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.