Fig.9. Fig.12. Fig. 7. Fig.4. PLATE I DQ A London Published by Thomas Tegg, 13 Cheapside. Sept 1.1827. J.Shury, seu ductor a number of metallic points are attached, to collect the electric fluid which flows on the surface of the cylinder. M M is a rod of brass inserted in the prime conductor, having a joint by which may be raised or lowered, to suit the height of the apparatus; this rod is a most useful appendage to the prime conductor. To the upper part of the rubber a piece of black silk is attached, which proceeds from thence over the top of the cylinder, to within about half an inch of the points of the wires inserted in the prime conductor; by which means the fluid that is brought into action by the attrition of the cylinder and rubber is prevented from being dissipated in the air, and carried round with the cylinder to the prime conductor. The action of the silk on the cylinder tends very much to increase the excitation of the machine, as may be seen by removing the cushion a little back from the cylinder, and leaving the silk to act upon it alone, in which case the excitation will often be found to be scarcely less than when the rubber is also in contact with the cylinder. 81. The plate machine, like the cylindrical, has had a few varieties introduced into its construction at different times. The largest specimen of this instrument is that which was constructed by Mr. Cuthbertson, for Tyler's museum, at Haarlem. The following will convey to the reader's mind some idea of the powers of this immense machine; and its construction will be readily understood by the description which we shall add of the most improved form of the instrument. It consists of two circular plates of glass, each sixty-five inches in diameter, and made to turn upon the same horizontal axis, at the distance of seven inches and a half from one another. These plates are excited by eight rubbers, each fifteen inches and a half long. Both sides of the plates are covered with a resinous substance to the distance of sixteen inches and a half from the centre, both to render the plates stronger, and likewise to prevent any of the electricity from being carried off by the axis. The prime conductor consists of several pieces and is supported by three glass pillars, fifty-seven inches in length. The plates are made of French glass, as this is found to produce the greatest quantity of the electricity next to English flint, which could not be produced of suffic ent size. The conductor is divided into branches which enter between the plates, but collect the fluid by points only from one side of the plate. The force of two men was required to work this machine; but when it is required to be put in action for any length of time, four are necessary. At its first construction nine batteries were applied to it, each having fifteen jars, every one of which contained about a foot square of coated glass; so that the grand battery, formed by the combination of all these, contained 135 square feet. The effects of this machine are aston shing: but Dr. Van Marum, who principally made experiments with it, imagining that it was still capable of charging an additional quantity of coated glass, afterwards added to it ninety jars of the same size with the former; so that it now contains a coated surface of 225 feet, and the effects are found to be proportionable. 82. The principal objection to the plate machine has hitherto been the difficulty of insulating for the purpose of producing negative electricity. This objection is now removed; and, by using the machine as constructed by Mr. Cuthbertson, both kinds of electricity are produced with ease. The following description of this beautiful and powerful instrument is given by Dr. Brewster in the Appendix to his new edition of Ferguson's Essays. Fig. 3 is a representation of this machine: AB is a circular disc of plate glass, supported upon an horizontal axis, the ends of which rest upon two upright pillars EF. By turning the winch or handle, W, a rotatory motion is given to the plate, which is excited by two rubbers X, Y, fixed at the opposite ends of a diameter of the circle. Each rubber consists of two cushions, which embrace the outer margin of the glass plate, as shown in the figure. The conductor, which is made of brass, is shown at CD, and it is attached to the upright pillar E by the glass rod R. To the right hand of C, and above B, are seen the points for the receiving the electricity from the machine. From each rubber there proceeds to within a little of the points, a flap or double piece of oiled silk, which prevents the dissipation of the electricity. As it is difficult to insulate the rubbers of the plate glass machine, without giving it an unseemly appearance, Mr. Cuthbertson introduced the practice of insulating the whole machine by the glass supports G, II, I, K, so that negative electricity can thus be obtained as easily as positive. 83. The cylindrical machine, however, being at present in more common use than the plate machine, it is hoped the following remarks may be of use to those who possess such an instrument, and particularly to such as may be disposed to provide themselves with it for the sake of economy. A cylindrical electrical machine ought never to be less than ten inches in diameter; there are cylinders of six, seven, eight, and nine inches diameter, often neatly mounted, and sold by the philosophical instrument makers, but they are of no manner of use for the purpose of experiment, and serve only as a kind of philosophical toy, for the amusement of children, and that too on a very narrow scale. To construct such machines, therefore, is an absolute waste of the materials. With a cylinder of ten inches, properly managed, a tolerable exhibition may be made, but the most convenient size is from twelve to sixteen, the length of the cylinder being in proportion. 