ELECTRICITY. COMMON. 



THICITY, COMMON. 



7V 



mall drop* acts M th rUxr. So aluo when current ( air ii ill 

 noted gint a plate of glass, the later will acquire |witive eleo- 

 tricity, and therefore the air negative, and the rapid agitation of 

 piece of silk in the air oommunioaUM to the latter |M>tive electricity 

 while the silk acquire, negative. 



The difference of temperature of a substance often determine* the 

 species of electricity which it acquire, by friction. Oenerally an in 

 crease of temperature ^-fi " to negative eleotricity, and polish or 

 amoothneei to positive ; pressure on many crystal* will produce opposite 

 electricities, a< will also heat (a* in tourmaline), and even the alight 

 adherence which a piece of gUnd taffeta would have to an isolated 

 metallic plate which it cover* i* ufficient to give the plate negative 

 eleotricity, which if the more remarkable from the fact that the 

 friction of the two would have made the taffeta negative and the plate 



Moreover, both the electricities are produced in most of the chemical 

 compositions and decompositions, in the ludden fracture of substance*, 

 ia evaporation*, 4c. ; and the higher couches of the air are in a *tato 

 of positive electricity when unoccupied by clouds, which are found in- 

 differently charged with either. [KLF.OTRICITT, ATMOSPHERIC.] 



When a body is positively electrised, we can procure the negative 

 electrisation of another conducting substance by the influence of the 

 former on the neutral electricity of the latter : this is what is under- 

 stood by t/ettrical imlnfliun. Let the conductor be placed ill the 

 vicinity of the influencing body, but not so close as to receive any 

 positive electricity by sparks or other direct communication. The 

 natural electricities of the conductor will be then separated by the 

 influence of the positively electrised body, towards which the negative 

 electricity must be attracted and the positive repelled ; the part of the 

 conductor nearest the influencing body must therefore be covered with 

 negative electricity, and that more remote with positive. If, now, this 

 end of the conductor be made to communicate with the ground, the 

 positive electricity will escape into this great reservoir, and moreover 

 sufficient negative electricity will be communicated from the ground 

 to the conductor to render the point of contact neutral : thus the con- 

 ductor acquires a double charge of negative electricity, and when 

 isolated will be found negatively electrised after it has been removed 

 from the vicinity of the isolating body. 



The effects of influence or induction, as above described, may be 

 easily observed in the following manner: Place a long and narrow 

 insulated conducting cylinder before a body strongly electrised, and 

 from different equi-distant points of the cylinder suspend pairs of pith- 

 balls by cotton threads, which will acquire the electricities of the parts 

 of the cylinder with which they are connected. We shall observe a 

 considerable divergence in the pair suspended nearest the influencing 

 body, because they are strongly charged with an electricity of a con- 

 trary nature to that of the body : going along the cylinder, the diver- 

 gence diminishes, and at a point not as remote as the middle of the 

 cylinder there will be no divergence. Beyond this neutral line the 

 cylinder has an electricity of the same kind as the influencing body, 

 increasing in intensity towards its farthest extremity, and therefore the 

 strings commence to diverge more and more as we approach that end. 

 In making this experiment a single pair of pith-balls moved along the 

 cylinder will be sufficient if we secure them from the direct influence 

 of the body by a piece of glass interposed near them. 



This is the direct influence which the electrised body has on a neutral 

 body, very different from the methods of developing electricity by 

 excitation and communication already referred to. Here is a charged 

 body exerting an inductive action on an insulated cylinder, and de- 

 veloping on it not merely one electricity of an opposite kind to itself, 

 but equal quantities of the two electricities. We shall return to the 

 phenomena of induction presently, hi order to point out some of the 

 results obtained by Faraday and the modern electricians : but having 

 now seen the action of an electrised on a neutral body, wejmny remark 

 that the neutral body must again re-act on the original body, sensibly 

 decomposing its electricity if it be a conductor ; and thus the true 

 arrangement of the electricity, in two surfaces influencing each other, 

 although instantaneously effected, may be regarded as the final effect 

 of a succession of direct and reflected influences between the bodies. 

