560 BELL SYSTEM TECHNICAL JOURNAL 



In this arrangement the sample is clamped in a vertical position 

 between two ebonite plates. These plates are recessed in such a 

 way that a closed cell is formed on each side of the sample. An air 

 vent is provided at the top of the cell and the mercury is introduced 

 through a capillary U tube connected to the bottom of each cell. 

 Thus the mercury is caused to rise slowly along the surface of the 

 sample displacing the air completely. Also a slight head of mercury 

 can be maintained in order to force it into all angles. However, this 

 electrode is subject to the errors of the confining ring of insulating 

 material in an exaggerated form on account of the mercury electrodes 

 being entirely enclosed by the ebonite clamping plates. This would 

 unquestionably lead to appreciable errors in the power factor especially 

 in the case of thick samples of low loss material. 



Foil Electrodes 



Another form of electrode which has been widely used consists 

 simply of a sheet of tinfoil applied to either side of the sample usually 

 with a thin film of wax or petrolatum to serve as an adhesive. This 

 has the advantage that the thickness of the electrode can be made 

 negligible thereby practically eliminating the error due to the field 

 from the vertical surface of the electrode passing partly through air 

 and partly through the sample. The capacitance from the upper 

 surface of the electrode to ground however is not eliminated. 



The question of the size of the electrodes also arises. If both the 

 electrodes are extended entirely to the edge of the sample the edge 

 correction for the capacitance is greatly reduced since the fringing 

 all takes place in air having a dielectric constant of 1 instead of the 

 higher dielectric constant of the sample. On the other hand the 

 fringing through the air does produce a small effect on the power 

 factor which does not exist when the fringing is all through the sample. 

 However, the biggest objection to this arrangement from the prac- 

 tical standpoint probably is that the samples very frequently are 

 not uniform in thickness near the edge and this makes it difficult 

 to determine the effective thickness of the sample. 



Another possibility is to make both electrodes the same size but 

 smaller than the sample. This should result in a comparatively 

 small edge correction but requires careful manipulation to insure 

 the electrodes being exactly opposite each other. The simplest 

 arrangement from a con\-enience standpoint is to have one large and 

 one smaller electrode. This howexer, results in a further increase 

 in the edge correction. 



