ELECTRICITY [THK DIELECTRIC. . (53). and the capacity per unit of length of same is 1 6 2a log j This result has important applications in the theory of telegraph cables, and to a form of graduated accumulator, invented by Sir William Thomson, and used by Messrs Gib son and Barclay in their experiments on the specific induc tive capacity of paraffin (see Maxwell, vol. i. 127). ON THE INSULATING MEDIUM. It has been assumed hitherto that the medium inter posed -between the conductors in the electric field is in all cases air the most prevalent of all dielectric media ; or, where any other medium actually occurred, as in the case of the Leyden jar, it has been assumed that the result is the same as if the glass were replaced by air. Experimenters soon recognized, however, that the capa city of a Leyden jar depends very much on the quality of the glass of which it is made. But the nature of this action was very little understood, until Faraday showed by a number of striking experiments that the dielectric has a specific function in all phenomena of induction. Fara- Faraday used in his experiments two identical pieces of apparatus, day s which were virtually two spherical Leyden jars. The outer coating expert- EF (fig. 16) was divided into two hemispheres, which could be monts. fitted together air-tight. The lower hemisphere F was fitted to a perforated stem, provided with a stop-cock G, so that it could be screwed to an air- pump while the apparatus was being exhausted, and afterwards screwed into a foot H. The upper hemisphere was pierced by a tube, into which was cemented a shellac plug 13. C is a metal wire passing down through B, which supports the hollow metal sphere D, forming the inside armature, and carries the metal ball A, by means of which D can be charged and discharged. To give an idea of the size of the ap paratus, it may be mentioned that the diameters of the inner and outer spheres were 2 33 in. and 3 57 in. re spectively. Two jars were made on the above pattern, as nearly alike as possible. The equality of their capa cities was tested as follows. Both were filled with air at the same temperature and pressure. Apparatus I. was then charged, by bringing A in communica tion with the knob of a Leyden jar, while the coating EF was connected to earth. I. and II. were then placed at a moderate distance from each other, as Fig. 16. symmetrically as possible with respect to the observer and other external objects, the outer armatures in both cases being in con ducting communication with the earth. The ball of I. was touched by a small proof sphere, the repulsion of which on the movable ball of a Coulomb balance was measured; after a short interval this measurement was repeated. The balls of I. and II. were then brought into communication, and the charge divided between the internal armatures. The ball of II. was immediately tested as before, and then the ball of I. ngain. Finally I. and II. were discharged and tested for permanent "stem effect." The result of one such series of measurements was I. II. o, 254,250 124 122 1 2. Neglecting the slight dissipation of the charge, and taking a.-.count only of the "stem effect" in I., we see that the charges en I. and II. after division are represented by 122 and 124, each of which is not far from the half of the whole disposable charge in I., viz., 124 5 ; so that the capacities of the two jars must be equal. This will perhaps be clearer if we consider what would happen were the capacities unequal. Let the capacities be C and C", the potential of I. before division V, and the common potential after U, the charge on I. Q, and on I. and II. q and / after division. Then Q = CV, ? = CU, ^ = C U, and q + ^-Q,. The indication of the torsion balance is proportional to the charge of the proof sphere, that is (owing to the symmetry of the arrange ments), to the potential of the knob with which it was in contact ; or at all events this is true if we consider only read ings taken from the knob of the same jar, and that is all we shall ultimately want. But (0 + 00 U-CV; hence 9" V ~ U c" ~u Hence the ratio of the capacities is equal to the ratio of the excess of the first over the last reading to the last reading, both being taken from the knob of I. Thus, taking the unconnected values in the above experiment, the ratio of the capacities would be (250-124)^-122, i.e. 1 02. By various experiments of this kind, Faraday convinced himself of the equality of his two jars. To test the sensibility of his method, he reduced the distance between the lower hemispheres and the ball in II. from 62 in. to 435 in., by introducing a metal lining. The capacity of II. was then found to be 1 09 (the mean of two observations). He next com pared the capacities of the jars when the lower half of the space between the armatures of one of them was filled with shellac. The ratio of the capacities was found to be 1 - 5 (mean of several experiments), the shellac jar having the greater capacity. It appears, therefore, that, other things being equal, the Spedti capacity of an accumulator is greater when the insulating induct medium, or, as it is called, the " dielectric," is shellac, ca P ac1 than when it is air. The ratio of the capacity in the former case to that in the latcer * is called the Specific Inductive Capacity of shellac. This we shall in general denote by K. According to this definition, air is taken as the standard, and its specific inductive capacity is unity. Properly speaking, we ought to state the tem perature and pressure of the air ; we may assume C. as our temperature, and the average atmospheric pressure (760 mm.) as our standard barometric pressure. It is easy to obtain an approximate value of K from the above result for the shellac apparatus. Remembering that the shellac occupies only one hemisphere, and assuming that the lines of force are not disturbed at the junction of the air and shellac, we have, if p denote the ratio of the capacities, 1 4- K l^T~ p> K = 2 P- 1 This gives for shellac K = 2 - 0, the real value being pro bably greater. Similar experiments gave for glass and sulphur K= 1-76 and 2-24 respectively. Thus the specific inductive capacities of shellac, glass, and sulphur are considerably larger than that of air. Faraday was unable to find any difference in this respect between the different gases, or in the same gas at different temperatures and pressures, although he made careful experiments in search of such differences. It would lead us too far to discuss in detail the pre cautions taken by Faraday to remove uncertainty from his experimental demonstration of the existence of a specific dielectric action. The reader will find a minute descrip tion in Faraday s own surpassingly lucid manner in the eleventh series of the Experimental Researches. His discovery of the action of the medium led Faraday to invent his well-known theory of the dielectric. Ac- Fara- cording to him, the fundamental process in all electrical day s action is a polarization of the ultimate particles of matter ; tneor 7- this polarization consists in the separation of the positive and negative electricities loithui the molecules, exactly as the two magnetic fluids are supposed to separate in the theory of magnetic induction. In this view a dielectric is supposed to consist of a number of perfectly conducting particles, immersed in a medium or menstruum, which is either a non-conductor or a very imperfect conductor. When electrical action starts, the tvo electricities separate in the molecules ; but, in the first instance at least, there is no interchange of electricity between different molecules. 1 It must be noticed that the assumption is tacitly made that the air is to be replaced by shellac everywhere, or at least wherever there
are Hues of force.