Abstract: A detector for detecting conductive materials contained in a stringy material sensitively, which is a cylindrical microwave cavity resonator (1) comprising a peripheral part (9) and two circular plates (10) covering the ends of the peripheral part. At least one of the circular plates has cylindrical protrusion (4) at its center. In the peripheral part, an antenna (5) for the generation of discharge is provided. In the protrusion (4) a detecting means (6) for discharge is provided. Paths (3,8) or a notched part (7) for the stringy material to be measured which passes through a high electric field formed by the protrusions and penetrate the cylindrical microwave cavity resonator oscillating at a fixed frequency, discharges are generated by the strong electric fields concentrated on the very small metals. Since sounds, light, and electromagnetic waves are generated at this time, the very small metals can be detected sensitively with discharge detecting means such as a microphone, a photodetector and an antenna.

Abstract: An apparatus for measuring physical quantities comprises a microwave cavity resonator (1) provided with a protrusion (4) on its axis. An object (2) to be measured is placed in the central portion (17) of the protrusion (4) in parallel with it, or the object (2) to be measured is placed vertically against the protrusion (4), so that an electric field can be confined to the area to be measured. This results in precise measurement of physical quantities on a minute portion of the object to be measured. A method of measuring physical quantities comprises obtaining moisture content x (g/m.sup.2) and basis weight y (g/m.sup.2) of the object to be measured from equations having at least a product term of moisture content x and basis weight y, and by measuring the frequency and the voltage of microwave energy at the maximum resonance point on the object to be measured. This enables obtaining easily and accurately the moisture content and basis weight of the object to be measured.