Fig. 1.
This is the ordinary Æolian harp, but in this country and on the Continent there are many more complicated forms of the instrument in existence. The Æolians of the four Strasburg Cathedral towers, for instance, are well known to tourists. At the castle of Baden Baden also the harps are a great attraction, and we here give a sketch of one of the loudest of these celebrated instruments.
Fig. 2.
Fig. 2 enlarged (89 kB)
It is set well back in the gallery, and the window opening is gradually contracted by the curious shed, of which one side is removed to show the construction, the air passing out through the grating, which is only slightly wider than the harp. Of the harp itself we give the plan and section, and to avoid fractions we retain its original measurement in mètres and centimètres—sixty-one centimètres being as nearly as possible two feet, and a mètre being a hundred centimètres, or thirty-nine inches and three-eighths.
It will be noticed that this pattern of the instrument has strings on both sides, and that the inner edge of the box is fitted with narrow sound-holes. The front of the box is of thin wood steamed into shape, and fitted round the curved ends as carefully as the sides are built into the back and belly of a violin.
Fig. 3.
In Kircher’s harp, the older form, the screen fits into a window, the instrument is hung on an iron rod, and has a great many strings stretched over broad sound-holes. The case is freely perforated, and is hung so as to half overlap the aperture which gives admittance to the air.
Fig. 4.
Kircher for a long time had the credit of being the inventor of the Æolian harp, but it is of much earlier date. It is, in truth, a very obvious contrivance, easily made, and not susceptible of much improvement. In our last figure we give its latest form, which differs from the others only in the arrangement of the screens. These are devised to throw a strong draught on to the strings, without having to be fitted into a window frame; but in this, as in all the other forms of the wind harp, it requires a pretty strong breeze to bring out its full tone.
The best penny whistles are tuned in D, and we shall assume that ours is so. Occasionally, however, they are in a different key, but this does not alter the fingering, as the intervals are the same, and the same air will be played with the same stopping. There are six holes, which, commencing from the mouthpiece end, we will number 1, 2, 3, 4, 5, and 6. Of these holes, 1, 2, and 3 should be worked by the fingers of the left hand; 4, 5, and 6 by those of the right.
The lowest note of the instrument is sounded when all the holes are stopped—the reason, of course, being that the vibration takes place along its whole length. To get this note is, however, not easy, as there is a great tendency to blow too strongly, and so get into overtones. ‘The very gentlest breath will give the dulcet note we seek.’ Having got the D, and it must be a good full note, unstop 6, so as to keep only 1, 2, 3, 4, and 5 shut, and you will with the same strength of wind sound E, the note that comes just above it in the scale.
F-sharp, the next note, is got by unstopping 5 and 6; G, the next, by unstopping 4, 5, and 6; A, the next, by unstopping 3, 4, 5, and 6; B, the next, by unstopping 2, 3, 4, 5, and 6; C-sharp by unstopping all the holes.
Nothing can be easier of remembrance than this. The fingers are lifted from the holes one after the other, beginning at the bottom of the instrument, and with every finger you lift you rise to a higher note. But we have not quite finished the octave. How do you get the D? By leaving 1 open and closing the rest. And one note we passed, C-natural, how is that obtained? By unstopping 1, 5, and 6.
We have thus gone from D to D and got our first octave. How do we get the next? By blowing a little stronger, a very little, and unstopping on the same principle as before. Beginning with D, we have 1 unstopped, and then closing 1 and opening 6 we get E; opening 5 and 6 we get F-sharp; opening 4, 5, and 6 we get G; opening 3, 4, 5, and 6 we get A; and opening 2, 3, 4, 5, and 6 we get B, just as we did before, the fingering being the same, but the notes, owing to the stronger blowing, being an octave higher. The next note, C-natural, is obtained by unstopping 1 and 6; the next, C-sharp, is given by clearing 1, 5, and 6; the next, D, by clearing 1, 4, 5, and 6. And so we have completed our second octave. But we have four more notes yet that can be safely sounded without giving our audience the ear-ache, and of these E is got by unstopping 3 and 6, F-sharp by unstopping 2 and 5, G by unstopping 2, 4, 5, and 6, and A by unstopping 1 and 6. We thus have a range of twenty-one notes, including the two C-sharps and three F-sharps, so that our instrument is by no means a defective one, and the only difficulty in playing it is the avoidance of overtones where the artistic merit comes in at the middle D. It is, however, easy to remember that if you blow softly you get the lower octave, if you blow firmly you get the higher octave, if you blow wildly you get the peculiarly metallic screech which has made the penny whistle the abhorred of civilised men.
