210 



THE POPULAR EDUCATOR. 



a ping of wood inserted in the mouth of the bottle serves for 

 a bearing at the other end. 



When a cylinder is used, caps should be turned of mahogany, 

 or some hard wood, so as to fit loosely on the ends of the cylinder ; 

 these are then fixed on by the cement above referred to, and 

 great care must be taken to ensure the cylinder being mounted 

 so as to run perfectly true. If the frame-work of the machine 

 be made first, it can be put in before the cement is fully set, and 

 carefully watched while it is being turned round ; it must not, 

 however, be left in its bearings to set. As the machine is usually 

 warmed before use, an aperture must be left for the escape of 

 the air ; a hole is therefore drilled through the cap at the end 

 away from the winch, and, while the cement is being poured 

 into the cap, this is filled by a greased wire, which may bo 

 removed as soon as the cement becomes hard. 



A better plan of mounting the cylinder has, however, lately 

 been tried. The inside is first rendered perfectly clean, and 

 then thoroughly dried by exhausting the air, and allowing a fresh 

 supply to enter through a drying tube. When it is thus pre- 

 pared, the caps are put on so as hermetically to seal it. The sur- 

 face of the cylinder is rubbed before use with a rag wetted with 

 turpentine, so as to remove all grease, lumps of amalgam, etc., and 

 it is then polished with prepared chalk. This imparts a greatly 

 increased power to the machine, for sparks of a much greater 

 length can be in this way obtained, and there ia little need of 

 warming the cylinder before use. Another advantage is that 

 the damp does not condense so readily upon it, and thus 

 it can be used at a lecture-table or in a room filled with 

 people. 



The frame-work of the machine can easily be made. It should 

 be formed of thoroughly dry wood, baked for a little time ; and 

 in making it, great care must be taken to avoid all points and 

 Bharp edges which draw off much of the electricity. The support 

 at the winch end should be made with a cap, so that the cylinder 

 may be removed when necessary, and the under side of the 

 cylinder should be five or six inches above the board. 



It is simpler, too, to make the rubber in a way rather different 

 from that shown in the figure. As it is not often required to 

 obtain negative electricity, the support may be of baked wood, 

 and the whole should then be shaped like the letter TJ the 

 lower end being hinged to the edge of the board, and a hole 

 made about two inches up, through which a thumb-screw may 

 pass into a wooden block placed a little way from it, so that 

 by means o2 the screw the rubber may be pressed firmly against 

 the cylinder. Care should be taken in the construction of the 

 rubber, as much of the power of the machine depends upon it. 

 It should be about one and a-half inches wide, and rather 

 shorter than the cylinder. Wash-leather answers well for its 

 covering, and the horse-hair or tow in it should be so arranged 

 as to give a uniform pressure. The silk flap is fixed to the 

 under side, and passes up in front of the rubber. At the back 

 is a small hook, to which a piece of chain may be attached, and 

 a wire should come from this to the under part of the rubber, 

 and there be connected with a piece of tin-foil running the 

 whole length of it. This is often omitted, but as the materials 

 of the rubber are not good conductors, this simple addition 

 greatly increases the power. The chain from the hook should 

 be allowed to touch the ground, or, better still, be connected 

 with a gas pipe, as thus a plentiful supply of electricity will be 

 obtained from the earth, which is the great reservoir of it. The 

 conductor of the machine may be made as shown in Fig. 6, 

 but is rather more convenient, if mounted on a separate stand. 

 The points, too, instead of being placed along the side, may be 

 fixed on a separate piece fitting into one end of the conductor. 

 A very good fork may be made by rounding the ends of a piece 

 of wood about the size of a small ruler, covering it with tin- 

 foil, and insertiag a row of needles along one side. The prime 

 conductor, also, may be made of wood covered with tin-foil, 

 and should have its end somewhat bulging, as shown in the 

 engraving. 



When the machine is wanted for use, every part should be care- 

 fully rubbed dry and clean with warm cloths. The rubber screw 

 should be loosened, and the rubber turned back, so as to allow 

 of the old amalgam being scraped off, and a, fresh supply placed 

 on the cushion under the silk flap. It may then be replaced 

 and pressed firmly against the cylinder by means of the screw, 

 or by the pressure of the hand on it. 



