344 Scientific Proceedings, Royal Dublin Society. 



2. The insulation resistance of unmachined erinoid is greatly dependenc 

 upon the value of the applied voltage when mercury is used as the electrodes. 

 Table 6 and Fig. 2. 



3. When machined, the insulation resistance of erinoid is only slightly 

 dependent upon the value of the applied voltage. Tables 3 and 4. 



4. Unmachined erinoid with water electrodes is practically independent 

 of the value of the applied voltage. Table 6. 



5. Above a voltage of 500, the insulation resistance of unmachined erinoid 

 with mercury electrodes is roughly independent of the value of the applied 

 voltage. Fig. 2. 



This dependence of the value of the resistance of unmachined erinoid upon 

 the value of the voltage thus appears to be very largely a surface effect, as it 

 practically disappears when the material is either machined, or its surfaces 

 are in contact with water electrodes. 



In conjunction with the preceding deduction it appears from Fig. 2 that 

 increase of voltage improves the contact between electrode and material. 

 This improvement is very rapid at first, but very slow after the voltage has 

 exceeded 500 : the voltage of the megger. 



The resistance of the sample of unmachined white erinoid, represented by 

 curve P, Fig. 2, was found by the megger to be 29'5 megohms at 16 - 8° C. with 

 mercury electrodes, and 14 megohms with water electrodes. It follows that 

 a much higher voltage than 500 would be required to obtain the same contact 

 effect with mercury as with water electrodes. The latter agreement would, 

 in all probability, never be realized on account of the advantage possessed 

 by the water of becoming absorbed into the skin of the material. 



Evershed 1 investigated the change of resistance with voltage for certain 

 porous materials such as paper and cotton cloth, and found that a similar 

 change took place to that shown in Fig. 2. Copper plates were used as 

 electrodes. A theory of this change which depends upon the condensation 

 of moisture in the pores of the sample was given by him. 



In the case of erinoid, viewed from such a theory, it would appear that 

 the porosity of the surface of the material is mainly responsible for its change 

 of resistance with applied voltage. 



The value of the galvanometer deflection for 600 volts in the case of the 

 white erinoid with water instead of mercury electrodes was 212, which was 

 found to remain constant for over three minutes and was quite independent 

 of the direction of the applied voltage. This showed an absence of dielectric 

 absorption and an unappreciable absorption of the water by the surface of the 

 material during the time stated. 



1 Journ. Inst. Elec. Eng., vol. lii, 1913, p. 51. 