And now, having cleared the ground—for it is not our place here to teach the ‘rudiments of music,’ and in showing how to produce the notes we have gone as far as we need in a ‘monograph’ such as this—we will unfold the little scheme we had in view when we started on this description, and introduce to our readers the Boy’s Own Mechanical Penny Whistle!
The principle of the whistle, and, indeed, of all instruments of the flute and flageolet type, being that certain of the holes in different combinations should be left open in order to give the different notes, and that the expression should be given by the modulation of the wind strength, it follows that the fingering is merely mechanical. A substitute for the fingering can therefore be found, and the simplest substitute we have come across is a sheet of wrapping-paper!
Take a strip of brown paper or manilla paper, just wide enough to cover the holes on the whistle, or rather overlapping about half an inch on each side of the end holes. Mark off on the paper at each end of the strip where the centres of the holes come, and rule parallel lines the whole length of the paper, so that as it pulls over the whistle each of the six lines will pass exactly over the centre of each of the six holes. On each side of these six lines draw a line so that the space between the two new lines on each side of the central one may be half as wide again as the diameter of the hole across which it is to move.
Now rule the paper crossways in lines three-sixteenths of an inch apart parallel to each other, and strictly at right angles to the lengthway lines. The strip is now ready for you to stop out your tune on the principle of the Jacquard loom or the American organettes now so common amongst us.
Fig. 1.
First find the shortest note the air contains—in our example, the ‘Blue Bells of Scotland,’ this is a quaver—and each of the ruled spaces cut by the lines through the whistle-holes must represent this interval of sound. Double the space will give double the interval of sound, and hence, if one space represents a quaver, two spaces will represent a crotchet. In the Blue Bells the first note is D, a crotchet; and as D is produced by unstopping 1, we fill up on the first line a double space. The next note is G, a minim; and, as G is produced by unstopping 4, 5, and 6, we fill up space on those lines, making them double the length of the first space, the note being double as long. The third note is a crotchet, F-sharp, and this is marked by blacking in 5 and 6. There is no need to continue this explanation in detail, as the method is sufficiently clear, and the notes are given in Fig. 1, and can be compared with the scale. One space equals a quaver, two spaces a crotchet, four a minim, in this instance; but should a quicker tune be selected the spaces may have to be given values of less interval. The simplest plan is to find the shortest note, and then, seeing how many of it would go to a bar, to mark off the bars along the edge of the scale, and then fill in at your ease. In our example eight spaces go to a bar, because the shortest note is a quaver, and eight quavers make the semibreve. Having filled in the notes, take a sheet of glass, lay the paper on it, and with a sharp penknife cut away all the spaces you have blacked—in short, make a stencil of your brown paper.
We are now ready to commence. Hang the stencil over the whistle so that the holes you have made in it pass over the whistle holes, and blow gently as you drag it along. As the holes are cleared one after the other the notes are given forth, and the whistle can be played almost as easily as a barrel-organ—if you can only keep the paper straight and flat on to the tin. But this is not always easy to do, and so we require a further invention, which the accompanying sketches sufficiently describe.
Fig. 2.
Fig. 3.
Fig. 4.
Fig. 2 is a piece of deal, the shaded part of which shows where it is to be cut away. Two of these blocks, each of them about three inches long and two inches wide, are required. Fig. 3 shows one of the blocks after it is in shape. The top groove in one must be larger and deeper than that in the other, owing to the tapering form of the whistle—for the whistle must fit firmly. Two rollers, made by sawing pieces off a broomstick, are taken of sufficient width to carry your stencil easily, and these are fixed as shown in Fig. 4. One has a handle made of bent wire, with the point that is driven into the roller flattened out and hammered in straight, so as to give a firm hold; the other has two spindles only.
