486 Mr. E. Appleyard on 



Conclusions. 



(a) Except in the case of homogeneous dielectrics, it is misleading to 

 deduce specific values referred to unit cube of the material, from the 

 results of tests on sheets. 



(b) With tinfoil electrodes, the apparent resistance of press-spahn 

 diminishes as the load increases, and it attains a fairly constant value at 

 a load of about 400 grammes per cm. 2 



(c) If, with tinfoil electrodes, the load is gradually diminished 

 after a load of 543 grammes per cm. 2 , the resistance gradually rises, but 

 the rise is less rapid than the diminution in the former case (/;). 



id) When the full load with tinfoil electrodes is again restored the 

 resistance falls to its minimum value. 



(e) For small loads, with tinfoil electrodes, the 2nd-minute deflexion 

 is in general greater than the 1 st-minute deflexion. As the load increases, 

 a point is reached at which these deflexions become approximately equal. 

 For loads greater than about 360 grammes per cm. 2 , the lst-minute 

 deflexion is in general greater than the 2nd-minute deflexion. 



(/') Increase of voltage, with tinfoil electrodes, especially with small 

 loads, behaves like increase of load, apparently increasing the contact 

 area, and diminishing the observed dielectric-resistance. Load, voltage, 

 and the normal effect of "absorption" thus combine to determine the 

 ratio of the lst-min. deflexion to the 2nd-min. deflexion. 



{(/) When mercury electrodes are used, the dielectric-resistance, as 

 measured at different voltages, is sensibly the same, even for abrupt and 

 great changes of vultage. 



(h) When mercury electrodes are used, the 2nd-minute deflexion is in 

 general never greater than the lst-minute deflexion. The inference is 

 that when, with tinfoil electrodes, the converse is the case, it arises from 

 imperfect contact, and not from the material itself. 



{i) When mercury electrodes are used, the dielectric-resistance, as 

 measured with a voltage applied in a given direction, is sensibly the same 

 as that measured with the voltage reversed, and this equality appears to 

 become greater after a few reversals. 



(j) There is a critical load at which tinfoil electrodes yield fairly 

 accurate results. With greater loads there is danger of crushing the 

 material. With a less load the contact is faulty. 



(k) With mercury electrodes under the application of 750 volts, with 

 earthing and reversing, respectively, at every fifth minute, the resistance 

 of a sample of press-spahn fell from 43 - 2 to 41*7 megohms in 51 minutes, 

 thus indicating the limits of precision of resistance tests on such material 

 on a prolonged test where the conditions have time to change. 



(I) The earth-readings, in such an experiment as that described under 

 (k), fall steadily towards zero, and their pairs of maximum values are 

 approximately equal, corresponding to the two directions of voltage. 

 These maximum earth-readings are only about 0*2 per cent, of the 

 maximum readings with the battery. Consequently, if only 1 minute is 

 allowed ior a sample to recover after the application of a given voltage, 

 the degree of precision of the results will be of the order 0"2 per cent., 

 on account of the previous charge left in the sample. 



(???) The retentive force between a disk of tinfoil and a sheet of 

 dielectric, forming part of a condenser, increases rapidly with the voltage ; 

 and it vanishes, or becomes extremely small, when the voltage falls 

 to zero. 



(n) Small variations between the observed capacity of a condenser 

 tested at different voltages may be due to more intimate contact between 

 the electrodes and the dielectrics at the higher voltages. 



