Variation of conductivity with direction of current (transformer oil). Variation of the ratio of viscosity to resistance with temperature (paraffin oil). the main fact to notice here is that the resistance decreases much more rapidly than the viscosity. Fig. 11 shows the difference in the rate of change of resistance with temperature of an oil film of 0.46 inch thickness when measured 15 seconds after the application of the voltage and after a steady deflexion had been obtained; and in addition the values of Ro obtained from the resistance/current relationships are shown for the different temperatures. It will be noticed Effect of temperature on resistance and viscosity of on engine oil. that the resistances measured after 15 seconds are less than the values of Ro in each case, suggesting the presence of a contact resistance before the application of any voltage. The viscosity/temperature curve is seen to be more nearly parallel to the lower curves than to that obtained for steady values of resistance at 144 volts. Some experiments have been made in an attempt to determine the effect of moisture on the resistance of an oil, and some of the results are of interest in the light of the present theory. Moisture was brought into contact with the oil in the form of water vapour, at the same temperature as the oil, by connecting a small flask of water to the testing apparatus. The amounts absorbed were very small, and it was not found practicable to measure them with an ordinary chemical balance. In fig. 12, curve I. shows the resistance/ voltage relationship before exposure to moisture, curve II. the same after 60 hours' exposure to moisture, and curve III. after 16 hours' vacuum drying after the exposure. Showing effect of moisture on transformer oil (temp. 60° C.). Curves IV. and V. show the resistance/time relationships at 144 volts corresponding to curves II. and III. respectively. The main effect of the moisture seems to be to reduce the value of the contact resistance built up without changing the resistance of the oil to any great extent, since all the five curves appear to cut the ordinate at approximately the same point. It may be asserted that the above phenomena could also be explained by the assumption that space charges are set up in liquid dielectrics. However, this seems to be negatived by two main facts :-(1) The absorption curve of an oil may take some hours to reach a steady value. If this were due to the gradual formation of a space charge, one would expect that, on removing the applied voltage and short-circuiting the film, the charge would take a similar length of time to disappear, and would give a residual current/time curve comparable with the original absorption curve. Such is not observed in oils. (2) If a space charge is built up giving an opposing E.M.F., then on reversing the applied potential this space effect should be acting in the same direction, giving a larger current than that observed at the beginning of the original absorption curve. That such is certainly not the case is indicated by the reversal curves shown. No mention has so far been made of the probable nature of the contact resistance. There is a certain amount of evidence to suggest that it exists in the form of a gas layer on one or both electrodes; but until further experimental support is obtained it must be regarded as a tentative suggestion only. an Finally, it should be pointed out that, when a contact resistance is built up, the layer so formed may have a large capacity, and thus, when the final steady current has been attained, the combination of oil and contact layer may act as ordinary two-layer dielectric, following ordinary Maxwellian theory. In the case of ordinary insulating oils, however, the residual effects so obtained would be small in comparison with the change in the resistance of the contact layer. It is conceivable that such an effect might be detected with an oil of very high specific resistance. Summary. A theory is put forward to account for the absorption current in liquid dielectrics. It is assumed that contact resistances are formed at one or both electrodes by the passage of an electric current, this causing the well-known decay of current with the time the potential has been applied across the electrodes. Experimental support of the theory is given, and it seems capable of explaining in a simple manner the anomalous conductivity phenomena observed in liquid dielectrics under D.C. stresses. My thanks are due to the International Standard Electric Corporation for permission to publish these results. I am also indebted to Mr. J. D. Cockcroft of the Cavendish Laboratory for much helpful criticism. XLI. The Action of X-Rays on Colloidal Ceric Hydroxide. By J. A. V. FAIRBROTHER, B.Sc.* IN INTRODUCTION. Na previous joint paper with Professor Crowther † we reported upon the action of X-rays on colloidal Iron, Copper, Silver, and Gold. It appeared that cationic sols, that is, sols in which the particles are attracted towards a negative pole, are coagulated, whilst anionic sols become more stable. Since then we have irradiated many more positive and negative sols and have found no exception to this rule. From the numerical investigation of a Bredig copper sol, it was suggested that coagulation was due to the destruction of the electrokinetic potential by ionization produced by the rays in the diffuse double layer surrounding the particles. By measuring the viscosity changes produced in a viscous sol by X-rays, it was hoped to study and follow the changes taking place in the electrical state of the particles. The researches described in this paper were undertaken with this end in view. Smoluchowski has developed the following formula for the viscosity of a colloidal solution: is the volume of the particles in unit volume of the solution. r is the mean radius of the particles. A the specific conductivity of the sol. the electrokinetic potential. If it be assumed that 4, X, and r remain constant, we can deduce a very simple relation between the viscosity of the sol and the electrical charge on the particles." * Communicated by Professor J. A. Crowther, Sc.D. Phil. Mag. S. 7. Vol. 6. No. 36. Sept. 1928. 2 C |