Now turn the cylinder, and if the machine be in a darkened 



room, flashes of blue light, accompanied by the peculiar smell 

 of electricity, and by a faint crackling noise, will be observed 

 passing round the cylinder and issuing from the edge of the 

 silk flap. Now bring the conductor so that the points of the 

 fork may nearly touch the cylinder, the electricity will then be 

 collected, and sparks several inches in length may be obtained 

 by holding the knuckle or any conducting substance near it. 

 If the rubber were insulated, similar sparks might be drawn 

 from it, but they would be of negative electricity. The great 

 point to be remembered in using all electrical apparatus is to 

 have every part of it perfectly dry and free from dust. 



This form of machine is by far the most common. In it, as 

 we have seen, friction is the exciting cause ; hence the power will 

 be found to depend upon the extent of rubbing surface. With 

 a view to increase this, two rubbers opposite to one another 

 have sometimes been affixed to the same machine, and two con- 

 ductors placed between them ; this, however, adds so greatly to- 

 the complication of the machine, that the plan is nearly dis- 

 carded. 



If we remove the chain or wire connecting the rubber with 

 the ground, and place one so as to make a communication 

 between the rubber and conductor, we shall find that no spark 

 can be obtained from either, showing that the quantities of 

 positive and negative electricity are exactly equal, and therefore 

 neutralise each other. 



Though the cylinder machine is that most generally used, its 

 place is sometimes taken by the plate machine (Fig. 7), especially 

 when great size and power are required. By some this form is pre- 

 ferred as being more compact and ornamental, and the power is 

 usually supposed to be about equal for an equal rubbing sur- 

 face; but when the cylinder is mounted on the plan mentioned 

 above, the advantage in point of power is on its side. 



A plate of thick glass has its edges carefully smoothed and 

 a hole drilled through its centre for the axle to pass through. 

 This is made of brass, with flanges to press against each side 

 of the plate and hold it firmly ; but as brass is a conductor of 

 electricity, a part of the winch is usually made of glass, to- 

 prevent the electricity being conducted away. Two rubbers, 

 F, F, are employed in the machine, and they are made double, so 

 as to grip the plate between them, and thus cause friction on 

 each side of it. Quadrants of silk are also affixed to the rubber 

 to prevent the escape of the electricity before it reaches the 

 conductor, just as a flap of silk was used for the same purpose 

 in the cylinder machine, and the power will be much augmented 

 if this be covered with varnish. The main disadvantage in this 

 machine is the difficulty of insulating the rubber so as to draw 

 negative electricity from it. 



The conductor is sometimes made in two pieces, as shown at 

 c, c, and the further ends are then connected by a brass rod, but 

 more frequently it is semicircular, and supported by a single glass 

 upright. The fork should be bent round so as to collect the 

 electricity from both sides of the plate, instead of from one only, 

 as is frequently the case. Great care is required in warming 

 this machine, lest the plate should become unequally heated and 

 crack. The best plan is to lay some silk handkerchiefs over 

 it, and let it stand a little way from the fire. 



Ebonite or vulcanite is now sometimes used in place of glass 

 for the plate, and possesses many advantages over it. A larger 

 amount of electricity may be obtained from it, and it is not 

 liable to crack as a glass plate is, nor does the damp condense 

 on it so readily. Still, it is much softer, and therefore will 

 not wear quite so long. For an ebonite plate, hareskin is one 

 of the best materials for use in the construction of the rubber. 

 It must be thoroughly cleansed from grease, and the amalgam 

 used must be softer than that used with glass. 



Plate machines have occasionally been constructed with two 

 or more plates fixed* parallel to one another on the same axis. 

 A much greater increase of power is, however, obtained by the 

 use of a very large plate. At the London Polytechnic there was 

 one with a plate about seven feet in diameter, and driven by a 

 small steam-engine, from which sparks of great length and 

 power may be obtained ; and even this size has been exceeded, 

 for some time ago there was one at the Panopticon with a ten- 

 feet plate. Machines of this size require, of course, great care 

 in their use, as a spark from one of them would be nearly suf- 

 ficient to knock a man down, and injurious effects might pos- 

 sibly be produced. 



Several other machines for producing electricity, or modifies.- 





