Abstract: A method for manufacturing a measuring device includes: a step of preparing a structure; a step of fixing a piece to a housing by first welding on a distal end face of the structure; and a step of polishing a welded portion formed by the first welding and the distal end face. The structure includes: the housing having a first end and a second end which are open in one direction; a thermocouple extending in the one direction in the housing; and the piece provided in the housing, exposed from the first end, and configured to hold the thermocouple. The piece has a side face facing an inner surface of the housing. In the step of preparing the structure, the thermocouple is accommodated between a pair of grooves provided on the side face and the inner surface of the housing by inserting the piece into the housing.

Abstract: A mass spectrometer comprising an ion source for producing an ion beam, an ion optical system having an entrance and an exit slit, and an ion detector. The ion optical system comprises a series combination of an energy dispersing unit and a mass dispersing unit. The energy dispersing unit comprises a toroidal electrical field having a deflection angle of 85.degree. to 95.degree. while the mass dispersing unit comprises a homogeneous magnetic field having a deflection angle of 85.degree. to 95.degree. and an entrance end face made concave as viewed from the entrance side of the ion beam when the apparatus is operated as a double focussing mass spectrometer of the reverse geometry type and an exit end face inclined 6.degree. to 14.degree. to the negative side from a position perpendicular to the axis of the ion optical system.