18 ELECTRICITY [EJLECTKIC QUANTITY. and the attraction of tlie separated + electricity on B stops it, and the + electricity recedes in similar fashion. When electrical equilibrium has been attained the action of the + electricity of A on the - electricity of B will exceed its action on the + electricity of B, which is on the whole more distant, 1 the electromotive force on the electricity of B will be on the whole attractive, and hence the pondero- motive force on B, will be also attractive. The above explanation involves of course the general explanation of electrification by induction. Experimental investigation of Electrical Quantity, Distribution, and Force. Electro- ID what follows we shall suppose that we have an scopes instrument which will serve as an electroscope and to ft . n ^ some extent as an electrometer ; that is, which shall tell meters." us readily whether a body brought into communication with it is + or - electrified or not at all, and also enable us to tell when one body is more strongly electrified + or - than another. The gold-leaf electroscope of Bennet or the dry pile electroscope of Bohnenberger will meet these require ments, and have been much used in electrical researches. We shall, however, suppose that we are using the rudimen tary form of Thomson s electrometer constructed by Elliot Brothers for lecture-room experiments, which is now much used in England, and answers very well. For a description of these and other electroscopes and electro meters, see article ELECTROMETER. We shall also assume for the present that we have the means of producing and communicating to any body as much of either kind of electrification as we please, and pass on to consider the data of experiment regarding the distribution of statical electricity in conducting bodies. We are thus at the very outset brought face to face with the idea of electric quantity. Electric Quantity. We have to explain how the introduction of the term quantity into electrical science is justified by experiment, and how we can multiply and subdivide quantities of electricity. Although it is no doubt possible to introduce the notion of quantity independently of the measure of electric force, yet the most convenient and practical measure of quantity de pends on the measurement of force, and the absolute electrostatic unit of quantity is stated in this way. We are naturally led, therefore, to combine with the study of quantity and distribution the experimental study of the laws of electric force. We shall have occasion to allude to two leading experi mental methods that have been used in investigating the present subject. These might be called the old method and the new. The old method, which did so much for electrical science in the master hand of Coulomb, depended on the use of the torsion balance and proof plane, both invented by Coulomb himself. This method was used by Reiss and others up to Faraday s time. (j ou . Michell, about Coulomb s time or a little before, first lomb s suggested the idea of measuring small forces by the torsion torsion of a wire. He proposed to apply the method to measure balance. j^e attraction of gravitation between two bodies of moderate size, thus finding the mean density of the earth, and the method was actually carried out by Cavendish ; but Cou lomb was in all probability unaware of Michell s suggestion. He made careful preliminary experiments (the first of the kind) on the torsion of wires, and found that the couple 1 It is here tacitly assumed that the attraction between two elements of electricity decreases as the distance between them increases. FIQ. 4. Torsion Balance. required to twist a straight wire through a given angle varies as the angle of torsion multiplied by the fourth power of the diameter of the wire directly, and as the length of the wire inversely (Mem. de I Acad., 1784). The balance used by Coulomb in most of his experiments is represented in figure 4. ABDC is a cylinder of glass 1 foot in diameter and 1 foot high. This cylinder is closed by a glass lid pierced centrically and eccen. trically by two openings, each about 20 lines wide. Into the middle opening is cemented a glass tube 2 feet high, to the upper end of which is fitted a torsion head; the sepa rate parts of the head are shown larger at the side of the figure. H is a collar cemented to the glass tube ; MO a metal disc, divided on the edge into 360 degrees ; this disc is fastened to a tube N, which slips into the collar II. K is a button whose neck turns easily in a hole in MO ; to the lower part of the button is fastened a small clamp, which seizes the wire of the balance. I is an arm with a small projecting piece which slips over the edge of the disc MO. This piece has a fiducial mark on it, which enables us to read off the position of the arm on the graduated edge of MO. The horizontal arm Id consists of a silk thread or fine straw covered with sealing wax terminated by a thread of shellac at b about 18 lines long, which carries a pith ball 2 or 3 lines in diameter. At the other end of the arm is a vertical disc of oiled paper, which serves as a counterpoise to the pith ball, as a damper to the oscillations, and as an index by means of which the position of the horizontal arm can be read off on a graduation carried round the glass cylinder. The eccentric hole in the cover of the balance allows the introduc tion of the fixed ball a ; this is carried on a shellac stem fastened to a clamp P, which by means of fiducial marks can be placed in a fixed position on the cover. The wire in Coulomb s balance was of silver, about 30 cm. long. Its diameter was 0035 cm., and it weighed about 003 gm. He found by the method of oscillations that a couple equivalent to the weight of 17 milligramme, acting at the end of an arm a decimetre long, would keep the wire twisted through 360. Besides this form of balance Coulomb used others, some more delicate for electroscopic purposes, and others less so, but of larger dimensions, into which he could introduce electrified bodies of considerable size. Faraday used Coulomb s balance, and Snow Harris used the bifilar balance, which is a modification of Coulomb s. In the second volume of his Experimental Researches, however, Faraday gives a general method of experiment ing, which to a great extent has superseded the older method. This may be called the " cage method ; ; it de pends for its success on the use of some delicate instrument for measuring differences of potential; this was supplied by the quadrant electrometer of Sir William Thomson, which has thus completely revolutionized the whole system of electrostatic measurement. Faraday s experiment was as follows (Exp. Res., vol. ii. p. 279) : Let A (fig. 5) be an insulated hollow conductor with an opening C to allow admission to the interior. Faraday used a pewter ice pail, 2 m 10 J in. high and 7 in. in diameter Connect the outside of A with one electrode of an electrometer E, which may for most purposes be the rudimentary form of Thomson s electrometer mentioned above. Connect the other electrode of the electrometer with the earth. If now we introduce a positively electrified body, say a "brass ball C,
- A cylinder of wire gauze will answer equally well, and allows the
experimenter to see better what he is doing. Such a cylinder we shall
call for shortness aa " electric cage."