562 BELL SYSTEM TECHNICAL JOURNAL 



may depend considerably on the skill and patience exercised in the 

 handling of the samples and electrodes, they may vary considerably 

 with different observers. Hence, a comprehensive discussion of this 

 point is beyond the scope of this paper. It has been the experience 

 of the writer, however, that there is little choice between the two 

 in this respect and that a decision between them rests primarily on 

 other factors. 



Since the magnitude of the ground capacitances and fringing effects 

 both for foil and for mercury confined by a metal ring can be deter- 

 mined from a single series of tests, such an experiment will now be 

 described. 



Samples of insulating material 6 inches square were entirely coated on 

 both sides with tinfoil using petrolatum as an adhesive. After the foils 

 were in place, a A.Y2 inch circle was described on each foil from the 

 center of the square and cut through so that the inner and outer 

 portions were not in electrical contact, although the separation 

 between them was very small. This left two 43/^ inch disc electrodes 

 L and M, on opposite sides of the sample surrounded by the annular 

 pieces N and O respectively. When the inner and outer sections are 

 connected, we have the condition of foil electrodes covering the entire 

 surface of the sample. Then if N is used as a guard ring, it is possible 

 to obtain measurements between L and M under uniform field condi- 

 tions with all fringing effect eliminated. If N is removed and M 

 and O are connected, we have the condition of one large and one 

 smaller electrode. If a metal ring 43^2 inches outside diameter is 

 placed on the foil, we have a condition similar to that of a mercury 

 electrode confined by a metal ring. If both N and O are removed, 

 we have the case of two equal foil electrodes smaller than the sample. 

 All of these variations can be obtained without any variation what- 

 ever in the conditions of contact between L and M and the sample 

 and therefore are directly comparable. 



The method of making these measurements '" by means of a com- 

 pletely shielded capacitance and conductance bridge ^ is the same 

 as that described by Campbell for the measurement of direct capaci- 

 tance and will not be described here. The measurements were 

 made at a frequency of 1,000 cycles as fewer difficulties are encount- 

 ered than at radio frequencies and the general results are the same 

 for any frequency. The capacitances were balanced to 0.1 mmf. or 

 better, and the conductances to 0.0001 micro-mho. 



^ G. A. Campbell, Bell System Technical Journal, July, 1922 and Journal of the 

 Optica! Society of America and Review of Scientific Instruments, August, 1922. 

 «G. A. Campbell, Electrical World, 43, 1904, 647-640. 



