summer amplifier 52 and relay 43 to the power drive circuits 37 resulting in a constant electrical power supply to the booster heater element 11 this is because with a zero input to the fixed rate integral amplifier 50 its output remains constant and has a value equal to that immediately prior to its input voltage level becoming zero and because the output of the fixed gain proportional amplifier 49 whilst also constant will be zero this mode of operation of the control system is particularly advantageous when as sometimes happens the temperature at which the plug end is maintained at the datum position as determined by the light receiver circuits 47 changes during the course of time from the predetermined datum value as set at the digital potentiometer 31 on control panel 41 when in the 34 stand by 34 mode prior to switching to the 34 run 34 mode therefore when the yarn breaks the temperature of the texturising medium is maintained at or very near the required level throughout the interval between the occurrence of the break and re threading up thus ensuring that yarn of correct bulk is produced immediately or substantially immediately subsequent to re threading up nevertheless the alternative mode.