A yoke main body formed of an upper yoke and a lower yoke accommodates therein a magnetic substrate, a plurality of line conductors, a plurality of capacitor substrates, a magnetic member and a spacer member. The dielectric body and the ground plane are disposed coaxially to the same center axis with the gap therebetween in the axial direction less than or equal to the thickness of the dielectric in the axial direction. Three C-shaped electromagnets are connected to the body of the vehicle to alternatively attract the steel elements, maintaining wheel rotation. Injected agents, such as reducing agents, are injected in a manner that prevents unwanted reactions with the test gas. The present invention makes it possible to record input data to an entire area of a disk more rapidly without fatal errors such as imperfect writing. It also concerns the said aqueous dispersions andor suspensions thus obtained, together with their uses in the mineral, paper and paint industries.