 [ELECTRICAL IMAGES.] This principle has been shown by Mr. Murphy 

 materially to facilitate the actual calculation of the distribution of 

 electricity on two electrised surfaces in presence of each other. 



The effect of the influence of a near electrised cloud has been felt by 

 several persons, among other* by the writer ; and in many cases fatal 

 reunite liavc followed, not from the direct discharge of the electricity, or, 

 as it u called, the lightning, but from the sudden re-union of the elec- 

 tricities which bad been separated by influence, and which, upon the 

 discharge of the cloud, is effected by means of a corresponding electric 

 charge brought through the body from the ground. 



Prom the power of separation of the neutral fluid in bodies at a 

 distance which is exercised by electricity, an easy means hss presented 

 iUelf by which a much greater quantity of electricity may be collected 

 upon a conducting plate than that which could be directly communi- 

 cated by a conductor. Wo shall therefore now endeavour to explain 

 the principle of the condenser, which wo think is very inaccurately 

 stated in Blot's Physique,' in which the subject of electricity is 

 treated, generally (peaking, in a very luminous manner. 

 The following investigation the author of this article gives, on his 



own responsibility, with the desire of placing the power of the condenser 

 on iU true basis : 



I 



c 



Suppose two equal conducting plates, of which the axes are A B, c n 

 to communicate respectively at A and D with known sources of elec- 

 tricity, and have their opposite faces B, c, near to each other and 

 parallel, the whole being surrounded by a non-conducting medium, the 

 known sources of electricity communicate quantities K, E' of eleotricity 

 to the bases B, D, and the mutual influences of the system generate 

 other quantities x, x' on the second base* B, c, these quantities are 

 dependent on r., E', on A B, c D, which for simplicity we shall suppose 

 both equal to c, and on the mutual distance B, o of the plates, which 

 we shall call a. Our problem is to find x and x' from these data, 



Consider the total action on a point r, taken anywhere within the 

 first plate and on its axis ; this must be equal to tern, in order that 

 the neutral electricity at that point may not be further decomposed. 

 Let i'B = r. 



The action arising from the base A and the adjoining portion of the 

 sides of the plate included between A and a parallel drawn through r is 

 E/(c z) ; the form of the function /is unknown, since it depends on 

 the law of the distribution of the fluid at the different parts of the 

 base and aides. 



Similarly, the action arising from the base B = x/(:) 



' 



Our first equation of condition must therefore be 



(1); 



and if we consider in precisely the same way the equilibrium f n 

 point Q within the second plate and in its axis, we obtain (putting 



(2). 



The equations (1) and (2) must hold true for all values of s and :' 

 between o and c, and they serve'to determine the form of the function 

 and the values of x, x'. 



If the bases were infinite, / (z) would be constant (' Principia,' 

 book xiv.) 



Now/(z) =/ (o) + /' (o) . z + f (o). 2 + &c. by Maclaurin'n 

 Theorem : = /(o) [ 1 + -\ nearly ; for i being very small, we reject 

 the powers higher than the first, and put - for abridgment, instead of 



yrj\ ! c to introduced for homogeneity. 



Wo may observe that n is necessarily a very small fraction in UK- 

 actual case ; for it depends on , R being the linear dimension of the 



base, and it vanishes when R is infinite. 



The equations maybe thus simplified; and dividing them by/(o) 

 they become 



(X -t- X' -r B + E') = . . . . (8) 



X' + X f 1 + J E' (1 + ) + E ^1 + + 



+ -jf- (X + X' + K + tf) = (4). 



HA 



Hence by subtraction and dividing by , we obtain 



Since r tf may be positive, negative, or zero, and yet thin equation 

 always true, we must have separately 



2 e 2 r\ 1 



*" o" l "o") ! (9 > 



X'-rX= -(+') /....(/) 



It will be useful to make a few remarks before proceeding further 



