793 



ELECTRICITY, COMMON. 



ELECTRICITY, COMMON. 



794 



bodies. [ATTRACTION.] We adopt at present the theory of two 

 fluids, but all the phenomena may be readily expressed also on 

 Franklin's theory. 



The pressure of the electricity on the surrounding medium, when 

 the body is perfectly conducting, determines the direction of the 

 motion under the influence of foreign electrised or non-electrised sub- 

 stances, which, by rendering this pressure unequal on the different 

 parts of the surface, produce motion by the unequal reaction of the 

 medium. But imperfectly conducting bodies have in themselves a 

 certain retentive or coercive force, and the electrical particles, instead 

 of then freely obeying the external impressed force by a corresponding 

 law of arrangement or accumulation amongst themselves, communicate 

 the forces impressed to the particles of matter by which they are 

 restrained. In imperfect conductors the force is partially exercised in 

 each of these ways. The circumstances of the motions of electrised 

 substances therefore vary with their conducting faculty. 



We can now understand the mode in which light substances are 

 attracted to a stick of sealing-wax which has been made electrical by 

 friction ; the electricity of the wax is in this case negative ; and when 

 brought near a small piece of paper, which is a conductor, it acts upon 

 the neutral fluid of the paper, attracting some of its positive electricity 

 to the side next it, and forcing the negative to the farther surface, 

 which, being in communication with the ground or a conductor, is 

 carried off; so that the paper is thus by influence made positively 

 electrical, whichjbeing of a contrary kind to that of the wax, is attracted 

 by it, and therefore the paper flies to the wax, and having touched it 

 communicates its positive electricity to it, thereby neutralising a 

 portion of its free fluid ; after which it shares a part of the surplus 

 of negative electricity remaining on the wax, when it is of course 

 repelled ; and if it become neutral by again touching the ground, and 

 the electrical force has sufficient energy, it will again fly to the wax 

 and the same results will be repeated. 



When a body is of an irregular figure, and is electrised, the elec- 

 tricity of its surface will be differently accumulated at the different 

 parts, projecting points having the most, and portions of small curva- 

 ture the least in convex surfaces ; and it is a mathematical problem of 

 considerable difficulty in some cases " to find the law of the distribu- 

 tion of free electricity on the surface of a perfectly conducting body 

 of a given form." The datum for the solution is, that the whole 

 action of the electric envelope on any point interior to the body is 

 zero : we have shown that it would be so in the case of a sphere 

 by a uniform distribution on the surface : but in other bodies this 

 distribution cannot be uniform to produce the same effect. The 

 next case in the order of simplicity is the spheroid, or more gene- 

 rally the ellipsoid, for a spheroidal shell, bounded by two similar and 

 concentric spheroidal surfaces, and attracting by the law of the 

 inverse square of the distance, will exercise no action on an internal 

 point; hence the accumulation of electricity on the surface of a 

 spheroid at any point is proportional to the normal breadth of the 

 stratum at that point, which it may be easily proved is proportional 

 to the perpendicular drawn from the centre on the tangent plane, or 

 inversely as the diameter parallel to the tangent at that point. 



Hence we see why the accumulation of electricity at points is 

 go great, which are therefore part of the armature of prime con- 

 ductors ; for if we conceive the axis minor of an ellipse to diminish 

 indefinitely, while the axis major remains invariable, the breadth of 

 the spheroid generated will be correspondingly diminished while the 

 length remains the same, and ultimately it will approximate to the 

 form of a needle pointed at the extremities of its axis major, the 

 breadth of the electricity at the point is then to that at the middle of 

 the needle as the length of the needle is to its greatest breadth. Now, 

 in consequence of the law of force being the inverse square of the 

 distance, we find the pressure against the air is as the square of the 

 accumulation , and consequently is very much greater at either ex- 

 tremity of the needle than at or towards the middle ; and therefore, 

 on being overcharged, the electrical spark is given from the extremity, 

 when not otherwise determined by the influence of external bodies. 



Moreover, when several conducting bodies, some or all of which are 

 electrised, are placed near each other, a new distribution of electricity 

 takes place on then- surfaces, caused by the decomposition of the 

 neutral fluid of each by the action of the extraneous substances : thus, 

 the principle for calculating the distribution in this case on every body 

 is to suppose it such that the total action on any point within each of 

 the conductors shall be zero ; for if not, the neutral fluid at that point 

 would be decomposed, and the separated fluids proceeding to the 

 surface of the body would alter the distribution. When the dis- 

 tribution ia ascertained, then the motions of the bodies may be 

 calculated according to the laws of dynamics, the pressure against 

 the surrounding medium being as the square of the accumulation. 



Two spheres placed in contact and electrised will have the point of 

 contact neutral. This result of theory (founded on the principles 

 above detailed), with many others, has been fully confirmed by expe- 

 riment. Those who wish to follow up the mathematical principle here 

 noticed, may ee Poisaon's ' Memoirs on Electricity ' (' Memoires de 

 1'Institut '). _and an English treatise expressly on this subject by Mr. 

