the operation is maintained at a relatively cool temperature since the impulse duration is short on the order of 0 008 seconds and the circuitry 52 to be described in greater detail below maintains a minimum time duration between impulses of approximately two seconds consequently the temperature level of the exposed shield 30 does not rise to a level which might be uncomfortable or dangerous to the operator the impulse generating circuit 52 and its manner of operation will now be described with reference to fig4 and the waveforms of fig5 the impulse generating circuit 52 is preferably mounted on a printed circuit board contained within the main tube 24 with the main power switch s2 closed no current will flow through the heating element 50 until a silicon control rectifier scr 122 is gated into conduction by the impulse generating circuit 52 when the silicon control rectifier is gated into conduction a single pulse of current on the order of 300 amperes will flow through the heating element 50 for a time duration corresponding with one half cycle of the alternating current voltage source 120 the impulse generating circuit 52 becomes operative upon closure of switch s1 when the seal arm 12 is moved downwardly sufficient that electrical contacts 110 and 114 fig2 close the circuit 52 will provide a current pulse for the heating element 50 and also operates to prevent a second pulse from occurring for a time delay which is preferably on the order of two seconds the.