Page images
PDF
EPUB

numerous.

46. Of the first kind, viz. friction, the instances Mr. Singer, is of importance to the practical are most numerous, and, as Mr. Singer remarks, electrician, and may be thus effected :-Sealingunder certain limitations, universal; they may wax when rubbed on woollen cloth is negatively indeed be obtained by rubbing any of an exten- electrified. Glass, when rubbed with silk is sive list of resinous and silicious substances; positively electrified. Let an electrometer he and of dry, vegetable, animal, and mineral made to diverge by its being approached by an productions. The electricity thus excited, is most excited stick of sealing-wax: while in this state, readily rendered visible by its effects on the gold approach it with any excited body, the electrileaf electrometer.

city of which is to be determined. If the diver47. Examples of the second kind are also very gence of the electrometer increase, the presented

If a small quantity of sulphur be body is negative; if it be diminished, the premelted and poured into a conical wine glass, it sented body is positive. In other words, all will contract a little, and become electrical in those substances that lessen the divergence occacooling. A silk thread with a small hook at the sioned by excited wax, are positive; and such as end of it, or a rod of glass should be inserted in increase it, negative: whilst those which lessen the sulphur while in a fluid state, to serve as a the divergence produced by excited glass, are handle for separating it from the glass when cold. negative; and such as increase it positive. On being separated from the glass, the sulphur Examining, by this test, the effects produced in will exhibit other signs of electricity; if kept in some of the instances of excitation already conthe glass it will retain its electric virtue for years, sidered, we find the truth of the preceding stateand ev.nce it very perceptibly on every attemptments, and the relation of the different electrical to separate the two substances.

states to the processes by which they are pro48. Mr. Henly discovered that chocolate, fresh duced, become more intelligible. Care ought to from the mill, becomes strongly electrical, as it be taken to destroy the divergence of the eleccools in the tin pans. It soon loses this property, but trometer after every experiment of this nature; recovers it once or twice, by being melted in an this is best effected by touching its cap with one iron ladle and poured into the tin pans. When end of a piece of brass wire. the mass becomes dry, the electricity cannot be 53. As an illustration of the doctrine here restored by melting, unless olive oil be mixed advanced let the following simple and easily perwith it in the ladie; in which case it completely formed experiments be made. recovers its electric power. M. Chaptal observed (1.) Roll up a warm and dry piece of Aannel, the same circumstance during the congelation of so that it may be held by one extremity, while a glac.al phosphoric acid. Calomel also, when it stick of sealing-wax is rubbed with the other. fixes by sublimation to the upper part of a glass After a slight friction present the flannel to an vessel, has been found strongly electrical. The electrometer, which will instantly diverge; while condensation of vapor, and the evaporation of this divergence continues, bring the stick of fluids, though apparently opposite processes, are sealing-wax near the cap, and the leaves of the alike sources of electrical excitation.

electrometer will quickly collapse. Both these 49. Various crystallised gems, and a stone substances, it is obvious, are electrified by mutual called the Tourmalin, become electrical by the friction, but their electricities are opposite; that mere application of heat; but no other sub- of the wax being negative, and that of the fannel stances have yet unequivocally manifested the positive. same property; though the effects of friction are (2.) The electricities thus produced are equal generally increased, if it is preceded by a mode- to each other: for if the friction be repeated, rate elevation of temperature.

and the two substances be both presented to the 50. The contact of dissimilar bodies is pro- electrometer at the same time, no signs of elecLably in all cases the real primary cause of elec- tricity appear: the opposite electricities, when trical excitement, but it is rarely employed alone, applied together, producing a reciprocal counterfor electricity is known to us only by its effects, action of effect. which are constantly the result of an artificial (3.) If a black and a white silk riband be exarrangement, and consequently may not immedi- cited in contact, in the manner already described, ately succeed the primary cause of electric the black riband will be found to be negatively, powers, similar in their separate action on the and the white one positively electrified. electrometer, and other indifferent matter; but (4.) Take the sulphur cone described at 47, e verting a mutual influence on each other, apply it and the glass separately to the elecdestructive of their individual properties. trometer; the cone will be found to be negatively

