ELECTRICAL ALARUM. 



ELECTRICAL ATTRACTION. 



782 



Dry gases, atmospheric air, highly elastic steam. 



Ice at Fahr. 



The substances in the above list are also called non-conductors or 

 insulators. 



The following is a list of electrical conductors ; or, as they are some- 

 times called, non-electrics. 



LIST OF NON-ELECTRICS. 



Metals. 



Well- burnt charcoal. 



Plumbago. 



Acids, acid solutions, and saline fluids. 



Water, and moist vegetable matter. 



Living animal matter. 



Flame, smoke, steam. 



The term electric, although having a definite substantive meaning, is, 

 from the general love of contracting long words, often used as an 

 adjective in the place of electrical. In arranging the following articles, 

 we have consulted the reader's convenience by restoring in many cases 

 the words to their original meaning, although in compliance with 

 custom we shall use the expression electric tdtyraph, &c., instead of 

 >-al telegraph, &c. 



ELECTRICAL ALARUM. [TELEGRAPH.] 



ELECTRICAL ATTRACTION. Under this designation it is in- 

 tended to notice the apparent attractions and repulsions which take 

 place when bodies are in particular states with respect to electrical 

 power. 



From a very early time it was known that amber, when excited by 

 being briskly rubbed, seemed to draw towards it such light bodies as 

 straws and feathers : and the phenomenon, as well an that of an 

 apparent repulsion, is now exhibited in a more striking manner by 

 presenting a cylinder of excited glass or of excited wax to a pith ball 

 or a downy feather suspended by a silk line. [ELECTRICITY, COMMON.] 



In attempting to explain by what means these actions might arise, it 

 is imagined that there may exist in all material substances a fluid, or 

 fluids, whose particles exercise upon one another attractive or repul- 

 sive powers, while a mutual attraction may take place between the fluid 

 particles and those of the substance with which they are combined. 



The hypothesis of Watson and Franklin was that there exists but 

 one kind of such fluid : it was supposed that, in exciting glaus and 

 some other bodies by friction, the fluid was abstracted from the earth 

 through the material by which the friction was produced, and accumu- 

 lated upon the surface of the body, which thus obtained more than its 

 natural quantity. It was supposed that this excess readily passed into 

 any body near it, provided the latter hail less than, or only its natural 

 quantity, or at most had not so great a quantity in proportion to its 

 magnitude. In the excitement of resinous bodies it was supposed that 

 the fluid passes from those bodies, through the material employed in the 

 friction, to theearth; and thus the resinous bodies were supposed to have 

 less than the natural quantities. Hence arose the distinction between 

 positive and negative, or redundant and deficient electricity ; and the 

 theory of ^Epiniis and Cavendish relating to the distribution of elec- 

 tricity on the surfaces of bodies is in conformity with this hypothesis. 

 But the hypothesis is open to some objections : thus, when bodies are 

 deprived of the fluid which, when they are in the ordinary state, is 

 combined with them, they are observed to repel each other mutually ; 

 and it must therefore be inferred that the particles of bodies exercise 

 upon each other a strong repulsive power, a circumstance which can 

 scarcely be reconciled with the general attraction existing among the 

 bodies in nature, or with the cohesive power by which the particles of 

 all bodies are held together. It is moreover impossible to conceive, as 

 Biot observes, why negative electricity, that is, a mere absence of fluid, 

 should be developed on the surfaces of bodies according to the hydro- 

 statical laws by which a real fluid would be developed. 



To this hypothesis succeeded that which is now generally received. 

 It was proposed about the same time by Symmer in England and 

 Du Fay on the Continent, and consists in the assumption of two fluids, 

 of directly opposite qualities, existing at the same time in combination 

 with the particles of all bodies in nature : the particles of each fluid 

 are conceived to exert on one another a strong repulsive force, while 

 the particles of the unlike fluids mutually attract each other. The 

 experiments of Coulomb with the electrical torsion balance have proved 

 that the attractive and repulsive forces vary in intensity inversely as 

 the squares of the distances between the particles, and that at equal 

 distances the attractions and repulsions are exactly equal. 



The manner in which the mutual actions of electrical particles pro- 

 duce the phenomena of attraction or repulsion in bodies with which 

 they are combined, omitting the consideration of the effects arising 

 from the decompositions of the natural electricities of the bodies, is 

 conceived to depend in part on the pressure of the atmosphere, and in 

 part on the conducting powers of the bodies. Thus, if by any excite- 

 ment electrical particles be collected on the surface of two bodies 

 (suppose two spheres) which are non-conductors, they will be retained 

 there, by the pressure of the surrounding air, and by the impermea- 

 bility of the material : hence the bodies must take the motions which 

 result from the repulsion or attraction of the fluids on them, according 

 as those fluids are of the same or of unlike kinds. If one of the bodies 

 be a conductor and the other a non-conductor, and they be both 



charged, for example, with electricity of the same kind, while that 

 which is on the surface of the latter suffers little change of disposition, 

 that which is on the other moves freely through its substance, and is, 

 by the mutual repulsion of the particles, driven in abundance to the 

 opposite side, so as to be more dense, or to form a thicker stratum 

 there than on the side nearest to the non-conductor. But the re-action 

 of the electrical fluid against the surrounding atmosphere being pro- 

 portional to the square of the thickness of a stratum, it follows that 

 there is an excess of force tending to make the conducting body recede 

 from the other ; and it is easy to perceive that a like explanation may 

 be given of the contrary movement which takes place in the conducting 

 body when the two are charged with unlike kinds of electricity. 