The rollers are fitted with an elastic band, so as to keep them close together and make them act as a miniature mangle. A slip of wood is fastened beneath the blocks to keep them in position. If it is intended to play the air through only once, and to shift for each repetition, a weight is affixed to one end of the paper to keep it flat; if, however, the air is to be repeated without a pause, the ends of the stencil have simply to be pasted together, and a flanged roller hung in the loop, as shown in the cut.
This is all the contrivance consists of. It is effective, and easily made. The only difficulty in playing with it is the need of the stronger blow in the upper octave, a difficulty soon mastered after a little careful practice. The principle of the perforated keyboard is applicable to so many instruments that these rough notes on its construction may prove valuable, even if it be not applied to the humble whistle. The humble whistle! Alas! But let it not be imagined that squeals and screeches are the sounds the poor whistle was made to produce. Any other instrument, if improperly used, will give forth its appalling overtones. Treat it properly, gently, and firmly, and you will find it as sweet-toned as a flageolet.
HOW VERY FUNNY!
In the whole history of science, from the Dark Ages down to the present time, there has been no record of any parallel to the extraordinary progress which electricity has made of late years.
It is comparatively but a short time since that people were marvelling at the telegraph, and the newspapers used to write gushingly about ‘compelling the lightning to bear our messages,’ and all that sort of thing. I dare say many boys who read this can remember what a sensation the electric light made when displayed on the top of one of the buildings in the Strand—they need not be very old boys to have seen it there. Nobody would be very much attracted by such a light anywhere now.
There is scarcely a single art, manufacture, or science into which electricity has not been pressed to do good service. Electric lighting has become a matter of course, both indoors and out; and, while it has been proposed to annihilate night in the city of Washington by setting up four huge electric ‘suns’ on the hill of the Capitol, so rendering any other illumination in the streets and houses as unnecessary as in the day-time; a modified lamp of a few ‘candle-power’ has recently been devised for small rooms, supplied by a little battery which might stand on the mantel-piece. Tennis is played and photographs are taken by the electric light; electric bells are as common as door-knockers; electricity is proposed as a means of killing sheep and bullocks in the slaughter-house and criminals on the scaffold, and is used by the physician as a remedy for the preservation of life.
On board some of our great men-of-war the captain can sit in his cabin and not only see the position of the helm, the speed of the ship, and the direction in which she is steering, but can fire every gun she carries—all by electricity. Electricity springs the deadly mine on the field of battle, and animates a sixpenny toy sold in the Lowther Arcade. Even the railway engines, tram-cars, and screw-boats propelled by electric force which have been lately invented cause but little surprise now, so habituated have we become to the gigantic strides of this nineteenth-century infant!
I am not going to preach a sermon upon it, however, as you may be expecting from this terrific introduction; nor am I going to bore you with a lecture on coils and currents and poles and induction, or any other technical details. But it occurs to me that a brief mention of one or two of what may be termed the minor applications of electricity—one or two only out of thousands—will perhaps interest you, as illustrating how widely spread the influence of the science has become, and how it penetrates into nearly all the affairs of life. To my mind, the fact of telegraph and telephone wires stretching for hundreds of miles across uncleared jungles and through virgin forests, as they do, is not half so strong an evidence of the pitch to which it has arrived as its being adapted to a conjuring trick.
At one of the places of amusement in Paris some ‘sprites’ carry wands which sparkle out and fade again as required, flashing in time to the music. But a much prettier and more elaborate arrangement has been brought out since, though I believe it has not yet been presented to the public. The performer—magician, fairy, or whatever he or she may be—wears a fancy dress, which is embroidered all over with what look like large glass beads or imitation pearls. These are in reality tiny electric lamps, all connected with each other by wires covered with silk in the texture of the dress, and communicating with two little iron plates in the heels of the fairy’s boots. Nothing remarkable, of course, is seen until these two iron discs come into contact with a certain spot—which is reached just at the appropriate moment—when every bead bursts into dazzling light, and the fairy becomes clothed with white living fire in an instant! Then she steps away from the communication with the batteries below, and the beads are as suddenly dead again.
