Contact with Dielectrics. 487 



IN the course of: some experiments described by Mr. E. H. 

 Rayner*, the electrical resistance between the faces of 

 sheets o£ dielectric was measured, using electrodes o£ thin 

 tinfoil. The dielectric sheet to be tested was laid between 

 two circular soft indiarubber disks, each disk being covered, 

 on its contact-face, with tinfoil, and the whole was surmounted 

 by a brass disk of the same diameter, upon which lead weights 

 could be placed. This interesting arrangement may be 

 regarded as a compromise between the use of unyielding 

 contact-plates and mercury contact. It is here proposed to 

 investigate the capabilities of this compromise ; and as the 

 material known as " press-spahn " was examined by Mr. 

 Rayner in his tests, it is here selected as a convenient dielectric 

 for the present purpose. 



Specific Values. — .Mr. Rayner reduces his results to kilo- 

 megohms per cm. 3 , but it is perhaps better to state the total 

 megohms measured in each case, without reducing them to 

 specific values. The reason for this is that many materials 

 of this class have a varnished surface, and this varnish has a 

 higher resistance than the middle layers, so that thin sheets 

 yield higher specific values than thick sheets, and specific 

 values cannot be said to be representative of the material 

 generally. Specific values should therefore be avoided, except 

 in the case of homogeneous dielectrics. 



Size of Electrodes. — The disks used by Mr. Rayner were 

 50 cm. 2 This corresponds to a diameter of 7*98 cms. 

 (3' 14 ins.). The electrodes used throughout the following 

 tests are 1533 cms. 2 , corresponding to a diameter of about 

 14 cms. (5*5 ins.). The soft indiarubber disks are of the 

 same thickness as those employed by Mr. Rayner, i. e. 1 cm. 



Load on Electrodes. — The weight used by Mr. Rayner to 

 press upon the electrodes was 20 kilogrammes, so that the 

 average load was 400 grammes to the cm. 2 In order to give 

 an idea of this load expressed in British units, it may be noted 

 that it is almost exactly the same mean pressure as would be 

 exerted by 1 cwt. distributed over a 5-inch disk. To obtain 

 the same mean pressure on a 5^ in. disk, it had to be loaded 

 with 135*2 lbs. (61*3 kilogrammes). It is clear that with 

 some dielectrics a load of this magnitude is likely to reduce 

 the thickness of the sample or alter its texture, and so 

 invalidate the results ; as press-spahn is fairly hard, it is 

 probably not seriously affected, but it will be necessary to 

 examine the effect of changes of load. 



* " Report on Temperature Experiments carried out at the National 

 Physical Laboratory." By E. H. Rayner. Journ. Inst. Electrical 

 Engineers, vol. xxxiv. p. 613, May 1905. 



