ELECTRODE EFFECTS 557 



which terminate on the vertical surfaces of the electrode and whose 

 paths are partly through the sample and partly through air. These 

 introduce a slight error into the power factor also. They also make 

 the capacitance depend to a slight extent on the thickness of the 

 electrodes. These edge effects vary with the thickness of the sample 

 and also with the ratio of the perimeter to the area of the electrode 

 and hence with its size and shape. They are also increased materially 

 when one electrode is larger than the other. 



The third inherent source of error is the capacitance from the 

 ungrounded electrode ^ to earth due to lines of force which pass out 

 in all directions other than through the sample. This also is illus- 

 trated in Fig. 1. This increases the measured capacitance by an 

 amount depending somewhat on the nature and position of surround- 

 ing objects. If this capacitance is due to fiux passing entirely through 

 air it makes no difference in the dielectric loss,^ but if the path of the 

 flux includes other material such as the wood, brick, plaster, etc., 

 in the walls and floor of the room as is often the case, it may add an 

 appreciable loss. 



We will now consider the above errors and also the question of 

 convenience as applied to certain specific types of electrodes. The 

 following types will be considered. 



1. Plain metal electrodes. 



2. Mercury electrodes. 



a — Confining ring of metal. 



b — Confining ring of insulating material. 



3. Foil electrodes. 



a — Both same size as sample. 



b — Both same size, but smaller than the sample 



c — One materially larger than the other. 



4. Conducting paint electrodes, 



5. Fixed gap electrodes. 



Plain Metal Electrodes 



One of the simplest forms of electrode would be two similar blocks 

 of metal between which the sample would be placed. If the surfaces 

 of both the electrode and sample were true planes, this would be 

 fairly satisfactory. However, as the surfaces of samples of insulating 

 material usually available for test are not true planes, the air-gap 



1 Assuming one electrode to be grounded as is usually the case. 



^ The apparent power factor is reduced by the increase in capacitance but the loss 

 factor is not affected since the dielectric constant is increased to the same extent that 

 the power factor is reduced. 