Electric alarums for the detection of burglars have long been in vogue in the shape of bells and gongs, so arranged as to be sounded directly the fastening of a door or window is tampered with, and electric ‘booby-traps’ have even been tried, designed to give the thief a severe shock or take him prisoner—the result generally being that the master of the house or the servants get caught in the snare themselves half-a-dozen times, after which its use is discontinued.
The weak point in all these things has been that, from their costly and intricate nature, they could not conveniently be applied to every accessible situation, and that the mechanism was always liable to be thrown out of order. The burglars would carefully avoid meddling with the shutters and doors to which these appliances were known to be affixed, and would gain an entrance at some unprotected spot. Now, however, somebody has patented an electric mat, which can be put down anywhere at night, and which sounds an alarm directly an intruder steps upon it.
Galvanism is employed, as is well known, by medical men, to restore power to paralysed limbs, to revive people who are faint almost to death, and to cure diseases. Dentists owe a good deal to electricity, and their patients owe still more. When a surgeon wants to cauterise some very small spot deep down in the flesh, instead of cutting and burning all the way down, he now inserts a wire, which is shielded, except just at the part which will come in contact with the bad place; an electric current is sent through it, and the wire becomes red-hot.
Neater still is the way in which a needle is detected underneath the skin. I dare say you know that such a thing often gives a doctor a great deal of trouble, and it is an accident which you should be very careful to guard against. It frequently occurs to boys who run about the house with bare feet. The needle, having no head like a pin to stop it, slips right into the flesh. Sometimes the patient is not certain whether it is there or not, as it may have worked out again, for the danger in these cases arises from the tendency of the needle to travel through the flesh, doing great mischief as it goes along. What is the doctor to do? If he is quite sure that it is there and can feel it, he will of course cut it out; but he has to be very cautious. A needle is so fine and slender, that sometimes, even when he thinks he can feel it with the point of a probe, he finds himself mistaken. It has been suggested that a magnet hung over the part will turn if any steel lie concealed beneath—a very pretty theory, but one that does not answer when put into practice. But one may make quite certain about it by probing the flesh with a little instrument which is connected with a battery in such a way that directly the point touches metal the circuit is completed and a bell rings.
Perhaps this was founded upon the very ingenious probe, by means of which the great French surgeon, Nélaton, discovered the bullet in Garibaldi’s foot. He could feel something there, at the bottom of the wound; but whether it was only the bone, or a bullet embedded in it, he could not say. So he made a slender probe of rough, unglazed porcelain, and rubbed it against the hard substance. On withdrawing it, he found it marked with lead!
Still more wonderful are the medical uses of the electric light. Not only is it made to illuminate the eye to the very back, but the throat as well. A little glass-bead lamp at the end of a rod is passed into the mouth, the current turned on, and there you can see the tonsils, gullet, windpipe and all, a great deal more distinctly than the interior of St. Paul’s Cathedral on a foggy day: while, to a bystander, the patient’s cheeks and throat look as if they were made of pink glass and filled with fire inside. Further, a similar rod and bead have been actually lowered into the stomach of a very thin person, and were found to be plainly visible through the semi-transparent skin; and it is thought that this may be valuable at times in the detection of disease.
From surgery to sleight-of-hand is a long step, but we find conjurers quite as eager to avail themselves of the assistance of electricity as doctors. Robert Houdin’s book on Magic gives an account of the marvellous adaptations of this science, wherewith his private house and park were furnished. He invented many of the tricks performed with electric apparatus by his successors at the present day—not such comparatively simple ones as ‘spirit-rapping’ hammers and drums which answer questions; but clever mysteries like the iron chest which a child can lift, yet which defies the strength of a man, and the crystal cash-box. These are things which might puzzle even scientific electricians who are not in the secret. By means of the first the great wizard acquired extraordinary influence over the Arabs in Algeria, because it seemed to them that he could at pleasure take away the strongest man’s power in a moment and cause him to become as weak as a baby, restoring it again as suddenly. It depends upon the fact that a current of electricity passed through a bar of soft iron makes it into a huge magnet for the time being. The little iron box, which is to be raised or remain immovable as the conjurer wills, is placed upon a pedestal, within which is the iron bar, connected with wires to a machine outside in charge of an assistant, who, at a given signal, turns on the current.
