Abstract: A simple and precise method of controlling the gap position (obliqueness and height) of magnetic recording installations with n tracks (n=1, 2 . . . ) is obtained by subdividing at least one track of a recording tape into two auxiliary tracks in accordance with the given recording format, which auxiliary tracks are recorded at the center to center track distance of d, using a special head with correct gap position. The recording wave length and magnetization are preferably equal due to the width of the auxiliary tracks being equal. The auxiliary track 1 is recorded continuously while the auxilary track 2 is recorded at time intervals with a signal of equal magnitude but with different phase angles. Each time interval has a phase angle associated with it. The magnitude of correction of the gap required can easily be determined from a graph showing the voltage level difference measured at two different phase positions.

Abstract: In flue gases or other humid exhaust gases which contain NO.sub.x and SO.sub.2, the NO.sub.x content is reduced and the SO.sub.2 content is oxidized to SO.sub.3 by a catalytic processing and the SO.sub.3 content is subsequently condensed as sulfuric acid. In order to prevent a clogging and deterioration of the catalyst and a contamination of the sulfuric acid the flue gas is subjected before the catalytic processing to a fine dedusting by a scrubbing with hot dilute sulfuric acid. In dependence on the water content of the flue gases the concentration of sulfuric acid in the dilute sulfuric acid and the temperature of the latter are adjusted so that no water will be transferred from the flue gases to the dilute sulfuric acid and no water or only a small amount of water will be transferred from the dilute sulfuric acid to the flue gas.