A current mirror circuit that uses only a single seed current, and thus only a single current source. At the outermost end of the rescue boom, according to the invention, there is a piercing device which comprises an elongated piercing tool provided with at least one longitudinal channel for supplying the fire-retardant medium to the nozzles of the tool and further to the object on fire. The C-section bridge provides support for the members and reduces the clamshell effect generated by application of the breaking force. Bridge circuits are supplied for controlling input of DC voltage into the primary windings of the transformers. The masking structure may be used as a mask in an etching process during fabrication of semiconductor devices.