The crystal cash-box is a casket, the top, bottom, and sides of which are made of glass, bound with wire at the edges. No deception seems possible; it is transparent right through, and is suspended over the heads of the audience by four slender wires attached to little hooks at the corners; yet several half-crowns are seen and heard to fall down inside it at the word of command.
You will naturally guess that the entire affair is under the influence of a battery ‘behind the scenes.’ The coins are first concealed within a ground-glass ornamental design in the lid, the glass of which is double. The lower slip would be just loose enough to allow them to fall, but is kept up by a bit of black thread, which rests against the wire. Just at this point the wire is made of platinum, which becomes heated by electricity much more quickly than copper or iron, being a bad conductor. Almost the instant the current passes this bit of platinum becomes red-hot, while the connecting wires are not affected; the thread is burnt through, down drops the slip of glass, and the half-crowns fall or slide out with a jingle.
We know that by the telegraph wire we can read what people write hundreds of miles away, and can hear what they say through the telephone. At the time when all these ‘phones’ and ‘graphs’ were being invented, one after another, almost daily, an American paper announced another novelty—the telegastrograph! You were to hold one end of a wire in your mouth and taste the orange, plum-pudding, or glass of wine into which the other end was stuck a thousand miles off! But although this was a hoax, it would hardly have been more wonderful, had it been true, than many real facts among the curiosities of electricity.
Nothing can be easier than to make the Leyden jar. Procure a smooth glass bottle, that is to say an unpatterned one; and let it have a wide mouth, though this is not essential. Thoroughly clean it and dry it, and paste on to it inside and out to the height shown in the illustration some sheets of tinfoil. Let the tinfoil cover the glass two-thirds or what not from the base, and leave no breaks below the line.
The best plan is to coat the inside first. Cut a circular piece of tinfoil a little larger than the bottom of the bottle, and paste it down with the edge pressed up against the side. Then drop into the bottle a well-pasted strip of foil the height you have selected, and just a trifle longer than the internal circumference of the glass on which it is to be stuck.
Having finished the inside, do the out. Cut a circular plate for the bottom, press it up round the edge and paste on the glass the strip for the exterior circumference, which should be of the same height as that inside. Then insert a piece of brass through a cork or mahogany stopper, fix a brass ball to one end and a brass chain to the other just long enough to rest on the bottom, wax or varnish the stopper, and the jar is complete.
Instead of lining the bottle with tinfoil, thin gold leaf or copper leaf can be used; and instead of the brass ball and bar a ball of baked wood and a copper tube. It was Harris who first used the baked wood; Hopkinson has experimented with Leyden jars in which sulphuric acid has taken the place of tinfoil! The form we have described is, however, the usual one, and as it is the cheapest it would be best to start with it.
To charge the jar the outside tinfoil is connected with the ground, and the inside is excited by means of the knob from the prime conductor of the machine. The electricity is, as the phrase goes, ‘bottled off,’ though ‘the fluid’ is no fluid, and is not ‘poured’ at all. Two conductors of large surface are separated by a rigid insulator, and hence the conditions are favourable for powerful attraction. That is all.
This simple apparatus, which takes such a prominent part in electrical experiments, obtained its name from having been invented at the old Dutch University, where Muschenbroek was at the time professor. In Germany it is called Kleist’s jar, from the name of another inventor, but it has been the custom amongst us to ascribe the honour of invention to either Muschenbroek, or Cuneus, his assistant.
It seems that Muschenbroek had noticed that excited electrics soon lost their electricity in the open air, and that this loss was quickened when the atmosphere was charged with moisture. Hence electricity was retained by surrounding its retainer with bodies that did not conduct it. To prove this he poured some water into a glass flask, put it into communication with the prime conductor of an electrical machine, and for fear of accidents judiciously handed it over to Cuneus to hold. When they thought it was charged enough, Cuneus tried to disconnect the chain from the conductor, and thereupon received such a lively shock in his arms and chest that he dropped the bottle and smashed it to pieces.
