mercury with respect to atmospheric pressure that depression causes a certain amount of ink to flow out of the nozzles 21 the ink carrying away any solid impurities or clogging material present in the tubes 22 after a time t sub 1 of around 5 seconds of just a suction effect has been set by a timer 116 the microprocessor 120 activates the control circuit 118 see fig5 by means of three signals e 1 e 2 e 3 fig6 each of a duration of 6 seconds and spaced by about 2 seconds at a repetition rate which is in the ultrasonic band to expel the drops of ink the circuit 118 can operate without distinction at a frequency of between 1 and 40 khz the control circuit 118 is connected to a frequency generator 122 which is arranged to generate a first frequency of between 5 and 10 khz which is used for the printing operation and a second frequency of around 30 khz which is used to expel drops of ink during the suction cycle the generator 122 is controlled by the microprocessor 120 by way of an electronic switch 124 for selectively generating one or other of the above mentioned frequencies at the end of the signal e 3 fig6 the suction effect is caused to continue for around a further 5 seconds up to the time t 2 set by the timer 116 in order to ensure that all.