570 BELL SYSTEM TECHNICAL JOURNAL 



in micro-microfarads are probably somewhat more con\enient for 

 actual use in making the necessary calculations. 



Consider the case of mercury electrodes with the lower electrode 

 grounded. On the basis of the foregoing discussion the total capaci- 

 tance C which is measured may be considered as made up of four 

 parts, namely, Cx the capacitance between the electrodes which 

 would exist under uniform field conditions as when a guard ring is 

 used. Cei the edge effect which would exist if the upper electrode 

 were a thin disk. G2 the additional edge effect due to the height of 

 the metal ring. Cg the capacitance to ground of the upper surface of 



the electrode. The dielectric constant i^=4.46 -^ where Cx is in 



A 



micro-microfarads d and A are the thickness and area of the sample 



in inches and square inches, respectively. To compute K, Cx must 



therefore be obtained from the relation 



Cx= C— {Cel-\-Ce2-\-Cg). 



Values for d, Ce2, Cg are given in the tables for certain particular 

 cases and may be determined in a similar manner for any other cases. 

 For foil electrodes, conditions are similar except that G2 is zero. 

 Cei which is the largest of the corrections is plotted in Fig. 5 for 

 various values of K and several thicknesses of the sample as taken 

 from the previous tables. It will be seen that in order to apply this 

 correction the approximate value of the dielectric constant of the 

 sample must be known in advance. This can always be obtained by 

 making a preliminary computation neglecting the corrections entirely. 



As an example of the above method suppose measurements have 

 been made on a 3^'8" sample of material having a dielectric constant 

 of about 4.5 using mercury electrodes with a shallow metal ring. 

 From Curve A, Fig. 5 we get 9.2 mmf. for Cei. Cei is estimated by 

 interpolation from item 7 of Table II at about 2.7 mmf. Cg is taken 

 as the average for item 6 of Table II or 0.8 mmf. This makes a total 

 of 12.7 mmf. to be subtracted from the measured capacitance in 

 order to obtain Cx from which the dielectric constant is computed. 

 If 434" foil electrodes were used d would be taken from Curve B, 

 Fig. 5 as 4.0 mmf. and Cg from the average of item 11, Table II as 

 2.4 mmf. making a total correction of 6.4 mmf. 



The values given in Fig. 5 for d are, of course, applicable only 

 to a given size of electrode, namely ^Yi' in diameter. If the edge 

 effect capacitance is considered equivalent to that of an additional 

 ring electrode surrounding the main electrode, this capacitance would 

 be proportional to the mean radius of this ring, or to the radius of the 



