16 ELECTRICITY [GENERAL PHENOMENA. units of time, mass, and length, was one of the greatest steps of our time. The impulse came from the famous memoir of Gauss, Intensitas Vis Magneticce Terrestris ad Meiisumm absolutam revocata, 1832. In conjunction with Weber, he introduced his principles into the measure ment of the earth s magnetic force. To Weber belongs the credit of doing a similar service for electricity. He not only devised three different systems of such units the electrodyuamical, the electrostatical, and the electromag netic but he carried out a series of measurements which practically introduced the last two systems. The funda mental research in this subject is to determine in electro magnetic measure the resistance of some wire from which, by comparison, the electromagnetic unit of resistance can be constructed. Measurements of this kind were made by Kirchhoff in 1849 ; more carefully in two different ways by Weber in 1851; by the committee of the Bri tish Association in 1863, Jkc. ; by Kohlrausch in 1870; and by Lorenz in 1873. Accounts of these important re searches will be foand in Wiedemann and Maxwell, and in the collected reports of the British Association on " Elec trical Standards." The ratio of the electrostatic to the electromagnetic unit of electric quantity is a velocity (ac cording to Maxwell s electromagnetic theory of light it is the velocity of light), the experimental determination of which is of the greatest theoretical and practical import ance. Such determinations have been made by Weber and Kohlrausch in 1856, by Maxwell in 1868, and by Thom son in 1869. The results are not so concordant as might be desired, but the research is a very difficult one. For convenience in practice the British Association com mittee have recommended certain multiples of the absolute unit, to which they have given names e.g., the Ohm, the Volt, the Farad, &c. These have become current to a great extent among practical electricians in this country. For practical purposes, an empirical standard of electromotive force has been introduced by Latimer Clark, whose value in volts is given as T457. It is very important, in order to be able to reduce chemical to absolute measure, to know accurately the electro-chemical equivalent of water. Values for this have been found by Weber (1840), Bunsen (1843), Casselman (1843), and Joule (1851). Kohlrausch (1873) made a careful determination of the electro-chemical equi valent of silver, from which the electro-chemical equivalent of water can be calculated. GENERAL SKETCH OF PHENOMENA. Funda- If a piece of glass and a piece of sealing-wax be each mental rubbed with a dry woollen cloth, it will be found that expert- ^ Q ^_ the glass and the wax have acquired the property 3llt> of attracting indiscriminately any small light body in the neighbourhood; and it will be further observed, in many cases, that the small bodies, after adhering for a little to the glass or wax, will be again repelled. These actions have at first sight a likeness to the at tractions and repulsions of magnetic bodies, but they are sufficiently distinguished from these 1st, Bytheir origin, being excited by friction and other causes in a great variety of bodies, whereas magnetic action is powerfully exhibited and communicated only by certain varieties of iron and iron ore, by nickel and cobalt, and by certain arrangements which we shall have to mention by-and-by; 2d, By the nature of the bodies acted on; for these may be, in the case of excited glass or wax, light particles of any substance, whereas the only bodies powerfully acted on magnetically are either magnets or their equivalents, or iron, nickel, and cobalt ; and 3d, By the fact that every magnet has two poles possessing opposite properties, whereas an electrified body may have similar properties in every part of its surface. If the experiment were carefully tried it would be found that a piece of glass excited as above repels another piece of glass similarly excited, but attracts an excited piece of wax. A convenient way of exhibiting these actions, which also brings under our notice another fact of fundamental importance, is as follows. Two gilt balls of elder pith are fastened to the ends of a light needle of shellac, which is balanced horizontally on a point carried on a vertical stand (fig. 1). To the stand a stop is fixed for con venience, to prevent the needle from spinning more than half round. If we touch the ball A with a piece of excited glass, and B with a piece of excited sealing-wax, and touch a ball C, fastened to a shellac stem, with a piece of excited glass, then C will chase A away till it is brought up by the stop, while it will, on the other hand, attract B. If, again, C be touched with a piece of excited wax, it will attract A and repel B. Pieces of glass or wax excited in this way are said to be Dcfir electrified, and the balls which by contact have acquired tion properties similar to those of the originally electrified bodies electl are said to be electrified by conduction. It appears from the above experiment that the electrifi- ducti cations of glass and sealing-wax, when rubbed with wool len, have opposite properties, which they communicate to bodies brought into contact with them. A body which has similar electrification to a piece of glass rubbed with wool len is said to be vitreously or positively electrified; a body with similar electrification to a piece of sealing-wax rubbed with woollen is said to be resinously or negatively electri fied. The result of the above experiment may then be summarized thus : Bodies similarly electrified, whether positively or nega tively, repel each other. Bodies oppositely electrified attract each other. We have seen that a pith ball becomes, by contact with a Cone positively electrified piece of glass, itself positively electrified. tors If we take two pith balls, electrify one of them positively, " on and then touch both simultaneously by a piece of thin wire, suspended by white silk, and test them with the electroscopic needle described above, they will be found both positively electrified ; each will repel A and attract B, though less powerfully than the originally electrified ball did, before the connection between them was made. The success of the experiment will be found inde pendent of the length or shape of the wire, and will be equally good with silver, gold, iron, lead, or any other metal. But, if we use a thread of glass or shellac to con nect the balls, the electrification of the first ball will be found unaltered, and the second will remain neutral that is, it will not attract or repel another neutral ball, and will equally attract both balls, A and B, of the electroscopic needle. The difference in the power of transmitting elec trical properties from one body to another, or of aiding in electrification by conduction, leads us to divide all sub stances into two classes conductors, which do very readily, and non-conductors ; which do not, or do not very readily, transmit electrification from one body to another. If we connect an electrified conductor by means of another con ductor to a very large conducting body, such as the earth, it will be found that so much electrification has been carried away from the small body that it is left sen sibly neutral. If, accordingly, we wish a conducting insu: body to preserve its electrification unaltered, we must sup- tion > port it on some non-conducting substance. When thus insul
supported the body is said to be insulated, the non-con- ors>