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.