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XLIX. On the Resistance of Dielectrics, and the Effect of an 

 Alternating Electromotive Force on the Insulating Properties 

 of India-rubber. By A. W. Ashton, B.Sc, Royal Exhibi- 

 . ' tion Scholar *. 



1. TN undertaking the following research an endeavour has 

 A been made to ascertain : — 



First. What relation exists between the resistance of 

 different dielectrics and the electromotive force at which the 

 resistance is measured. 



Secondly. What form is taken by the curves showing the 

 variation of the current with the time when (a) a condenser is 

 charged by a battery of given E.M.F., (b) the condenser is 

 discharged by placing it directly on to the terminals of a 

 galvanometer. 



Thirdly. What effect is produced on the insulating pro- 

 perties of ind r arubber by the continued application of a high 

 alternating pressure. 



2. The question whether dielectrics can be said to strictly 

 obey Ohm's law in regard to electrical conductivity has been 

 considered by previous observers, but at present very definite 

 conclusions do not appear to have been reached. The varia- 

 tion in the insulation resistance of several cables was examined 

 by Heim (see 'Electrician/ vol. xxv. p. 751). The method 

 of measurement used was that known as the " direct de- 

 flexion " method, in which the current is measured by the 

 deflexion of a sensitive galvanometer joined in series with a 

 battery and the plates of the condenser formed by the cable 

 tested. Readings were taken after one or two minutes' elec- 

 trification, and after a reading at any one pressure the inner 

 and outer conductors of the cable were connected for one or 

 two hours in order to get rid of the slowly disappearing 

 " residual charge." From these experiments, Heim concluded 

 that the dielectrics tested showed a decided deviation from 

 Ohm's law, the conductivity being greater at the higher 

 pressures. In the discussion on a paper by Preece (see 

 Journal Institute of Electrical Engineers, Dec. 1890), some 

 results were given by Alexander Siemens of tests taken on 

 cables with cores of different materials, viz., guttapercha, 

 indiarubber, and impregnated fibre. The charging pressure 

 was varied over a wide range (from 100 to 1200 Leclanche 

 cells), and it was found that the resistance calculated from 

 the current after one minute's electrification became less as 

 t\\c pressure increased. Leick (Wiedemann's Annalen, No. 13, 



* Communicated by the Physical Society : read May 31, 1901. 

 Phil. Mag. S. 6. Vol. 2. No. 11. Nov, 1901. 2 h 



