Allen — The Insulating Properties of Erinoid. 357 



After the air-drying, the sample with its contained water was heated to 

 just above 80° C, and during this heating it is fair to assume that very little 

 of this water would escape, but would go ou penetrating a little farther into 

 the material as time went on. This extra penetration, as shown by the rough 

 parallelism of line D with B during the time of the last test, is apparently 

 very small. 



The break-down voltage of erinoid and fibre. 



This has been determined for erinoid at the National Physical Laboratory. 

 The electrodes used were two inches in diameter and the sample about two 

 millimetres thick. Tests were made on unsoaked erinoid and on this material 

 after soaking for two weeks in distilled water, mineral oil, and castor oil. 



The respective r. m. s. values found for the break-down voltage in these 

 four cases were 13,000 ; 1,500 ; 7,000 ; and 8,000. 



The break-down voltage of vulcanized fibre has been investigated by a 

 number of workers, but as this value mainly depends upon the degree of 

 dryness, and partly upon the thickness of the sample, the results given show a 

 very large variation. For a thickness of about two millimetres, and a well- 

 dried sample, the value of the break-down voltage of fibre may, from the 

 results referred to, be somewhere within the range 10,000 to 16,000 ; that is 

 roughly the same as the value for erinoid. 



Summary of deductions. 



1. "When dry, erinoid is a good insulator of fairly constant insulation 

 resistance. It is slightly hygroscopic, but not so much so as fibre. 



2. Erinoid does not absorb water when in direct contact with it so readily 

 and to such an extent as red fibre, neither is it so retentive of the water 

 absorbed as the latter. 



3. When its surface is not machined its resistance greatly depends upon 

 the value of the applied voltage, unless water electrodes are used. This 

 dependence is very slight in the case of unmachined fibre, whether mercury 

 or water electrodes are used. In the case of machined erinoid the resistance 

 is almost independent of the voltage. 



4. It is practically free from dielectric absorption, but the latter is 

 appreciable in the red fibre. 



5. The relation pointed out by Easch and Hinrichsen between tempera- 

 ture and resistance is true for erinoid and fibre. 



6. Of the varieties of erinoid tested, red is the most absorbent of water 

 and generally of the lowest resistance. 



