The effects of electrodes on measurements of permittivity and power factor on insulating materials in sheet form

Measurements of permittivity and power factor made on sheet materials with different electrodes are apt to give widely different results. The way in which these discrepancies depend on the properties of the contact film between the electrode and the sample is investigated with particular reference to tests at audio and radio frequencies, and it is shown that in certain cases electrodes consisting of graphite and brass plates in the form frequently used, give rise to very large errors. In particular cases such errors may be very large at audio frequencies and small at radio frequencies, or vice versa. Mercury electrodes are considered to be the most satisfactory for general purposes, although a film of graphite between the mercury and the sample may improve the contact slightly, as previously noted by Churcher. Thin tinfoil applied with a trace of vaseline as an adhesive is a good substitute. Graphite electrodes backed with metal plates are easy to apply, but should be used with caution at audio frequencies, and not at all at radio frequencies.