Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/48

38 E L E C T K I C I T Y [THE DIELECTRIC. Schiller Method of elec trical oscilla tions. Silow. Results for fluids. was instantaneous or lasted for a considerable time. The case was different with the imperfect insulators, glass, stearine, and gutta percha, for which he has given no results. To test still farther the influence of the time, Boltzmann measured the attraction between a sulphur and a metal sphere first, when the latter was charged continu ously positive or negative, and, secondly, when it was charged positive for ^w^ ^ a second, negative for the next Tj-^Q-th, and so on ; he found the attraction to be the same in both cases, provided the charges without respect to sign were equal. This experiment establishes beyond a doubt the existence, in the case of sulphur, of a specific di electric action, which is fully developed in less than ^gth of a second. From experiments of this kind values of K were deduced, which agreed fairly well with those obtained by other methods. A very important result which he obtained was, that for a certain crystalline sphere of sul phur the values of K were different in the directions of the axes, being 4773, 3 970, and 3 811 respectively. The result realizes an expectation of Faraday. 1 Schiller employed two methods the method of Siemens, which we have already described, in which the duration of charge was from ^th to -g^th of a second, and the method .of electrical oscillations devised by Helmholtz. In the latter method K is given by the equation K = (T 2 - T 2 ) -T- (T 2 - T 2 ), where T , T, T , are the periods of oscillation of a certain coil, firstly, by itself, secondly, when con nected with an air-condenser, and thirdly, with the same condenser when the air is replaced by the insulator to be tested (see below, p. 82). In this method the duration of charge varied from aTnbns-th to S0 l 00 th of a second. The following table gives some of the results of Boltzmann and Schiller: Boltzmann. Schiller. Kbonite 3-15 2-32 3-84 2-55 276 ( 1-92 j 2-47 2 34 2-94 6-34 2-21 1-68 1-81 2-12 2-69 5-88 Paraffin (clear) .... ) Do. (milky) ... j Sulphur Uosin . . Indiarubber (pure).. Do. (vulcanized) White mirror glass. . The first column of Schiller s results was obtained by Siemens s method, the second by the method of oscillations. It will be seen that the shortness of the time of charge has affected the value of K in the last column, reducing it considerably in all cases. Boltz- manu s results are on the whole the largest obtained by any physicist ; he attributes this to the care with which he constructed his plates. Gibson and Barclay found 1 97 for paraffin, and Siemens 2 9 for sulphur. Among the more recent researches on the theory of dielectrics may be mentioned those of Hood, 2 whose results for crystals are interesting, and Wiillner, 3 who has studied the course of induction when the charge is maintained for a considerable time. There are very few fluids which are sufficiently good insulators to allow an easy determination of their specific inductive capacity. Measurements have, however, been made by Silow. 4 He used (1) Siemens s method, and (2) a method in which he observed the deflection of a quadrant electrometer corresponding to the same potential, first, when the quadrants were filled with air, and secondly, when they were filled with the fluid to be examined; the ratio of the latter deflection to the former is the specific inductive capacity of the liquid. The instrument actually used was a glass vessel, inside which were pasted pieces of tinfoil corresponding to the quadrants of 1 Exp. Res. , 1 689. 8 Poyy. Ann., clviii. , 1876. 3 Poyy. Ann., N.F. i., 1877. 4 Poyy. Ann., clvi., 1875; clvii., 187$. Thomson s electrometer. The shape of the needle was also slightly different. A fine silver wire replaced the bifilar suspension, and the deflections were read off by means of a scale and telescope. The needle and one pair of quadrants were connected with the earth, and the other pair of quadrants charged to a constant potential by connection with a battery. The results were for oil of turpentine by method (1), 1 468; by (2), 1 473; for a certain specimen of petroleum, by (1), 1 439; for another specimen, by (2), 1 428; for benzol, by (1), 1 4S3. In the researches in which Siemens s method was used, the speed of the commutator was varied considerably, but no effect was thereby produced on the value of K, which is therefore, within certain limits at least, independent of the duration of the charge. Perhaps the most important of all the recent additions Oases, to our knowledge in this department is due to Boltzmann, 5 Koltz- who has succeeded in detecting and measuring the decrease "" "" of the specific inductive capacity of gases when rarefied. The principle of his method is as follows. Suppose we have an ordinary air-condenser inside a receiver, which we can exhaust at will. Let one of the armatures A of the condenser be connected with a battery of a large number n of cells (Boltzmann used about 300 Daniell s), while the other armature B is connected with the earth. If we now insulate 15, and if the condenser does not leak, then on connecting B with the electrometer no deflection will be indicated. If, however, we increase the number of cells by one, the potential of A will increase from np to (n + 1 )p , while that of B will rise from to an amount which is proportional to p. Let the corresponding electrometer reading be /3. Suppose now that we altered the specific inductive capacity of the gas from K 1 to K 2 , both armatures being insulated, A originally at potential np, and B at potential zero ; the potential of A will, by the mathematical theory, become ~ np , while that of B remains zero. If now we reconnect A with the battery of n cells, the potential of A becomes again np. If we then connect B with the electrometer we shall get a deflection a proportional to np i hence we nave - ft K n 1 - ^- 6 V K, Let us now assume, what experiment shows to be the case, that the increase of K is very nearly proportional to the pressure, then, Jj and > denoting the manometric reading in millimetres corre sponding to K x and K 2 , we may write ( i , nu i = C[ 1.+ =3 b, 760 Here X is a constant, the meaning of which is very simple, if we assume our law of proportionality to hold up to absolute vacuum ; in fact, 1 -t- X is in that case the specific inductive capacity 6 of the gas at 760 mm. pressure, at the temperature t of observation, and l + (l + at) is the corresponding coefficient at C. The formula written above becomes therefore - g-760 In this way Boltzmanu arrived at the following values for >/K at 760 mm. pressure, and temperature C. : for air, 1-000295; carbonic acid, 1-000473; hydrogen, 1 -000132; carbonic oxide, 1 000345; nitrous oxide, 1-000497; defiant gas, 1-000656; marsh gas, 1-000472. These results are of great importance in connection with the electromagnetic theory of light. Residual Discharge. When an accumulator, whose dielectric is glass or shellac, is charged up to a moderately high potential, and one armature insulated, a gradual fall of the potential occurs. This fall is tolerably rapid at first, but it gets slower and slower till at last it reaches a certain limit, after which it remains sensibly constant for a considerable time. This fall is not entirely due to loss by conduction or convection of the ordinary kind, for we fihd that if an accumulator that has been charged to potential V, and has been allowed to stand till the potential has fallen considerably, be again charged up to potential V, then I hem lliena 5 Poyg. Ann., lv., 1875.

6 K. is now taken to be 1 for absolute vacuum.