A paramagnetic measuring instrument comprises a magnetic circuit, formed from ferromagnetic material and having pole ends defining a discontinuity, and a gas measuring chamber disposed in the discontinuity between the pole ends. The measuring chamber includes a frame shaped outer part formed from nonmagnetic material and having a central opening extending between the pole ends; two ferromagnetic pole shoes each respectively disposed adjacent a separate one of the pole ends and extending into the opening of the outer part; two distribution channels disposed in the outer part on opposite sides of the measuring gap for supplying a measuring gas and a comparison gas, respectively, to the measuring gap; and inlet and outlet gas lines for supplying the measuring gas and the comparison gas, respectively, and removing the mixture formed from the measuring and comparison gases. According to the invention, the cross section of the opening of the outer part of the measuring chamber, as well as the pole faces, are rectangular. Furthermore, the distribution channels extend in straight lines, parallel to one another, at the two opposite shorter sides of the measuring gap. The distribution channels have a length corresponding to the length L of the shorter sides and have a height h and a depth t, respectively, which in each case is greater than the width W of the measuring gap.