84. The most powerful excitation of the machine is produced as follows:--Let the machine be placed within the influence of a good fire, but not so near as to injure any of its parts by the action of the heat. With a flat round pointed knife spread a little amalgam evenly along the cushion, and return it to its place: turn the cylinder a few times round; then take off the cushion, and observe carefully those parts on its surface that have not been touched by the cylinder while revolving; on these parts put a little more amalgam, and repeat the process of turning the cylinder, and supplying the defective parts with amalgam, till every point of that part of the surface of the cushion which presses on the cylinder appears to be properly supplied with amalgam, Take now a piece of leather about five or six inches square, and spread over one side of it a quantity of amalgam; throw back the silk flap, and, turning the machine gently round, apply the amalgam side of the leather to the cylinder for the space of two minutes or more, as circumstances may require, during which time the excitation will be observed to increase rapidly. The cylinder must next be wiped perfectly clean with an old silk handkerchief, and afterwards with a soft dry linen cloth. Let the cushion be again removed; and the amalgam which appears above and below the line of contact with the cylinder carefully scraped off, the silk flap wiped with a linen cloth, and the whole returned to its place and made fast. If now the cylinder be turned slowly round, streams of the electric fluid will be seen rushing from the silk flap round the lower part of the cylinder, attended with a hissing and snapping noise, while large brushes of the same, of several inches in length, may be observed flying off from the lower edge of the silk into the surrounding air. The machine is now fit for use, and may be fastened to the table, after which the whole of its parts are to be well wiped with a warm and dry linen cloth to free them from dust. 85. The operator, however, must not expect this high and rich state of excitation to be of long duration. The cylinder will soon cool; dust will be attracted by the action of the machine; and the moisture produced in the air of the room by the breath of his audience, will, by their united effects, render all his efforts to produce a copious supply of electricity entirely fruitless. 86. To remedy this defect, which gentlemen who deliver public lectures on electricity have often found to be a grievous one, provide a box of thin plate iron, ten or twelve inches long, four inches wide, and one inch and a half in depth, with a lid to fit very easily over it. In this box a piece of bar iron of about six inches in length, three in breadth, and half an inch in thickness, after being heated in the fire to a dull red heat is to be placed, the lid of the box put on, and the whole, on a suitable iron stand, placed under the cylinder, on the board of the machine in a longitudinal direction. The radiation of heat from the 'iron will effectually preserve the equality of the temperature of the surrounding air for a considerable length of time, and indeed for any length of time required, since, by employing two bars of iron, the one may be kept in the fire while the other is in the box, and thus no other interruption in the course of the experiments will be necessary beyond what is occasioned by the changing of the irons. By this means the machine may be made to act in full vigor under the most disadvantageous circumstances. 87. Dr. Brewster mentions a method used by Mr. Ronalds of aiding the excitation of the machine, which he represents as being attended with the greatest advantages. He thus describes it: In the cylinder machine, the rubber is placed in front of a half cylinder of copper, which communicates with a copper pipe, that serves for the support of the rubber. A very small spirit-lamp, whose burner consists of only one thread of cotton, is placed immediately beneath the mouth or lower end of the copper pipe, so as to keep the rubber and the parts adjacent to it always hot and dry. Besides this contrivance, Mr. Ronalds places his prime conductor upon a glass support, so that a similar spirit-lamp may be placed below it in order to convey heat to its interior. Mr. Ronalds remarks, that a cylinder machine thus constructed, of half the dimensions of one made upon the usual plan, is highly and permanently effective. The same principle may easily be applied to plate machines. The heat is supposed to assist the excitement by promoting the oxidation of the amalgam. 88. This method may certainly, in some degree, prove beneficial; but it will be found to be in many respects inferior to the simple method above mentioned. In the first place, the radiation of heat will be neither so general nor so great; in the second place, the flame of a lamp or a candle absorbs the fluid; and in the third place, the light, which must necessarily be emitted from two spirit-lamps, would prove highly detrimental to the effect of those experiments which require to be performed in darkness. To which may be added, the expense of the lamps, and the spirit to be consumed. 89. Where a plate machine of large dimensions is used, this additional article will not be required; for the great thickness of the glass renders it capable of retaining the heat much longer than can be done by a thin cylinder; for which reason it must be obvious that, if cylinders were made much stronger than they generally are, their action would be effectual for a greater length of time than it is. Opinion, however, runs in favor of thin cylinders, but the consistency of such opinion remains to be shown. It is well known that the old globular machines, which were made of thick glass, when once put in a state of powerful excitation, retain that state much longer than the modern thin cylinder will do under the same circumstances. 90. The following directions for preparing the amalgam for electrical machines are given by Mr. Singer, in his treatise, already mentioned. Melt in an iron ladle two ounces of zinc with one ounce of tin, and, while this mixture is in a fluid state, pour into it six ounces of mercury; let the whole be then put into an iron or wooden box, and agitated until it be quite cold. It must then be reduced to fine powder in a mortar, and mixed with sweet hogs' lard to the consistence of thick paste. This part of the process need not be performed till the amalgam is wanted for use. This amalgam, he remarks, answers exceedingly well, but, he afterwards adds, I have since made it with a still less proportion of mercury with equal effect. The proportions may be two ounces of tin, four ounces of zinc and seven ounces of mercury. The mercury must be heated to about 300° Fahrenheit, before the fused metals are added to it. When thy amalgam has been agitated until cool, and finely powdered, it is to be mixed with hogs'-lard be trituration in a mortar; and should it at any time become hard, more lard must be added and the trituration be repeated. 