The professor was pleased; the assistant was not. He was ill for two days afterwards. ‘I would not take another shock for the kingdom of France,’ he wrote to Reaumur. And all the first experimenters with electrical apparatus were much alarmed at shocks which to us would seem hardly worth noticing. Poor Winkler, for instance, was so frightened at the unexpected experience that he ‘betook himself to cooling medicines to allay the fever.’
The shock received by Cuneus soon led up to the jar as we now know it. First water was tried, then mercury, and finally tinfoil. Muschenbroek’s experiments took place in 1746; in the next year Watson began to come to the front. He first fired gunpowder by electricity, then he mixed camphor with gunpowder and discharged muskets by electricity. Then hydrogen and spirits of wine were fired by the spark by means of a drop of water or a lump of ice.
Watson it was who put the inside and outside tinfoil coatings on the jar. Bevis suggested the outside; Smeaton, of Eddystone Lighthouse fame, suggested the inside. Watson’s experiments before the Royal Society attracted much attention to the science, though he had in some things been anticipated by the French, who had sent a discharge through 12,000 feet, and on one occasion had used the great basin of the Tuileries, giving an acre of water as part of the circuit.
Nollet sent a discharge from a jar through a regiment of 1500 men holding each other’s hands, and they were all shocked in the arms and shoulders. But perhaps the best known experiment is that of Franklin with his kite.
Two strips of cedar, fixed crosswise, with a large silk handkerchief tied at the corners, and a sharp-pointed wire projecting a foot above the upright, was all that Benjamin Franklin’s famous kite consisted of. It had an ordinary paper tail, a bellyband, and a long fine string, with a short piece of silk ribbon tied at the end. Just where the ribbon was knotted to the string he hung a key.
It was in June, 1752, when he let his kite up in the thunderstorm. He and his son, after some little difficulty, got it out to the full length of the string, and then stood up inside a doorway to keep the ribbon dry. A thundercloud passed over, and nothing seemed to happen. The experiment promised to be a failure. Gradually, however, the loose filaments of twine began to stand out at right angles, and were found to be attracted by the fingers; then a knuckle held to the key extracted a spark from it, and as the string got thoroughly wet in the pouring rain the electricity became abundant. With it the experimenters charged the Leyden jar, whose discharges afterwards proved the identity of the electricity of the thundercloud with the electricity of the machine.
Another famous experiment is that known as Lichtenberg’s figures. It is generally performed as follows. Hold the jar, charged positively, in the hand, and with the knob draw on a glass plate, cake of resin, or sheet of vulcanite, a series of patterns. Then put the jar on an insulator, and, lifting it by the knob, trace another series of patterns with the outer coating, so as to cross and intertwine with those made by the knob. Having designed the patterns, make a mixture of red lead and flowers of sulphur and dust it on to the slab. A curious thing will happen. The red and yellow will sort themselves out. The sulphur will stick to the positive lines, the lead to the negative ones, and the pattern will be given in two well-marked colours. The sulphur will be in tufts, the lead in spots. In mixing the powder the sulphur became negatively electrified, the red lead positively so, and hence the disposition of the materials.
The terms negative and positive were first used by Symmer as alternatives for resinous and vitreous. Symmer was the man who discovered the electricity in his stockings and charged the jar by their aid. His experiments were the same in principle as those of Cigna with his silk ribbons, but were much more astonishing.
When Symmer pulled off his stockings he noticed that they often gave a crackling sound, and when he undressed in the dark he saw sparks issuing from them. When he wore silk stockings for show and worsted beneath them for warmth the effects were more powerful. When one stocking was drawn out of the other they appeared inflated, and attracted and repelled each other like electrified bodies!
