334 



NA TURE 



IFeb. 3, il 



pu np ; a modification of Gsissler's form. — A. Gro5se, a wire- 

 tapa rheostat. Fine gerinan-silver wires are spiralled around 

 cotton threads, which are then woven into a sort of tape, the 

 warps being thereby insulated from one another. A piece 2 

 cm. wide and 4 metres long has 1000 ohms resistance. — W. 

 Holtz, a Wheatstone's bridge for air and water flow. An 

 illustrative apparatus of tubes such as has often been used. 



In the ScoHisli GcJgrjpkka! Magazine for January there is 

 an excellent bathy-orographical chart of the Clyde sea area, 

 constructed for Dr. H. K. Mill by Mr. J. G. Bartholojiew. 

 The colou'-ing of the map is designed to show with special 

 effect the area anl depths of the Firth of Clyde and its inlets. 

 For this purpose the land and sea have been treated separately, 

 and coloured in strong contrast to each other. The system of 

 colouring is, however, uniform, and in both cases the lowest or 

 deepest areas are distinguished by the darkest tints, and gradu- 

 ated up to the lighter tints of the higher or shallower portions. 

 The same num'jer contains a paper on the configuration of the 

 Clyde sea-area, which was read lay Dr. Mill at the last meeting 

 of the British Association. 



SOCIETIES AND ACADEMIES 

 London 



Royal Society, December 16 — "Note on Specific 

 Inductive Capacity." By |ohn Hopkinson, M.A., D.Sc, 

 F.R.S. 



Consider a condenser formed of two parallel plates at distance 

 X from each other, their area A being so great, or the distance .v 

 so small, that the whole of the lines of force may be considered 

 to be uniformly distributed perpendicular to the plates. The 

 space between the plates is occupied by air, or by any insulating 

 fluid. Let e be the charge of the condenser and V the difference 

 of potential between the plates. If the dielectric be air, there 

 is every reason to believe that V oc <•, that is, there is for air a 

 constant of specific inductive capacity. My o.vn experiments 

 ([1880] Phil. Trans., vol. clxxii p. 355) show that in the case of 

 flint-glass the ratio of V to c is sensioly constant over a range of 

 values of V from 200 volts per cji. to 50,000 volts per cm. 

 From experiments in which the dielectric is one or other of a 

 number o'" fluids and values of V upwards of 30,000 volts per 

 en. are used. Prof Qui icke concludes (Wiedemann, AnimUn, 

 vol. xxviii., 1886, p. 549) that the value of ejV is somewhat less 

 for great electric forc.-s than for small. Fro n the experiments de- 

 s;ribed in that paper, and from his previous experiments (Wiede- 

 mann, Annalcn, vol. xix., 1883, p. 705, ct seq.) he also cmcludes 

 that the specific inductive capaci y determined from the mechan- 

 ical force resisting separation of the plates is 10 per cent, to 50 

 per cent, greater than that determined by the actual charge of 

 the condenser. The purpose of the present note is to examine 

 the relations of these imoorlant conclusions, making as few 

 assumptions as possible. 



In wjrds, the specific inductive capacity as determined by 

 charge or discharge of a condenser at any given potential and 

 distance between the plates is the arithmetic mean of the induc- 

 tive capacities dete.'mined by the forje resisting separation of the 

 plates and that determined by lateral pressure, the potential and 

 distance being the same. This is true whatever be the relation 

 between charge and potential difference, but it is at variance with 

 the experimental re.ult that K^ and Ks are both greater than K. 



The results obta'ned by Prof. Quincke are not easy to recon- 

 cile. For that reason it is the more desirable that their full 

 significance shxild be ascertained. Full information is given of 

 all the details of his experiments except on one point. It is not 

 stated whether, in the experiments for determining K b/ direct 

 discharge of the condenser, the capacity of the connection and 

 key was ascertained. It would in most ordinary arrangements 

 of key be very appreciable in comparison with the capacity of 

 the condenser itself If neglected, the effect would be to a 

 certain extent to give too low a value of K, the effect being most 

 marked when K is large. 