91. The application of the electrical apparatus to the purpose of experiment will afford the bes: illustration of its subordinate parts; and, in adopting this plan, we shall be enabled to give such an arranged view of the chief properties of the electrical fluid as may be of service in aiding the inexperienced electrician in making an orderly display, instead of a series of experiments which have no regular connexion, and in which sometimes one property, and sometimes another, is illustrated. We shall commence with some observations on the action of the machine itself, and for these we must acknowledge our obligation to Mr. Singer. 92. The machine being prepared according to the directions already given, and the cushion pressed moderately against the cylinder by the action of its adjusting screw, it may be put in motion, and the following phenomena will be observed. 1. Distinct lines of light, accompanied by lateral scintillations, pass from one conductor to the other, across that part of the cylinder which is not covered by the silk flap; these are called electrical sparks. 2. Bright sparks pass between either of the conductors and the knuckle, or any smooth uninsulated substance presented to them at a moderate distance; and if received on the knuckle, or any part of the body, produce a painful sensation. 3. These effects are more distinct, and the sparks from each conductor stronger, when they are taken from both at the same time. 4. The power of the spark from either the positive or negative conductor, singly, will reach its maximum when the opposite conductor is uninsulated, by sus pending a chain or wire from it to the ground. 5. If the two conductors are connected by a wire, or other conducting substance, the most vigorous friction of the cylinder will not electrify either. 6. If, instead of a wire, the conductors are connected by a silk string on which a number of shot or metal beads are strung, at the distance of a twentieth of an inch from each other, a series of bright sparks will pass between the beads as long as the turning of the machine is continued. It must be remembered, that the conductor to which the cushion is fixed shows the electrical phenomena of the cushion, and the opposite conductor, that of the electricity of the glass cylinder; hence the observation of their phenomena is properly an observation of the circumstances that occur in all cases when electricity is excited by friction. 93. Ön these very beautiful phenomena, Mr. Singer makes the following judicious observations. The first and second phenomena seem to show that the cause of electricity is corporeal; for sensation is affected by it, and a mechanical impulse experienced, which it is difficult to asscribe to any other than a material cause. The third phenomenon proves that there is a mutual action between the electricities excited in the opposite conductors; since their effects are more powerful when directed at the same time to one conducting body. The fourth phenomenon shows that the same relation which is observed between the opposite electrified conductors exists also between either of them and the ground, but in a different degree. By the fifth phenomenon it is seen that positive and negative electricity, if excited to the same extent, and united by con ducting matter, exhibit no electrical phenomena. The sixth phenomenon is observed to show that when the conductors are connected, the machine continues to excite electricity, but is prevented from displaying it by their mutual contact. 94. From these appearances the following explanation of electrical phenomena may be deduced:-1st. The cause of electrical phenomena is material, and possesses most of the properties of an elastic fluid. 2. This electric fluid attracts and is attracted by all other matter, and, in consequence of such attraction, exists in all known substances. 3d. The attraction of different bodies for the electric fluid is various, so also is that of the same body under different circumstances, consequently the quantity of electricity naturally existing in different substances may be unequal; and the same body may attract more or less than if alone, when combined with other matter: but its original attraction will be restored by destroying the artificial combination. 4th. From some peculiarity in the nature of the electric fluid, its attraction for common matter is more influenced by figure than by substance; and consequently is stronger in extensive than in limited surfaces. 5th. From this peculiarity, it moves with great facility over the surface, or through the substance of some bodies, and is arrested in its progress by others. 6th. When the attraction of any substance for electricity is equal to the electric fluid it contains, that substance will evince no electrical signs; but these are produced when there is either more or less electricity than is adequate to the saturation of the existing attraction: if there be more, the signs will be positive; if less, they will be negative. 95. Electrical excitation then, may be thus effected:-The bodies employed have each a certain quantity of the electric fluid proportioned to their natural attraction for it; this they retain, and appear unelectrified so long as they remain in their natural state. And, if two such bodies are brought in contact, their natural attractions will be altered, one of them attracting more than in its separate state, and the other less; the electric fluid thus diffusses itself amongst them in quantities proportioned to their relative attractions, and hence they appear unelectrified. But if they are suddenly separated, the new distribution of the electric fluid remains, whilst the original attractions are restored, and as these are not equal to each other the bodies will appear electrical; that of which the natural attraction was increased by contact, having received an addition to its quantity of electric fluid, will be positively electrified; and the other will be negative. ATTRACTION AND REPULSION. 96. The motion of light bodies produced by electricity, is usually called attraction and repulsion, and is occasioned by the mutual attraction existing between the electric fluid and common matter. In practical electricity there are numerous methods of illustrating this motion; the following are some of the principal. (1). Fix at the end of the prime conductor a knobbed rod, and hang on it two small pithballs, suspended by threads of equal length. The |