51. It was at first supposed that these pheno- and the glass positively electrified. mena were peculiar to the substances by which 54. From the above experiments it appears, they were produced; hence the power excited by that in all cases of excitation positive and negathe friction of glass was termed vitreous electri- tive electricity are produced at the same time, city; and that by the friction of sealing-wax, and may be observed by the use of proper means. resinous electricity. It has, however, long since But it also appears that by friction with the same been proved that both powers are produced in substance, different bodies are variously affected; every case of electrical excitation; and, because for glass rubbed with silk evinces positive electheir mutual counteraction of effect resembles tricity: but wax rubbed with silk" is rendered that of an affirmative and negative power, they negative. Again, polished glass, when rubbed have been styled positive and negative electricity. with silk, skin-wool, or metal, becomes positive ;

52. The determination of these two states of but if it be excited by friction against the back electricity in different excited bodies, continues of a living cat, it appears negative. Wool, silk,

{

or fur, rubbed against sealing-wax, are rendered showing at one view what kind of electricity positive; but gold, silver, or tin, are by the same will be produced by rubbing various electrics process rendered negative.

with different substances; the followiny Mr. 55. Electricians have drawn up tables for Singer gives us on the authority of Mr. Cavallo. Is rendered

By friction with
The back of a cat Positive Every substance with which it has been hitherto tried.
Smooth Glass Positive Every substance hitherto tried, except the back of a cat.

Positive Dry oiled silk, sulphur, metals.
Rough Glass

Negative {

Woollen cloth, quilis, wood, paper, sealing-wax, white

wax, the human hand.

Positive
Tourmalin

Amber, blast of air from bellows.
Negative Diamonds, the human hand.

Positive

S
Hare's skin

ş Metals, silk, loadstone, leather, hand, paper, baked

wood.

Negative Other finer furs.
White silk

Positive Black silk, metals, black cloth.
Negative Paper, hand, hair, weasel's skin.

Positive Sealing-wax.
Black silk

Negative

Hare's, weasel's, and ferret's skin, loadstone, brass, silver,

iron, hand, white silk.

Positive Some metals.
Sealing wax.

Negative

Hare's, weasel's, and ferret's skin, hand, leather, woollen

cloth, paper, some metals. Baked wood

Positive Silk.

Negative Flannel. In a note appended to the preceding table Mr. The COMMUNICATION OF ELECTRICITY. Singer says, Mr. Cavallo had inserted metals, 58. Mr. Cavallo, speaking of communicated which appeared to imply that the friction of all electricity, remarks, that under such a title falls metals electrified sealing-wax positively; this I almost all that we know of the subject : the find is not the case: iron, steel, plumbago, lead, passage of this virtue, says he, from one body and bismuth, render sealing-wax negative, and to another is what causes its light; by being all the other metals I have tried leave it positive. communicated to other bodies we see its attracI have therefore made a slight alteration in the tion; by its quick transition it melts metals, table. The least difference in the conditions destroys animal and vegetable life ; and, in of such experiments will occasion singular va- short, it is by this communication that the science rieties of result: with the same rubber (an iron is known and cultivated. chain), positive electricity may be excited in one The following observations and experiments stick of sealing-wax, and negative in another, if on this particular part of the subject we give the former have its surface scratched, and the from Mr. Singer, with a few necessary exceplatter be perfectly smooth. Many repetitions of tions, preferring them to any thing we have Each experiment are therefore essential to an ac- yet seen for their appropriateness and concisecurate conclusion.

