Precise Measurements on Dielectric and Magnetic Materials

Dielectric and magnetic properties are measured by assuming an electrical model for the specimen and its electrodes or windings; the accuracy of the final result depends mainly on the validity of the model. Distributed parameters may have to be assumed for frequencies above 10 MHz. Measurements on dielectrics may be of two different capacitances for specimen in and out, or of two capacitances adjusted to be nearly equal. British and American practices are different in allowing for edge effects in a disk specimen. The liquid-immersion method is of limited use in measuring dielectric loss. For frequencies up to 1 MHz, bridge methods are recommended; at higher frequencies, resonance methods. For frequencies at which distributed parameters must be assumed, there are coaxial line and various waveguide and resonator methods. For 10-5 Hz to 20 GHz there are time-domain methods of somewhat less sensitivity. Magnetic properties are not usually reproducible enough to justify accuracy better than a few parts in 1000. Bridge methods exist for both low- and high-flux densities; at high fluxes there is also the wattmeter method. A special method is used for measuring the properties of magnetic recording tape.