The aim of our work is to investigate a gliding discharge type reactor used for polymer treatment in which the plasma string runs between diverging electrodes. Starting in the point where the inter-electrode distance is minimal, the discharge moves along the electrodes in a channel formed by two dielectric plates spaced between 0.5 mm and 1 mm. The motion of the discharge is caused by a magnetic field of 20 mT to 140 mT that is directed perpendicularly to both the dielectric plates and the discharge current. The discharge is struck in air at pressures between 10 kPa and 100 kPa. A discharge visualisation study reveals that for these conditions the discharge is very stable. The discharge velocity and the time of discharge running in ignition-lengthening-extinction cycle are determined as a function of discharge current, magnetic field and pressure. The temperature of the gas is measured with a thermocouple inserted between the electrodes.