56. The result of experiments of the kind 59. From the few simple experiments which just described, Mr. Singer found to be much we have already described the reader must be influenced by the state of the bodies em aware' that electricity can be communicated or ployed, and the manner in which the friction conveyed from one body to another. But the was applied to them. In general, he remarks, faculty of electrical transmission is very different strong electric signs can only be produced by in different bodies ; some convey it with great the friction of dissimilar bodies; but similar rapidity; others more slowly; and there are substances, when rubbed together so that the some that appear absolutely to arrest its promotion they individually experience is unequal, gress. Examples of this fact are apparent in are sometimes electrified; and, in such cases, the most simple experiments. The divergence the substance of which the friction is lim'ted 10 of an electrified electrometer may be destroyed, the least extent of surface, is usually nesative. weakened, or maintained, by touching its cap This he farther remarks is the case with the with different bodies; now, as the divergence of strings of a violin, over a limited 'part of which the electrometer is caused by its electricity, such the bow passes in its whole length, and the hairs effects can only be produced by the relative of the bow become positive.

power of the touching bodies to deprive it thereof; 57. From these facts he draws the following for so long as the electricity remains the diverconclusions, viz. that positive and negative elec- gence will continue unaltered. tricity are concomitant phenomena, and that in 60. This may be shown most satisfactorily by all cases of electrical excitement, they are both the two following experiments. (1.) Touch the produced, though one only may occasionally cap of the electrified electrometer with a stick appear; and that these phenomena are not pe- of dry glass, sulphur, or sealing-wax; the diculiar to any distinct class of bodies, but may vergence of its leaves will continue; this shows be produced alternately in various substances, therefore that these substances do not transmit by changing the materials or method by which, electricity. (2.) Touch the cap of the electrified friction is communicated to them.

electrometer with a piece of wood, a rod of

.

ness.

any metal, a green leaf, or with the point of the The diamond and all transparent gems.
finger ; its divergence immediately ceases. Such Raw silk, bleached silk, dyed silk.
bodies therefore permit the transmission of elec Wool, hair, feathers..
tricity.

Dry paper, parchment, and leather. 61. By experiments of this kind it is found, Air, and all dry gases. that there is a gradation of effect from one class Baked wood, dry vegetable substances. of bodies to the other. Those which transmit Porcelain, dry marble. electricity with facility are called conductors; Some silicious and argillaceous stones. those whose transmitting powers are inferior, Camphor, elastic gum, lycopodium. imperfect conductors; and such as have no Native carbonate of barytes. power of transmission, non-conductors: but in Dry chalk, lime, phosphorus. general the various bodies in nature are divided Ice at 13° of Fahrenheit. into two classes only; the remote extremes of Many transparent crystals when perfectly dry, each forming the intermediate class.

Ashes of animal bodies. 62. In the following enumeration of the prin Ashes of vegetable bodies. cipal conductors, and non-conductors, the sub Oils, the heaviest are the best. stances are placed nearly in the order of their Dry metallic oxides. perfection; but the determination of this circumstance has not hitherto been accomplished 65. The most perfect non-conductors, contiwith much precision.

nues Mr. Singer, become conductors by the

accession of moisture; hence the necessity of CONDUCTORS.

preserving them clean and dry during electrical All the known metals.

experiments. Resinous substances, raw silk, and Well-burnt charcoal.

Muscovy talc, are least liable to attract moisture, Plumbago.

and are therefore most useful where perfect nonConcentrated acids.

conductors are required. Powdered charcoal.

66. Glass (becomes moist on its surface only, Diluted acids, and saline fluids.

and this moisture may be checked by covering Metallic ores.

it with sealing-wax or good varnish. Glass conAnimal fluids.

sequently enters most extensively into the strucSea water.

ture of an electrical apparatus; its strength, and Spring water.

the facility with which it may be procured of any River water.

form, fitting it most admirably for that purpose. Iee above 139 of Fahrenheit.

