connected or disconnected from the mains voltage 220 v output members u1 and u2 may comprise optoswitches for obtaining galvanic separation and amplifiers for operating the contactors these are shown in fig2 as electromagnetic electric switches but as an alternative they may consist of static coupling members for example thyristors fig3 shows the temperature t of the exhaust air as a function of the time t during a drying procedure in a tumbler dryer at t o the drying is started the motor m and the element e are switched on and a stream of hot air is blown through the drum tr the temperature t of the exhaust air first increases relatively rapidly according as the air channels the drum and the articles are heated the rate of increase of the temperature decreases and at time t 1 it reaches a relatively constant level t 39 the continuous curve in fig3 shows a typical characteristic at full load for the time t 1 t 2 the main part of the water contents of the articles is driven off when there is only a small amount of water left the temperature t starts increasing at t t 2 and approaches a new limit value which is considerably higher than t 39 if the hot air injection is not interrupted if there is a smaller quantity of articles in.