and partly on the load impedance 64 the voltage threshold for a desiccation output is set to cause trigger signals to be sent to the u201c on u201d time control circuit 70 and to the switched mode power supply 66 when the voltage threshold is reached the u201c on u201d time control circuit 70 has the effect of virtually instantaneously reducing the u201c on u201d time of the rf oscillator switching device simultaneously the switched mode power supply is disabled so that the voltage supplied to oscillator 60 begins to fall the operation of the generator in this way is described in detail in our european patent application no 0754437 the disclosure of which is hereby incorporated by way of reference referring back to fig2 when the instrument 12 is in use small particles of condensed tissue and other debris can become adhered to the edge electrode 2 and to a lesser extent the base electrode 3 if the instrument is used particularly aggressively it is possible that a conductive track of such debris can build up between the electrodes 2 and 3 across the ceramic insulator 4 such a conductive track is shown schematically at 11 in fig2 if no action is taken to prevent it this conductive track 11 develops into a u201c flare out u201d in which the current passing directly between the electrode 2 and the electrode 3 will cause the instrument to overheat and finally fail the following description explains how the generator 10 detects and compensates for just such a situation at regular intervals in this case every 1 10 ms the current is measured across the load 64 by the current detector 80 and the current value is sent to the.