The property of double refraction in liquids caused by electri- 

 fication is sometimes cited as showing that electrification is not 

 proportional to electromotive f irce. The fact that the double 

 refraction in a liquid under powerful electromotive forces is very 

 small would further sho.v that there is a close approximation to 

 proportionality, .and that the deviation frjm proportionality 

 would be insensdole to any electro-static test. Such conclusions, 



however, cannot be safely drawn in the case of bodies such as 

 caslor-oil, in which K ± /x". In such bodies, assuming the 

 electro-magnetic theory of light, the yielding to electromotive 

 force is much greater if the force be applied for such tine as 

 10— ' second than when applied for 10— ^^ .■■econd, and it is quite 

 possible that the law of proportionality might be untrue in the 

 former case, but very nearly or quite true in the latter. 



"On the Dielectric Constants of Fluids." (Addendum to 

 Dr. Hopkin en's "Note on Specific Inductive Capacity.") 

 By Prof. G. Quincke, For.Mem.R.S. 



In investig.ating the properties of dielectric fluids {Wiede- 

 mann's Aiiitalcii, vol. xix. 18S3, p. 707; vol. xxviii., i8i6, p. 

 529), I found the dielectric constants with the electric balance 

 or by the hydrostatically measured pressure of an air-bladder 

 greater than when measured by the capacity of a condenser sur- 

 rounded by air or the insulating fluid, and discharged by turning 

 a key through a ballistic galvanometer. 



The capacity of the key and of the short thin junction-wire 

 connecting the key with the condenser was, however, in that 

 calculation left out of account as being evanescently small. 



In consequence of a written communication from Dr. John 

 Hopkinson, I quite recently compared the capacity of the key 

 and the junction-wire with the capacity C of the condenser 

 by observations with the ballistic galvanometer with the same 

 difference of potential between the surfaces, and thereby found 

 the relation — 



— = 0"I762 ; 

 C 

 greater, therefore, than I had conjectured. 



Let there be subtracted from the observed galvanometer read- 

 ings .fj and i,i for the condenser in air and in the dielectric 

 fluid, the deflection calculated for the electricity on the key and 

 junction-wire, then there will actually be obtained from the ratio 

 of the readings thus corrected {s^ and (s^-^ values of the dielectric 

 constants (K) of the fluid almost exactly coinciding with the 

 measurements of the electric balance. The agreement is indeed 

 as perfect as might be expected, considering the difference in 

 the methods of observation employed. 



Thus, for example, it was found : 



Ether 



Carbon disulphide 



Benzole 

 Petroleum 



[Note added by Dr. Hopkinson. — Prof. Quincke's explana- 

 tion sets tlie questions I have raised at rest. There can be 

 little doubt that K, K,, and \^p are sensibly equal and sensibly 

 c instant. The question what will happen to K/ and Kj if 

 K is not constant has for the present a purely hypothetical 

 interest. 



Physical Society, January 22. — Prof. McL»od, Vice- 

 President, in the chair. — Dr. F. Wormack was elected a 

 Member of the Society. — The following papers were then 

 read: — The permanent and temporary effects on some of 

 the physical properties of iron produced by raising the 

 temperature to 100° C, by Mr. Herbert Tomlinson, B.A. — 

 On some new measuring-instruments used in testing mate- 

 rials, by Prof. W. C. Unwin, F.R.S. In most measuring- 

 instruments previously used, it has been considered sufficient to 

 make the measurement of elongation from one side of the bar, 

 but this, the Professor showed, w.as liable to serious errors owing 

 to the fact that test-bars are not always perfectly straight, and to 

 the p issibility of originally straight bars being bent by improper 

 fixing in the testing-machine. In such cases the modulus of 

 elasticity calculated from the apparent elongations are subject to 

 consilerable error. In endeavouring to overcome these diffi- 

 culties the author has devised several new forms of measuring- 

 apparatus, which a-e attached to two sides of the bar by steel 

 points, and the mem elongation of the two sides determined. 

 The first apparatus described consists essentially of sliding 

 calipers read by microscopes to 1/10,000 of an inch. Anothe.- 

 form has two clamps provided with sensitive levels. Each is 

 attached to the bar by two steel points, the line jo'.ning which is 



