CHAPTER LIII.—THE ELECTRICAL MACHINE, AND HOW TO MAKE IT.
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.
Fig. 6.
Fig. 7.
The next thing to make is the cushion, for which you require a block of wood, like E E ([Fig. 6]), an inch and a half wide, half an inch thick, and not quite as long as the cylinder. Cut a notch in the centre for the upright to be fixed in, and then procure a piece of coloured thin leather, or wash-leather, an inch longer than the wood, and wide enough to go barely round it. Glue it on to the top and bottom of the wood so that it is quite loose in front, and also glue up one end. When the glue is thoroughly dry, stuff the cushion with horsehair or tow, making it as uniform as possible. Glue up the open end. To the under edge of the cushion glue the silk flap, which should pass up in front of the rubber and over the top of the cylinder (see [Fig. 7]). It may be of oiled silk throughout, or it may be made as follows. Glue a piece of leather the same as that used for the cushion along the under edge of the wood. Should you have used a coloured leather for the cushion, you will have to glue it coloured side down, so that the softer surface may come next the glass. This piece of leather should be the length of the cushion and just wide enough to reach to the top. Along its upper edge you have next to glue a piece of black sarcenet, and the leather and the sarcenet should together be as long as the flap would have been had you made it entirely of silk.
Fig. 8.
Fig. 9.
The cushion being thus completed, you have next to fix it to an upright, and this upright should be just high enough to admit of the cushion pressing along the centre of the cylinder. To increase the power of the machine you can, if you like, glue a strip of tinfoil along the bottom of the rubber, connecting it by a small chain to a hook at its back, from which hook a chain hung on to the nearest gaspipe affords the best connection with the earth. The upright ([Fig. 8]) is affixed to a foot in which a slide has been cut as shown at K in [Fig. 9], where N is the position of the screw and L is the hole for the upright, so that it can be slipped backwards and forwards until the right pressure against the cylinder is obtained. A brass thumb-screw will keep it in position, as shown in our first illustration.
For the prime conductor use a wooden rolling-pin covered with tinfoil. If you have to prepare a piece of wood specially it is best to have it about two inches and a half in diameter, and instead of making it round to make it with an egg-shaped or pear-shaped section. This is, however, unusual at present, and a round stick is much easier to get. Its ends should be well rounded off. Tinfoil is cheap enough,—it costs about twopence-halfpenny a sheet, so that there is no need to be sparing of it; it should be carefully pasted on to the wood with ordinary paste, and have its edges notched so as to avoid wrinkles and folds. When it is dry, it should be burnished with a knife-handle to make it as smooth as possible. When it is finished off draw a line along it—in the case of a pear-shaped conductor let the line be along its thinnest edge—and at a quarter of an inch apart drive in stout pins, which should be filed off so that their points may be quite sharp and project about half an inch. The length of the row of pins should be rather less than that of the cushion; they should not run the whole length of the conductor, but stop about a couple of inches from the end. The conductor should be the same length as the cylinder, but it should be so fixed as to bring the row of pins immediately opposite the flap. It thus stands a little to the right of the centre, away from the handle end. At the longest end a thick wire is fixed, to which a brass ball is attached.
To support the conductor you require a glass rod, obtainable at the same time as the cylinder, and costing a shilling a pound. It should be half an inch in diameter and sufficiently long to fix well into the board and conductor, and bring the points level with the centre of the cylinder. To get it to remain fast you will have to roughen its ends on a grindstone and fasten them into holes with cement. It should be so fixed as to bring the points of the conductor about half an inch from the cylinder. Now cut the silk so that its edge is just above the row of pins; and then give all the edges and corners a final sand-papering, and set about getting the machine to work.
The first thing you have to do is to smear the cushion with amalgam costing sixpence an ounce, an ounce lasting for many weeks. It usually consists of five parts of zinc mixed with three parts of tin, and gradually mixed with nine parts of mercury, but an amalgam can be easily made by melting in a tobacco-pipe a piece of zinc about as large as a pea, and adding to it an ounce of mercury, stirring the mixture thoroughly and pouring it on the hearth to cool. If you have a silk flap over the cushion, smear the amalgam on to the cushion as evenly as you can manage; if you have a leather flap with silk top, spread the amalgam along the leather in a strip of about half an inch in width, which you have previously slightly greased with a tallow candle. To apply the amalgam you will have to unscrew the cushion from the stand.
When you have replaced the cushion and are ready to start, put the machine down to the fire to warm. Do not put it too near, or it may crack. The machine must be free from dust, so clean it well with a silk handkerchief; and it must be dry, more especially the cylinder where it is not covered with the flap and the glass support of the prime conductor. Unless these are free from damp the machine will not work. Do not turn the handle of the machine until you are assured that the cylinder is dry; by doing so you give the rubber the benefit of the whole circumference of moisture, and it may take you some time to get rid of the wet. If you ever have a plate machine, remember this most particularly, for your brushes will be made quite useless for a time by such thoughtlessness. And one other word of caution with regard to plate machines, and that is, never place them sideways to the fire, always dry them end on. The thicker the glass the easier it splits with inequalities of temperature.
All being dry and warm, clamp the machine to a table and turn the handle. If you have carefully followed these instructions, your knuckle will receive a spark as soon as you apply it to the ball of the prime conductor; and the handle of the machine will turn more stiffly as you progress. If your cylinder becomes too greasy, wipe it clean and apply a little more amalgam. If it does not work quite right at first, see that some pointed thing on the table is not drawing the electricity away; even a hair will affect the working, so keep a good look-out for fragments of cotton and fluff. When you get the sparks bring your jar into play, charging by simply holding it out with its knob touching, or close to the knob of the prime conductor.
The essentials of the cylinder machine are that it should be made of hard dry wood and of glass without an excess of alkali, that its workmanship should be firm and true, that it should have no unnecessary points and edges, and that it should be only used when free from damp. Those who are interested in the subject, and wish to see what a well-made machine is like, should visit the Science Collection at the South Kensington Museum. There is there, among other noticeable things, a plate machine seven feet in diameter. The exhibition is one of the best in London, and it is most easily got at from the Queen’s Gate side, the entrance being the door nearest the private houses on the way up to the gardens from the railway station. It is open free on the usual South Kensington days—Monday, Tuesday, and Saturday.