502 Mr. A. W. Ash ton on tlie 



1898) has tested certain dielectrics, viz., guttapercha, paraffin, 

 and sulphur, and has found an increase of conductivity at the 

 higher voltages. 



3. In all the above experiments, ihe resistance of the di- 

 electrics examined has been calculated from the values of the 

 current obtained at not more than a few minutes 5 electrifica- 

 tion. Unfortunately these values of the current are those 

 which are most affected by the previous electrical treatment 

 of the dielectric. In a paper by Ayrton and Perry, " On the 

 Viscosity of Dielectrics " (see Proc. Roy. Soc. vol. xxxvi.) , 

 it is suggested that more accurate values of the true con- 

 ductivity of a dielectric might be obtained by taking read- 

 ings of the current when it has become steady. An attempt 

 has been made to examine certain dielectrics by this method 

 in order to ascertain the conductivity at different pressures. 

 In the above paper, Professors Ayrton and Perry also drew 

 attention to the analogy existing between the behaviour of 

 bodies under mechanical stress and that of dielectrics under 

 electrical stress ; it is further pointed out that careful dis- 

 tinction must be drawn between that part of the energy 

 absorbed by the dielectric which is afterwards recoverable, 

 and the portion which is converted into heat, and which 

 determines the amount of true conduction taking place. The 

 form of the " residual charge " curves has been examined in 

 the case of glass by Hopkinson (Phil. Trans, vols, clxvi. & 

 clxvii.). In these experiments the condenser was charged 

 for a definite time and then discharged for an instant. The 

 plates of the condenser were then joined to an electrometer, 

 and readings of the residual potential were taken from time 

 to time. Amongst other results, Hopkinson found that this 

 residual potential was proportional to the exciting potential. 



4. In the experiments which are now described these re- 

 sidual charge effects have been examined in the case of 

 several dielectrics by means of curves showing the current 

 (1) when the condenser is charged with a battery having a 

 galvanometer in series with it ; (2) when the condenser is 

 discharged by connecting directly to a galvanometer. The 

 examination of rubber for deterioration in insulating proper- 

 ties when subjected to the continuous action of alternating 

 pressure, has been suggested by the failures which have some- 

 times occurred when rubber has been used as the insulating 

 material for alternating-current mains. The dielectrics upon 

 which experiments have been made are indiarubber, paraffined 

 paper, and mica. The indiarubber tested consisted of three 

 different kinds of sheet-rubber called Nos. 1, 2 f and 3 ? and 

 an " Okonite " cable. 