67. Many substances in the preceding list losé Snow.

their non-conducting power, and become conLiving vegetables.

ductors, when intensely heated. Such is the case Living animals.

with red hot glass, melted resin, wax, &c.; but Flame.

the most intensely heated air, if unaccompanied Smoke.

by flame, is not a conductor. Many fibrous Steam.

substances attract water so readily, that it is absoMost saline substances.

lutely necessary to dry and warm them before Rarefied air.

their non-conducting property appears; this is Vapor of alcohol.

particularly the case with paper, flannel, parchVapor of ether.

ment, leather, &c. The influence of heat on this Most of the earths.

property is, indeed, very remarkable. It is well Most stones.

exemplified in the following instance: Wood, in To the above Dr. Brewster adds powdered glass its natural state, is a conductor; if baked, its and powdered sulphur, which, he says, have been moisture is expelled, but its organisation is not found to be conductors by the experiments of altered : it is then a non-conductor. By expoVan Swinden.

sure to a greater heat its volatile elements are dis63. Many of the above mentioned substances sipated, and its indestructible base (charcoal fail to conduct electricity when they are made replete with alkali) only remains; this is a conperfectly dry; hence it is concluded that their ductor; but if exposed again to heat, with access conducting power arises from the water they con

of air, it suffers combustion, and is converted tain. . Indeed this faculty does not permanently into ashes and gases, which are non-conductors. exist in any of the bodies enumerated, but varies

68. There does not appear any definite relation and disappears with their modifications of tem

between the chemical characters of bodies and perature, &c. Thus hot water is a much better their conducting potvers; for the best conducconductor than cold water is; the same is the tors (metals), and the best non-conductors case with charcoal and other substances.

(resins, sulphur, &c.), are alike infiammable sub

stances. The products of combustion, too, are Non-CONDUCTORS.

dissimilar in this respect : acids and alkalis 64. These are also denominated electrics, as conduct electricity, but the metallic oxides do before remarked, and occasionally insulators; not. Neither does it appear that specific grabut the latter term will only apply to the most vity, hardness, tenacity, or crystalline arrangeperfect of them.

ment of particles, are connected with the power Shell-lac, amber, resins.

of electrical transmision; for similar characters Sulphur, wax, jet.

of this kind are possessed bodies of both Glass and all vitrifications, talc

classes. Thus platina, the densest of bodies, is

a conductor; but so also are charcoal, and rare- periments here detailed with brown, as well as fied air.

writing paper. This idea, however, does not 69. Whatever be the cause of non-conducting seem to have occurred to writers on the subject power, it is evident that without its existence as till very recently. When very coarse brown a property of air, and other substances, electrical paper is used, the effects produced are much phenomena would be unknown; for, if the faculty stronger; nor is there in this case any necessity of electrical transmission existed universally, the for the application of Indian rubber; for if a cause of every effect of this kind would be dissi- piece of coarse brown paper, of about twelve pated and lost at the moment of its prodaction. inches long and six inches broad, be made very But, by the property of non-conductors, any ex- dry and warm, and then drawn gently three or cited electricity which they surround is preserved; four times between the knee and the lower part and it is then said to be insulated. A support of the arm, both being covered with woollen, is of glass, sealing-wax, silk, or any non-conductor, will be found to be highly electrical, and will is, for the same reason, called an insulating sup- with considerable force adhere to the wainscottport, or an insulator; and a piece of metal or ing of a room. If any conducting substance be other conductor, so supported, is named an insu- applied to it immediately after the friction, lated conductor.

such as the knuckle of the folded hand, or a brass 70. The use of insulators and conductors in ball, a strong spark will instantly dart from the practical electricity may be exemplified by very paper to it, attended by the usual snapping sound. simple experiments, which, says Mr. Singer, will So powerful indeed is excited brown paper, when form no improper introduction to the consider- carefully managed, that a small jar may be ation of more important apparatus. He gives charged with it; and it has been recently prothe following :

posed as a covering for a circular board to be (1.) Hold a sheet of writing paper before a fire used instead of a plate of glass in constructing till it be perfectly dry and warm; lay it flat upon a cheap kind of electrical machine. a table and rub the upper surface briskly with Indian rubber. The paper will adhere to the

