. The Bell System technical journal . s between the electrode and the sample have a marked effecton the apparent dielectric constant. An air-gap. 001 in. thick inseries with a sample having a dielectric constant of 5 will have thesame effect on the capacitance as increasing the thickness of thesample by .005 in. If the actual thickness of the sample is .05 in. thisresults in an error of nearly 10% in the value of dielectric power factor will also be reduced and the loss factor or productof power factor and dielectric constant will be reduced by the factor (—y, or about 17%. Thus i
. The Bell System technical journal . s between the electrode and the sample have a marked effecton the apparent dielectric constant. An air-gap. 001 in. thick inseries with a sample having a dielectric constant of 5 will have thesame effect on the capacitance as increasing the thickness of thesample by .005 in. If the actual thickness of the sample is .05 in. thisresults in an error of nearly 10% in the value of dielectric power factor will also be reduced and the loss factor or productof power factor and dielectric constant will be reduced by the factor (—y, or about 17%. Thus it is evident that the elimination of all\55/ gaps between the electrode and the sample is one of the first require-ments both for reproducibility and accuracy. A second effect inherent in all electrodes which is a source of errorunless properly allowed for, is the so-called fringing of the electro-static flux, that is, the lines of force tend to spread out and includean area of the sample greater than that of the electrode. This is. Fig. 1 illustrated in Fig. 1. So far as the flux which is confined entirelyto the material of the sample is concerned, this produces an errorin the dielectric constant, which involves a determination of theeffective area of the sample, but not in the power factor which isindependent of the area. However, there are some lines of force ELECTRODE EFFECTS 557 which terminate on the vertical surfaces of the electrode and whosepaths are partly through the sample and partly through air. Theseintroduce a slight error into the power factor also. They also makethe capacitance depend to a slight extent on the thickness of theelectrodes. These edge effects vary with the thickness of the sampleand also with the ratio of the perimeter to the area of the electrodeand hence with its size and shape. They are also increased materiallywhen one electrode is larger than the other. The third inherent source of error is the capacitance from theungrounded electrode ^ to
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