ELECTRICITY. vcstigatoil by Hertz in ISSS. and more recently by Marconi antl others, who iiave made commer- cial use of them. To Faraday is due tlu" realization of the impor- tance of the properties of the nonconducliiij; media separating charged bodies. He proved that both electrostatic and magnetic actions take place thiutiyh the media, not 'at a distance.' He orig- inated the idea of lines of induction, snd estal>- lished their laws: he also explained tlie attrac- tion and repulsion of bodies caused by electrical or magnetic forces as due to the relative prop- erties of these bodies and of the surrounding medium. Clerk Maxwell expressed Faraday's ex- perimenUil discoveries in mathematical language and formuUv. and advanced the theory of elec- tricity and magnetism which is the basis of all modern theories. The idea of expressing all electrical and mag- netic f|uantities in absolute units, based on some definite system of standards of length, time, and mass, is due to Gauss (q.v.) and Weber (q.v.). The most important recent advances in the knowledge of the connection between electricity and matter have come through the observation of the ionization (q.v.) of gases. Most of the researches along this line have been made under the direction of Prof. .T. .J. Thomson, at the Cav- endish Laboratory, Cambridge. England. To understand the fundamental principles of electricity, it is well to start with the experi- ments that were performed by the early Greeks. Here it is found that, if two bodies are brought into intimate contact, e.g. by pressure or bv rubbing together, and then separated, they have certain new properties, the most marked of which is the power of attracting any small bits of matter, such as particles of dust or fragments of paper. Bodies possessing this power are said to be •electrified' or 'charged with electricity.' It is easily observed that if certain l)odies. such as glass, rubber, silk, are charged, the electrical effects are manifested only at the points where the contact was made with the other body; while Avith other bodies, such as pieces of metal, the electrical effects are manifested over the whole body regardless of where the contact was made. and the charge is said to spread over the surface. Bodies nf the latter kind are called 'conductors'; of the former, 'non-conductors.' It takes time — infinitesimally short — for a charge to distribute itself over the surface of a conductor; but in the end. imless there is furtlier charging, conditions come into a steady state; the charges are said to be at rest. The science of the distribution and other properties of electrical charges at rest is c.illed 'electrostatics.' while the study of the phenomena involved in varying and moving charges belongs to 'electric currents,' or 'electro- kinetics.' ELECTRO.STATICS. Fr.vDAMEXTAL Phe.no.mkxa. As said before, the primary fact of electricity is that, when two bodies are brought in contact and then separated, they acquire the power of attracting small por- tions of matter. Further, the two bodies which are thus charged attract each other, as may be sho'.vn by suspending one so that it is free to move, and brin^in^ the other near it. If other portions of matter are charged. e.<r. pieces of rub- ber, of dry wood, of cotton cloth, of paper, of fur, it will be observed that, when they are 753 ELECTRICITY. brought near a suspended glass rod which has been charged, there are some cases of attraction and others of repulsion. All charged bodies which attract a piece of glass charged by con- tact with silk, are said to be "negatively' charged; and all charged bodies which repel the charged glass are said to i)e 'ijositively" charged. It is found by experiment that any positively charged bodj- will repel any other Ijody [josi- tively charged: any negatively charged body will repel any other negatively charged body; any negatively charged body will attract any posi- tively charged body. It is foutid that glass rubbed with silk is I'ositivcly charged; but if rubbed with cat's fur it is negatively charged. It is possible to make a tabular arrangement of bodies in which any body when charged by contact with any other body lower down in the series becomes positively charged, the second body becoming negative. Such a table is called the 'elect ro^^tatic series'; a few of its members are the following: cat's fur, glass, paper, tlannel, silk, cork, metals, ebonite, sealing-wax, sulphur, hard rubber. As noted above, natural bodies may be divided into conductors and non-conductors; conductors being such bodies as allow chaiges to spread over them, wliile non-conductors keep the charges local- ized where the contact took place — a "charge' meaning simply a condition at the surface of a body such that there are forces of attraction or repulsion manifested there on other charged bodies or on light pieces of matter. Among the conductors are: all metals, water containing some salt or acid in solution, damp thread, the human body, the earth. Glass, sulphur, parallin, dry wood, silk, paper are non-conductors or in- sulators. Thus, if a piece of metal is held in the hand when it is charged, the charge will dis- tribute itself over the metal, the hand, and body, and sometimes over the floor if it is not dry; and, therefore, the charge left on the metal it--elf may be extremely small. It is necessary, then, in order to charge the metal strongly, to hold it in a piece of paper or other non-conductor. Charges may be transferred from a charged body to an uncharged one simply by allowing their surfaces to touch. If a charge is given a con- ductor and allowed to come to rest, it is found to be entirely on the surface, not in the interior. Thus there are absolutely no electrical forces inside a hollow, closed conductor which is charged, unless a separate charge on a different body is |)laced inside. If a charged body is brought near an un- charged one which is separated from the earth by a non-conductor, i.e. is 'insulated.' electrical forces may be observed near the latter of the two bodies; it is said to be charged by 'induction.' If the body which was originally chargeil was positive, the portion of the other Itody which is nearest it is now negatively chargeil, anil the ]ior- tion which is farther away is positively charged. Exactly the reverse is true if the body which was originally charged is negative. If the charged body is now removed to a distance, the 'induced charges' will disappear. If the body which was originally unchaiged is joined to the earth by a conductor, and if. after the charged body is brought near, this conductor is removed, (he former iiody will be foimd to be charged: and this charge will now remain even when the originally charged body is removed. To join
