492 



THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1957 



tanees are ininiiuized. Lateral displacemeiit.s are kept small by an annular 

 ring of steatite ceramic which is in turn surrounded b}- a ring of Ketos 

 steel. Pressures of 23,000 lb on the two-inch sample discs have been 

 applied without damaging the cell. 



Although measurements could be made with ease in this cell it is not 

 without disadvantages. The steatite ring has a rather high dielectric 

 constant which tends to increase fringing effects. These effects are fur- 

 thermore pressure dependent since the electrode separation varies as 

 pressure is applied. Also loss measurements could not be obtained as 

 leakage along the steatite surface was larger than the leakage through 

 the samples of the polyethylene-butyl rubber compound investigated. 



An attempt was made to eliminate fringing effects by making meas- 

 urements on samples of varjdng thickness and extrapolating to zero 

 thickness but results were too uncertain to be of cjuantitative value. 

 The uncertainty resulted from the inability to cast the sample discs 

 with uniform thickness an effect which becomes pronounced with verj^ 

 thin samples. It was possible, however, to estimate the total straj' 

 capacitance in this manner and it was found to be about 10 per cent of 

 the sample capacitance and only slightly dependent upon pressure. 



BRASS 

 ^--ELECTRODES 



Fig. 1 — Cell employed to measure {l/C)(dC/dP) with sheet specimens under 

 one dimensional pressure. 



