Abstract: A static magnetic field generator generates a non-magnetic field region. An AC magnetic field application instrument applies an AC magnetic field to the non-magnetic field region. A detection coil has an axis parallel to a direction of the AC magnetic field in order to detect a magnetization signal. A measuring instrument is connected to the detection coil. A resonance frequency variable device includes a capacitor connected in parallel to the detection coil in order to adjust a resonance frequency of the detection coil and the measuring instrument. A capacity of the capacitor is adjusted such that a resonance frequency of a closed circuit including the detection coil, the measuring instrument, and the resonance frequency variable device including the capacitor coincides with a frequency of a harmonic signal.

Abstract: A spark plug has an insulator having a through hole formed along an axial direction, a center electrode which is partially inserted into a portion of the through hole on a forward end side in the axial direction, and a glass seal portion which is in contact with the insulator and the center electrode within the through hole, in which the glass seal portion contains glass and an electrically conductive substance. The glass contains an Si component and a B component in a total amount of 50 mass % or more, as reduced to SiO2 and B2O3, a Zn component in an amount of 20 mass % to 35 mass % as reduced to ZnO, and an alkali metal component. The glass contains, as the alkali metal component, an Na component in an amount less than 1 mass % as reduced to Na2O.

Abstract: A spark plug has an insulator having a through hole formed along an axial direction, a center electrode which is partially inserted into a portion of the through hole on a forward end side in the axial direction, and a glass seal portion which is in contact with the insulator and the center electrode within the through hole, in which the glass seal portion contains glass and an electrically conductive substance. The glass contains an Si component and a B component in a total amount of 50 mass % or more, as reduced to SiO2 and B2O3, a Zn component in an amount of 20 mass % to 35 mass % as reduced to ZnO, and an alkali metal component. The glass contains, as the alkali metal component, an Na component in an amount less than 1 mass % as reduced to Na2O.