He experimented with a pair of white silk stockings and a pair of black silk stockings. When he wore both white or both black on the same leg, nothing happened; but when he wore a white and black on the leg, and pulled them off after ten minutes or so, they remained inflated, and showed the shape of his leg! Brought within eighteen inches of each other, they rushed together; then they were separated, and again became inflated, and again rushed together.
Experimenting with the two pairs held against each other, he found that they sorted themselves out, rushing each to each, until they gradually wasted away, and from legs substantial enough for the foundation of a family ghost story—a ghostly legacy—dwindled down into mere flabby pieces of silk. The electricity he obtained from these classical stockings was considerable. He charged a Leyden jar from the four of them, and secured enough electricity to shock himself up to his elbows, and to light a teaspoonful of spirits of wine!
One caution before we conclude. In every experiment, whether it be merely in shocking, in rendering luminous half-a-dozen eggs placed end to end by sending the shock through them, in perforating a card by passing a spark through it as it rests on the foil, in splitting wood by driving the wires in until their points are close to each other, in breaking a glass by passing a spark from knob to knob in water, however simple it may be, remember always to discharge by touching the outside first. Otherwise you may receive an unpleasant surprise, and, like Cuneus, come to grief with your Leyden jar.
In our chapter on the Leyden Jar we assumed that those of our readers who were likely to experiment in frictional electricity would be in possession of an electrical machine to start with. Many, however, may be desirous of building an electrical machine of their own. As this can be easily done, and as the cost of the materials is comparatively slight, we purpose giving a few practical hints on the subject which may be of use to those wishing to build and those anxious to repair if they only knew how.
Before we deal with the cylindrical machine we must, however, devote a few words to the electrophorus by which the jar can be charged if desired. An electrophorus is easily made. Choose the lid of a tin canister about eight inches in diameter and half an inch deep for your ‘form,’ or have a lid specially made by a tinsmith with its sharp edge turned over a wire ring, so that it may keep its shape and not be so likely to cut your fingers. Let the tinsmith also make you a thin flat disc of zinc or brass, smooth and rounded at the edges, and measuring about six inches and a half across. To this disc solder three loops of brass wire, and to the loops tie three silk strings of equal length, by which you can lift the disc. The silk should be quite pure, and if you like something else you can use a handle made of sealing-wax and stick it on to the centre. The silk strings, however, are the simplest, strongest, and most easily replaced.
Turn the ‘form’ bottom upwards, and run round it a strip of thick white paper, so as to project about an inch above the bottom. This will be the mould into which the mixture is to be poured, for the lid is always to be used bottom upwards. In the old days the mixture was poured into the tin mould and left there, but it was found that the cake would crack very easily under such circumstances, whereas when it is left to itself it lasts for months with ordinary care. Make your mould, then, with the lid for its bottom and the paper for its rim, and proceed to melt your mixture. This should consist of yellow beeswax and Venice turpentine in equal quantities by weight. Use an earthenware pot, and gradually warm up the mass, stirring it with a piece of wood so as to ensure its melting equally. When it is melted, you have to add to it five times its weight of shellac—that is to say, if you used two ounces each of beeswax and turpentine, you will have to use twenty ounces of shellac. The shellac is to be added to the melting mixture a handful at a time, and all lumps must be dissolved before any more of the flakes are added. Do not let the liquid get too hot, or it will become like india-rubber and spoil. When all the shellac has been got in, take off the earthenware pot, give the mass a stir, and carefully pour it out into your paper-edged mould, until the liquid is half an inch deep. When the cake is cold, wet and tear off the paper, and then lift it off the tin. If you drop the cake it will almost certainly break, but if you keep it free from hard knocks it will last a long time. Do not have the cake too thick.
