There are thus three methods of charging a body: (1 ) By contact with another body and then separation: (2) by contact with another charged body; (3) by induction as j114 The fact that, when a charged body is brought near an uncharged one. there are charges induced on the latter, of such a kind that an unlike charge is nearest the former, accounts for the observed attraction of small particles of mat ter by charged bodies. It should he noted par ticularly• however, that the forces of attraction or repulsion between charged bodies vary greatly with the nature of the surrounding medium. be ing greater in air than they would be in pure water: and the character of the induced charge; and the question of attraction or repulsion de pend upon the relatire properties of the sur rounding medium and the body on which the in duced charges are. The importance of the con sideration of the surrounding medium was first emphasized by Faraday. who gave the name 'di electrie' to the material medium around charged bodies, because the electrical actions evidently take place through it. All non-eonduetors can scrve as dielectrics, and so can certain poor con ductors under certain conditions. it will he shown later that the potential energy of charged bodies depends direetly upon certain properties of the dielectric; in the neighborhood: and when ever any motions of attraction or repulsion take place, they are always such that by the change the potential energy is decreased.
A special case of induction which is of great theoretical and practical importance is one due to Faraday and called the 'lee-pail Experiment.' because first performed with a metal ice-pail. If a positively charged body, suspended by an insulating. is lowered carefully into a nearly closed hollow conducting vessel which is insulated from the earth, in such a manner as not to touch the vessel, positive charges may be observed on the outside of the latter. These charges do not change in any way, however the charged body inside is moved about. if this body is removed. without having totwhed the vessel, the charges on the latter disappear. Similarly. if a negatively charged horly had been lowered into the vessel negative charges would have ap r.eared on the outside. Nmv. if a charged body, together with the other body with it was originally in contact and so eleelrified, is low ered into the vessel, there it no effect on the out sid• of the latter: the action of one elmpre neutralizes that of the other: they are said to be of 'equal quantity,' hut of opposite sign. Thn4. whenever a positive charge is produced. an equal negative charge appears also. In the first experi Mein, therefore, with the hollow conducting ves sel, when a positive charge appears on its outer surface. there must be an equal negative charge till its inner surface. which is nearer the positively charged body lowered in from above. If this last body is a conductor, and if it is allowed to touch the hollow conducting vessel. thus forming part of the conductor, two things may be ob served: (1) there are no longer any charges in side. everything is diseharged: (2) the charges on the outer surface have not changed at all, either in intensity or position. This experiment proves therefore that the positive charge lowered into the vessel induce; an cgior/ negative charge on the inside of the vessel, and an equal positive charge on the outside. The series of experiment;
shows. further. that the region outside a closed conductor is unaffected by any production or motion of charges inside. It has been stated be fore that a charged conductor has all the charge on its outer surface unless a charge is separately introduced inside: so if charges are moved out side a hulloes closed conductor there will be no electrical forees inside. Therefore, a closed con ductor separates space into two quite distinct portions.
L,tw OF ELECTROSTATIC ACTION. 111 order to give a numerical value to an electric eha•ge vari ous steps are necessary: (1 ) Two charges are defined to be equal if they produce the same effect of attraction or repulsion on any third body: in particular, if the two together produce no action, one is equal and opposite to the other. (2) A unit charge is defined to he such that it two particles of matter have each a unit charge and arc at a distance of one centimeter apart in a vacuum. the force between them is one dyne. 13) If two or three or four. etc., unit charge; are given a body (e.g. by lowering them into a hollow (dosed emulnetorl. it will exert a force two or three or four. etc.. times that which it would if it had is unit charge. To givo a numerical vaIne. therefore. to any charge. it is necessary to find that combination of unit charges either multiples or fractions—which has I he same action on any third body as does the charge for l which a number is desired. The num .wr of unit charges gives the numerical value sought.
now, two particles of matter are electrically charged. one with a quantity e. the other with a quantity el. and if they are at a distance apart of r cent i met erg. force of tion or repulsion is found by experiment to sat isfy the law that 'the force varies as the product er' and inversely as In symbols. the force eel = where N.' is simply a factor of proportionality, and is a different constant for different dielec trics. In particular. by the definition of a unit charge• if and r=1, 1 must equal one dyne if the medium is the mire ether: and there fore on this system of units K=I for the ether of :pave free front matter. This system. based on the above definition of a unit charge. is palled the 'C. C. S, electrostatic' system. (The 'dimen sions' (q.v.) of an electrical charge may be found at once from the above formula. e and el are hoth charge: and hence a charge has the same dimensions as the square root of Force has the dimensions therefore the dimen Until something is known of the nature of I:, there can be no mechanical understanding of an electrical charge.) That portion of this law of electrostatic action which says that the force varies as cc' and inversely as r', is known as •Coulomb's law.' It has been tested t'N pe r i mem ally in two ways: one directly, by placing charges at different distances: the other indi rectly. for it can be proved mathematically that only if this law is true will there be no elec trical force inside a closed hollow conductor, e.g. a spherical shell. Coulomb (17S5) applied the first method, which is not accurate; and the second was used by Cavendish (17;3) and later by Clerk Maxwell. The fact that the force de pends upon the medium was discovered by Caven dish (about 17721. and later independently by Faraday. who thoroughly investigated the sub j1.1•1 .