PART II. table, and if lifted up by one corner and pre

ON ELECTRICAL APPARATUS. sented quickly to any fat conducting surface, as the wainscot, &c., will be attracted by and adhere 72. Numerous, extremely beautiful, and deto it. This adherence is occasioned by the at- licate experiments may be made in electricity traction of electricity excited on the paper, which with but a small quantity of apparatus, and that in its dry state is an insulator or non-conductor; of a simple form, and at a trifling expense. Such the necessity of which circumstance to the suc an apparatus consists chiefly of a few glass cess of the experiment is rendered evident by tubes, of about an inch in diameter and two or the paper falling down as soon as it has attracted three feet long; one of two very large sticks of rroisture enough to destroy its insulating pro- sealing-wax; a few pieces of silk, old silk handperty, and is further apparent from the impossi- kerchiefs answer extremely well; a few pieces of búity of producing the same results by the new flannel; some wires and balls of different friction of paper in its ordinary state of dry- sizes; and half a dozen small balls made of the

pith of elder. (2.) Repeat the excitation of the paper in a But, when it is required to exhibit the more dark room; when the paper is lifted from the striking and important of the electrical phetable by its corner, present the knuckle of the nomena, we must have recourse to a much more other hand successively to various parts of its powerful, complicated, and consequently exsurface, a series of faint divergent flashes of light pensive apparatus. will ensue. This light is occasioned by the 73. The principal article, the very fountain, transmission of the electricity excited on the so to speak, of all electrical apparatus, is what paper to the hand; and it occurs at every con- is commonly denominated the electrical machine; iact, because the non-conducting power of the of the structure of this instrument we have various paper prevents its transmission from one part of accounts in different works ou electricity, but as the surface to another, the effect existing over the it is not our intention to swell our pages with a whole portion that has been subjected to fric- repetition of what others have said before us on tida.

what is now become obsolete, we shall not put (3.) Excite the dry sheet of paper as before, our readers to the trouble of travelling over an and place it upon an insulating stand, a piece uninteresting description of apparatus which has of apparatus to be described hereafter, present nothing to recommend it but its antiquity. the knuckle to the edge or under side of the metal 74. Of the Electrical Machine, there are now plate, and a bright spark will appear; but a various kinds in use; these, however, may be second approach will produce either a very slight classed under two heads, the cylindrical and the effect, or none that is perceptible; for the metal plate machine. But before entering on a paris a conductor, and it transmits the whole effect ticular description of these we feel strongly inof the excited electric at once. Hence insulated clined to lay before our readers the following conductors are employed in the electrical ap- remarks of Mr. Singer on electrical apparatus iu paratus to receive or collect the diffused elec- general. tricity of excited bodies, and to apply it to the 75. The structure of an electrical apparatus, purposes of experiment.

says this distinguished electrician, consists in the 71. It is rather remarkable that the ingenuity judicious arrangement of insulators and conducof Mr. Singer did not lead him to try the ex- tors, so that the former shall prevent the dissipa

Bess.

tion of the effects the latter are employed to col- its appendages. (4.) The conductor or conductors. lect or transmit; thus the cap and leaves of the The glass cylinder of the machine is put in mogold leaf electrometer form a conductor intended tion by a simple winch. This is less liable to be as a test of electrical action; but to fit this con out of order than those that are turned with a ductor for its purpose it is insulated, being sup- multiplying wheel, and also enables us to exported on the glass cylinder by which the leaves cite the machine more powerfully. The cylinder, are enclosed.

FGHI, is supported by two strong perpendi76. When electricity is excited by friction, the cular pieces, DĖ. The axis of one cap of the quantity of effect is, within certain limits, pro- cylinder moves in a small hole at the upper portioned to the extent of the rubbed surface; part of one of the supports. The opposite axis hence it appears that every part of that surface is passes through the upper part of the other concerned in the production of the general effect. support. To this axis the winch or handle is Now, that this may be the case, it is essential fitted. The cushion is supported and insulated by that every part of such surface be insulating; for a glass pillar; the lower part of this pillar is friction is a progressive process, a succession of fitted into a wooden socket, to which a regucontacts; and the effect produced by it in the lating screw is adapted, to increase or diminish first instant would otherwise be destroyed by the pressure of the cushion against the cylinder. conducting power, before a second operation A piece of silk comes from the under edge of the could contribute to its increase. For this reason cushion, and lies on the cylinder, passing between electricity is most usually excited by the friction it and the cushion, and proceeding till it nearly of a conductor of limited size, against the exten meets the collecting poin of the conductor. sive surface of a non-conductor.

