ELECTRICITY. 754 ELECTRICITY. a conducting body, e.g. a metal sphere, to the earth by a conductor is most simple; it is necessary to join it by a wire to a gas or water pipe; but to connect a non-conducting body, eg. a glass sphere, to the earth is not at lirst sight so easy; it can be done, however, by liolding a Bunscn burner so that the Ihinie s])reads over the whole surface of the body, because the gases in the fianie are good conductors. In both ca.ses, it is evident that the action of the connecting conduc- tor is to make the earth itself a portion of the originally uncliarged body: and so the induced charges arc distributed, one on the earth and one on the body itself. This last is of a kind oppo- site to that of the body w-hieh originally was charged, and it remains after the connection with the earth is removed. There are tlius 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 just described. 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 be noted par- ticularly, however, that the forces of attraction or repulsion between diarged bodies vary greatly with the nature of the surrounding medium, be- ing greater in air than they would be in pure Avater: and the character of the induced charges and the question of attraction or repulsion de- pend upon the rclatirc 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- electric' to the material medium around cliarged bodies, because the electrical actions evidently take place through it. All non-conductors can serve as dielectrics, and so can certain poor con- ductors under certain conditions. It will be shown later that the potential energy of cliarged bodies depends directly upon certain properties of the dielectrics in the neighliorliood : and when- ever any motions of attraction or reiiulsion take place, they are always such that l)y the change the potential energy is decreased. A special case of induction which is of great theoreticiil and practical importance is one due to Faraday and called the "Ice-pail Experiment,' because first ])erfoinied with a metal ice-pail. If a positively charged body, suspended by an insulating cord, is lowered carefully into a nearly closed hollow conducting ves.sel 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 al)Out. If this body is removed, without having touched the vessel, the charges on the latter disappear. Similarly, if a negatively charged body had been lowered into the vessel negative charges would have ap- peared on the outside. Xow. if a charged body, together with the other body with which it was originally in contact and so electrified, is low- ered into the vessel, there is no elTect on the out- side of the latter: the action of one charge neutralizes that of the other: they are said to be of 'equal quantity,' but of opposite sign. Thus, whenever a positive charge is produced, an equal negative charge appears also. In the first e.peri- nient, therefore, with the hollow conducting ves- sel, when a positive charge ajipcars on its outer surface, there must be an equal negative charge on 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, cverj-thiiig is discharged; (2) the charges on the outer surface have not cliangcd at :ill. either in intensity or position. This exi)erinicnt proves therefore that the positive charge lowered into the vessel induces an equal negative charge on the inside of the vessel, and an equal positive charge on the outside. The series of exiierinients shows, further, that the region outside a closed conductor is unatl'ected by any jiroduction 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 hollow closed conductor there will !« no electrical forces inside. Therefore, a closed con- ductor sepaiates space into two quite distinct portions. Law of Electrostatic Actio.x. In order to give a numerical value to an electric charge vari- ous steps are necessary: (1) Two charges are defined to be equal if they produce the same efiect 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 be such that if two particles of matter have each a unit charge and are at a distance of one centimeter apart in a vacuum, the force between them is one dyne. (3) If two or three or four, etc, unit charges are given a body (e.g. by lowering them into a hollow closed conductor), it will exert a force two or three or four, etc., times that which it would if it had a unit charge. To give a numerical value, therefore, to any charge, it is necessary to find that combination of unit charges — either multiples or fractions — which has the same action on any third body as does the charge for which a number is desired. The number of these unit charges gives the numerical value sought. If, now, two p.articles of matter are electrically charged, one with a quantity < the other with a cpiantity c and if they are at a distance apart of r centimeters, the mechanical force of attrac- tion or repulsion is found by exjieriment to sat- isfy the law that 'the force varies as the product ce' and inversely as r".' In symbols, the force where K is simply a factor of proportionality, and is a difl'erent constant for dilFercnt dielec- trics. In particular, by the definition of a unit charge, if c=r'=l and r=l, f must equal one dvne if the medium is the pure ether; and there- fore on this system of units K=l for the ether of space free from matter. This system, based on the above definition of a unit charge, is called the 'C. G. S. electrostatic' system. (The 'dimen- sions' (q.v.) of an electrical charge may he found at once from the above formula, e and c' are both charge; and hence a charge has the same dimensions as the square root of Kr'f. Force has the dimensions MLT-'; therefore the dimen-