 Murphy of Cambridge. We may however here introduce a few 

 popular details on the subject, chiefly with reference to the distribu- 

 tion of electricity or the densities of the electric fluid in different 



bodies placed so as to act electrically upon one another, or in different 

 parts of the same body when the latter had been subject to the 

 electrical influence of another body. 



When a body possesses equal quantities of the opposite kinds of 

 electricity, the fluid [may be conceived to be uniformly diffused 

 through it ; and two such bodies being placed in contact, will produce 

 no effect on each other, because the attractions existing between the 

 particles of the unlike fluids are counteracted by the equal repulsions 

 between the particles of the like fluids. But if a body acquire by any 

 means an excess of either the positive or negative fluid, a mutual 

 action will take place between this body and any other in its vicinity, 

 the fluids in the latter being previously in equilibrio or not. 



It was early found that two electrified spheres, equal in magnitude, 

 but of which one was solid and the other a thin shell, acquired 

 precisely equal quantities of the electric fluid; and hence it was 

 inferred that such acquired fluid existed only upon or near the surface 

 of an electrified body. The experiments also of Franklin on a charged 

 Leyden jar with interior and exterior coatings, which could be re- 

 moved and replaced at pleasure ; and those of Coulomb, in which, by 

 a proper apparatus, electricity was drawn from the surface of a 

 charged body when none could be obtained from the bottoms or sides 

 of holes sunk in it to certain depths, sufficiently prove that the 

 electric fluid exists in a stratum on the surface or rather within the 

 body at an infinitely small distance from the surface. 



If a sphere have uniformly diffused over its surface a charge of 

 either kind of electricity, and there be placed in contact with it a 

 sphere of equal magnitude in which the electric fluids are in equilibrio, 

 a mutual action between the spheres will take place ; the effect of 

 which will be that the fluid particles of opposite kinds in both 

 spheres will have their equilibrium disturbed, and equal quantities of 

 both kinds will flow to the surface of each : the density of the fluid, or 

 the thickness of the stratum at either surface, being different in the 

 circumferences of circles at different distances from the point of 

 contact. 



The investigation, by mathematical analysis, of the laws of the 

 distribution of electricity being too extensive for this place, it must 

 suffice to state the results of some of the experiments made by 

 Coulomb on that subject. 



When a sphere, charged as above supposed, is placed in contact 

 with another in which the fluids are in equilibrio, whatever be the 

 ratio between the diameters of the spheres, the density or the thick- 

 ness of the stratum of fluid on each is, at the point of contact, equal 

 to zero; and, at the opposite extremity of the diameter passing 

 through that point, the density is the greatest. Representing the 

 density opposite to the point of contact by unity, the results of the 

 experiments with two equal spheres show that, in an indefinitely 

 narrow zone at thirty degrees from the point of contact on each, the 

 density may be represented by 0'2 ; at sixty degrees from that point, 

 by 0'7b' ; and at ninety degrees, by 0'95 : it is found also that the 

 mean density on each sphere is represented by 0'79. 



When two spheres, in the circumstances just mentioned, but 

 having the diameter of one double the diameter of the other, are 

 placed in contact, the experiments of Coulomb show that, on the 

 smaller sphere, the density opposite to the point of contact is, to 

 the density at ninety degrees from that point, as 1 to 075 ; and at 

 sixty degrees, as 1 to 0'44 : they also show that, at ninety degrees 

 from the point of contact, the density on the smaller sphere is to 

 the density on the larger as 1 to 0'8 ; and that the greatest den- 

 sity on the smaller sphere is to the mean density on the greater as 



1 to 0'64. When the diameters of the spheres were to one another 

 as 4 to 1, it was found that the greatest density on the smaller 

 sphere was to the mean density on the greater as 1 to 0'43. 



If two spheres which have been placed in contact are subsequently 

 removed beyond each other's influence, the density of the fluid at the 

 surface of each becomes uniform ; but on the larger sphere the density 

 is always greater than on the smaller sphere. From Coidomb's 

 experiments it is found that when the diameters of the spheres are as 



2 to 1 the densities are as 1'08 to 1 ; and when the radii are as 4 to 1 

 the densities are as 1'3 to 1. That philosopher infers also from his 

 experiments, that in no case can the ratio of the densities on two 

 unequal spheres be greater than that of 2 to 1. 



When an electrified globe and a circular disc were placed in con- 

 tact, Coulomb found that the quantities of fluid on them were directly 

 proportional to their entire surfaces. The distribution of electricity 

 on the surface of a spheroid has already been noticed. 



When electricity is generated by the friction of two substances, one 

 acquires positive, the other negative electricity, but it is difficult to 

 judge a priori, fr9m the nature of the substances employed, the cha- 

 racter of the electricity which each will take ; and though most treatises 

 contain tables of substances in which each is positive to that which 

 precedes and negative to the succeeding, yet the nature of the electricity 

 is so liable to alteration, from very minute circumstances of the 

 friction, that it is better, even in each case, to try direct experiment. 

 The friction produced by liquids also produces electricity, the electric 

 light, when a barometer well freed from air is first filled with mercury, 

 having been remarked from the earliest dates of the use of that instru- 

 ment [BAROMETER] ; and we have seen in the case of the hydro- 

 electric machine [ELECTRICAL MACHINE], that water in the form of 