The more strongly this silk is made to adhere to 77. An apparatus, then, properly arranged for the cylinder, the stronger is the degree of excithe excitation of electricity, is called an elec- tation. Before the cylinder, or opposite to the trical machine. To excite positive electricity, a cushion, is a metallic tube, Y Z, supported by a glass tube, of about an inch in diameter and two glass pillar LM. This is called the conductor, feet long, is generally used; the excitation is and sometimes the prime conductor. For the produced by rubbing it lengthwise by a piece of more conveniently trying experiments with this dry oiled silk, held in the hand which is made machine, and exhib.ting the different states of to grasp the tube. In this way both the silk the cushion and conductor, there are two wires and the tube are electrified; but the electricity to be fixed occasionally, the one to the conof the silk is destroyed by the conducting power ductor, the other to the cushion; on the upper of the hand, and that of the tube only appears. part of these are balls furnished with sliding In a similar way negative electricity is procured wires, that they may be set apart from each by rubbing a large stick of scaling-wax with other at different distances. dry flannel or fur; the electrical power of the 80. It is matter of surprise that this simple sealing-wax being all that results.

and very powerful electrical machine should 78. Thus, with the most simple machinery, have totally escaped the notice of some modern two processes are employed to procure the op- writers on the subject, who have taken much posite electricities, although they are at the same pains to describe others of far inferior importtime both excited in each; but, to obtain them ance. This remark is equally applicable to both, it would be necessary to insulate the silk, another form of the cylindrical machine which or flannel, used as rubbers, either by employing is but little known, but which has many good them in a very dry state, rolled up, so as to pro- qualities to recommend it to the attention of duce the friction with one extremity, at a dis- those who prefer the cylinder to the plate matance from the hand, or by affixing them to chine. The principle on which this machine, as a glass or other non-conducting support; but well as that just described, is constructed, is that neither of these methods would be convenient adopted by Mr. Nairne; but whatever of imwhere many experiments are to be made. This provement it possesses is due, we believe, to Mr. difficulty does not occur when large surfaces of Bywater, author of an excellent little work on glass are employed instead of tubes as sources of electricity. The following is Mr. Bywater's own exc tation; for these may be made circular, and description of this machine: A A A A, fig. 2, is proper friction be communicated to them from the board on which the supporters and pillars a fixed cushion, placed on an elastic support, are erected, and by which the machine is made against which they are made to revolve. fast with cramps to a table. BBB B are two

79. We shall here give a brief description of wooden pillars or supporters, the lower ends of the two forms of the electrical machine which which are morticed into the board A A A A, and are most generally approved of, and shall begin in the upper ends of which the axis of the cylinwith the cylindrical machine. In point of power, der CCCC turns. DD is the winch by wbich the

very best kind of cylindrical electrical ma- the cylinder is turned on its axis. E E is a piece chine with which we are acquainted is the im- of wood, a part of which is slided into a groove proved one, as constructed by the late Mr. Geo. under the board A AAA, and made fast by the Adams, of London, an eminent lecturer, and thumb-screw f. GG is a glass pillar, which is author of several valuable philosophical works. fixed to the wood E E, and supports what is This machine is represented in plate I, fig. 1, called the negative conductor and rubber HH. ELECTRICITY. The parts of the machine, which II is another piece of wood, part of which is fall more immediately under our attention are, slided into a similar groove under the board (1.) The electric, or the glass cylinder which is to AAAA, and is made fast by the thumb-screw j. be excited. (2.) The mechanical contrivances by KK is a glass pillar fixed into the wood II, and which it is put in motion. (3.) The cushion and supports the prime conductor LL; to this con

« PreviousContinue »