if the tray is strongly charged. ”

“ 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 [Pg 40] 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. ”

“ 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. ”

“ 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 ”

“ 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. ”

“ When the arm of the switch is central neither lamp lights up, or, if you are fitting up the trumpet and drum, the press will not work either when the switch is in this position. This is an advantage, as when people get too inquisitive you can turn off the current, and then whatever they do they will not make the trumpet or drum work till you turn it on again, which you can do when you want them to work for you! ”

“ If the galvanometer is not at hand we must take our chance; the greatest possible care must be taken in winding the secondary wire, as this thin wire is extremely brittle. The insulation must now be improved by plunging the whole coil into a deep vessel large enough to contain it, which is full of melted paraffin. This must be placed near the fire, so as to keep the wax melted, and the coils must be left in it to soak for an hour or two. When the paraffin has thoroughly permeated through it it can be taken out and held above the vessel to drain. ”

“ To most boys electricity offers many attractions, and as I have recently constructed an induction coil out of materials which are cheap and easily obtained, I think I shall confer a benefit on many readers if I give them a short description of how this was accomplished, so that if like-minded they can proceed in the same way. ”

“ 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. ”

“ It is very confusing business setting up the wires so as to produce the right effect, which is to change the current from drum to trumpet and vice versa in a moment by merely altering the handle of the switch. ”

“ 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. ”

“ At the same time it is perfectly evident that there are heaps of our readers who are very anxious indeed to try, at all events, and as we must aim at more elaborate apparatus as we advance in electrical knowledge, it is a pity not to endeavor to supply them with the help they need. ”

“ A candle which has just been blown out, leaving the wick glowing, can easily be lighted by means of the charged Leyden jar if a piece of bent wire is held touching the outer coating and the other end on one side of the wick while the knob is approached to the other, so that the spark passes through the glowing wick. ”

“ 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. ”

“ 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) at [Pg 41] tired as a soldier or trumpeter, by all means do so. ”

“ Frictional electrical machines can be purchased from any electrical instrument makers, at a small price, and with these experiments mentioned are more readily performed. In this article I only mention experiments that can be performed with materials to be found in every house, or the necessaries for which can be procured from a shop for a nominal sum. ”

“ 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 [Pg 30] 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 ”

“ When this moves towards the magnet, contact is broken and the core is instantly de [Pg 16] magnetized, so that the spring flies back and contact is made again. The screw is adjusted so that the contact is broken just as the soft iron touches the core. When the battery is joined on, the contact-breaker will fly backwards and forwards with tremendous speed, making a loud, buzzing noise, while brilliant sparks will appear between the platinum wire and foil. ”

“ The best places are under furniture with rather long legs; the foot of the operator can then be placed on the springs, and so make them meet and the trumpet or drum sound without the least chance of detection. ”