press fitted in the hole 53 so that the handle is prevented from being disengaged even when vibrated during carrying a reference numeral 55 denotes a power supply cord having a plug 56 at its one end while 57 denotes a diode connected in series to the power circuit of the heater 2 and intended for performing half wave rectification a numeral 58 designates a heat radiation plate for radiating heat produced by the diode 57 as shown fig7 the diode 57 is soldered at its one end to the heat radiating plate 58 which in turn is fixed to the cover 16 by means of a screw 59 a reference numeral 60 denotes a switch spring made of a resilient material such as stainless steel and fixed together with the other end of the diode 57 to the conductive metal fitting 3 by means of a screw 61 a numeral 62 designates an externally operable switch which is adapted to be so as to open and close the contact between contacts 63 on the switch spring 60 and the heat radiating plate 58 the power supply cord 55 heater 2 diode 57 and the switch spring 60 constitute an electric circuit as shown in fig1 this circuit is switchable by means of the switch knob 62 between two modes namely a first mode in which the diode 57 is connected in series to the circuit so as to effect the half wave rectification and a second mode in which the diode is disconnected from the circuit so as to allow a full wave.