 These views, however, have of late years been greatly modified, in con- 

 sequence of the researches of Faraday. [ELECTRICITY, COMMON.] 



The circumstances relating to the distribution of electricity on the 

 surfaces of bodies agreeably to the hypothesis of two fluids, have been 

 determined with the aid of a most refined analysis, by La Place, Ivory, 

 and Poisson ; and the ' Theory of Electricity,' in the ' Encyclopaedia 

 Metropolitana,' contains an investigation of the distribution on the 

 surfaces of spheres. The results are there computed numerically, and 

 compared with those which Coulomb has obtained by direct experi- 

 ment, and the discrepancies are found to be only such as may fall 

 within the limits of the unavoidable errors in the experiments them- 

 selves. 



When a magnetised body is placed near one which is not so, a de- 

 composition of the natural magnetism in the latter takes place, similar 

 to that decomposition of the electrical fluids which occurs when a body 

 in its natural state is brought near one which has been excited by 

 friction [ELECTRICITY, COMMON], and all the phenomena exhibited by 

 magnetised bodies on one another are capable of being explained on the 

 hypothesis of two fluids, the particles of which repel or attract one 

 another according as they are of the same or of unlike kinds, the in- 

 tensities of the forces being inversely proportional to the squares of the 

 distances. 



The hypothesis of two fluids, moving in opposite directions, is also 

 adopted in order to explain the phenomena of galvanism and electro- 

 magnetism ; that which is considered as positive flowing from the zinc, 

 through the acid, to the copper, and that which is called negative, from 

 the copper, through the acid, to the zinc ; but the discovery that a 

 balanced magnetised needle was affected when placed in the vicinity of 

 the wire connecting the opposite ends of a galvanic battery, has giveu 

 rise to an hypothesis that the power supposed to exist in what was 

 called by the general name of an electric fluid was produced by a 

 spiral motion of the fluid current about the conducting-wire, or about 

 the particles of a magnetised needle. 



Oersted, the discoverer of the electro-magnetic action, conceived 

 that, when the circuit was completed by the wire passing from the 

 zinc end to the copper end of a battery, the positive and negative 

 fluids which, if unimpeded, would flow in straight lines, were com- 

 pelled by acting against each other to move spirally about the wire ; 

 and this opinion is still generally held, it being supposed that the turns 

 of the spiral are very near one another, so that they may be considered 

 as circles whose planes are perpendicular to the axis of the wire. The 

 course of either current, according to the explanation given by Mr. 

 Barlow, may be easily understood from the following description, 

 which relates in particular to that which is called positive : Let a con- 

 ducting wire be imagined to be so bent that part of it shall be in a, 

 vertical position, and shall pass perpendicularly through the paper, the 

 part above proceeding from the zinc or positive end of the battery, and 

 the part below to the copper or negative end ; the current of positive 

 electricity being thus from above downwards : imagine also, a small 

 needle whose north and south poles are represented by and to be 

 horizontally balanced on a pivot and placed successively at different 

 points in the circumference of a circle described about the intersection 

 of the wire with the paper. Then, the directive power of the earth on 

 the needle being neutralised by the presence of a magnet, the line n s will 

 assume in every part the position of a tangent to that circumference, the 

 direction being always such as it would be if the point revolved in ad- 

 vance of round the axis of the wire in the same order as the hands of a 

 watch move. [ELECTRO-DYNAMICS.] The negative current may be under- 

 stood to be revolving at the same time in like manner, but in a contrary 

 direction. The currents thus revolving are supposed to act on the 

 particles of the magnetised needle, disposing them to arrange them- 

 selves parallel to its axis, and so as to bring the latter in the position 

 of a tangent to the curve described by the fluid about the wire. 



The hypothesis of Ampere differs from that which has been just 

 stated : this philosopher having formed the conducting wire into two 

 parts, which could be so connected with the opposite poles of a galvanic 

 battery that the current of positive electricity might be made to pass 

 along the two parts in the same direction, or in contrary directions, 

 found that, in the former case, the wires (which were delicately sus- 

 pended in horizontal and parallel positions) seemed to attract, and in 

 the other case to repel, one another. He from thence inferred that the 

 fluids passed in rectilinear directions, or parallel to the axes of the 

 wires, and that the particles were so polarised as to present to each 

 other, laterally, when they moved in the same direction, sides which 

 had powers of attraction ; and when they moved in sontrary directions, 

 sides which had powers of repulsion : he furth a supposed, in order to 