To use this electrophorus, turn your lid upside down, as you did during the casting, and place the cake on the top, turning it also bottom upwards, so that the smooth surface which came nearest the tin when it set is now the upper one. Let the whole apparatus be warm and dry. Strike or rub the surface rapidly with a piece of warm flannel or fur—fur is the best; and while you are beating the cake, keep your fingers on it to prevent it slipping off its stand. When you think the cake is sufficiently excited, which it will be in a minute or so, lay the cover in the centre, holding it by the silk strings or handle. Touch the cover with your finger, and then lift it from the cake, and you will get a powerful spark, and each time you touch the cover, before you lift, the result will be the same. In dry weather the cake will remain electrical for weeks, but it is better to recharge it each time it is used. Do not let your clothes get too near the electrophorus during your experiment, and keep all pointed things as far away from it as possible. An eight-inch electrophorus ought to give an inch spark if properly made and charged. To charge the Leyden jar, all you have to do is to hold the knob near the cover and take from fifty to a hundred sparks. You should have the electrophorus raised so as not to have to lift the cover too high each time, and you should hold the jar by its bottom, thus giving the necessary connection with the earth.
Fig. 1.
The cylinder machine (Fig. 1) is a much more complicated affair. It consists of a stand A; a cushion, of which the upright is shown at B, and from which the silk flap is shown at the top; a cylinder, shown at D with its caps E E, and its handle at F; and a prime conductor G, insulated on a glass rod H. It is best to buy the cylinder. Glass confectionery jars, Winchester quarts, ordinary bottles, and even commoner vessels have been used, but the results have rarely repaid the extra trouble necessitated by the want of a cylinder with proper ends. Such a cylinder about six inches long will cost under two shillings, and one of a fair size, say ten inches long, can be obtained for five shillings from any chemical appliance seller, such as Griffin, of Long Acre, or Townson and Mercer, of Bishopsgate Street Within. Should a makeshift be adopted, the first step is to cement a disc of baked wood on to each end of the bottle so as to afford the needful fixing. The cement for the purpose should be made by melting rosin in an earthen pot, adding a little beeswax and raw linseed oil to toughen it. For half a gallipot full of rosin use a piece of wax about as big as a walnut, and a teaspoonful of oil. When the rosin is thoroughly melted, stir in some plaster-of-paris; and the more plaster you can manage to make it take up the harder will be your cement. The mixture must, however, be perfect. While it is liquid shake in some red lead to give it a good colour. The cement will have to be melted each time it is used, and the articles it is required to join should be warmed before it is applied, so that the change of temperature may be gradual.
The first thing to do is to provide our cylinder with an axis. The simplest way to mount it is to run a hard wood stick right through the centre, the stick being only just large enough to pass, but the most workmanlike way is to fit it with two hard wood ends. Unless, however, this be done with great accuracy, the cylinder will not be properly centred, and the result will be a failure. For beginners, therefore, it may be advisable to retain the central spindle. Before inserting it thoroughly clean the cylinder inside and out.
Fig. 2.
Fig. 3.
The spindle should project a few inches at each end; and to hide the glass collars and give a strong grip, a pair of ends (see Fig. 2) should be cut or turned which can be slipped down the stick. These can be fastened to the glass with cement, and further kept in place by a fine screw or French nail driven through to the centre so as to just avoid the glass. In the case of the spindle not being driven through, the ends are made longer, and on them the cylinder works. We have here (Fig. 3) the cylinder A duly fitted with the central spindle B, with the collar C shown by itself and in position, and the handle D slipped on to the squared end of the spindle. This handle should be made to fit firmly, and it is best to cut the square hole in it first and then to cut the spindle end to suit it. Like the rest of the machine it should be of hard wood, and should have all its corners and edges sand-papered off.
Fig. 4.
Fig. 5.
We must next make the stand for the cylinder to work on. Get a piece of board an inch or more thick, lay the cylinder on it, and mark where the two ends come at G G (Fig. 4). The cylinder is to revolve on the spindle, and the ends are to prevent its shifting from side to side, so that your marks will be at the junction. Now get out two uprights from the same thickness of board, and let them be twice the height of your cylinder’s diameter; let one have a hole large enough for the spindle to work in, and let the other have a slit from which the spindle can be kept from rising by a pin run through, as shown in Fig. 5. When your uprights are ready, cut the tongues at the end, and then cut the holes in the board for them to stand in. These holes will be to the outside of the lines given by the cylinder, and should be on one side of a line drawn through the centre parallel to the sides of the board. Fit in your uprights and try if the cylinder works freely in them. If all is true and level, take them out and glue them home.