1. Electric welding apparatus including an arc electrode, a workpiece, a power supply connected with said electrode and workpiece to establish an electric arc therebetween, and a monitor circuit for monitoring the arc length, said monitor circuit comprising: first means adapted to be coupled to said apparatus for providing an input signal proportional to the arc voltage or current across the electric welding arc; isolating means responsive to said input signal for providing an isolated output signal electrically isolated from said arc welder; reference level setting means for providing a reference signal level; comparator means responsive to said reference signal level and said output signal for providing a trigger signal when the level of said output signal is equal to or exceeds said reference signal level; indicating means responsive to said trigger signal for indicating that the welding arc length is reaching a limit set by said reference signal level; wherein said isolating means further comprises optical coupling means comprising a light-emitting diode in operative connection with a photo-transistor for electrically isolating the input signal from the reference level means, the comparator means and the indicating means, and wherein: said light-emitting diode is coupled to said first means and provides an optical signal in linear proportion to the amplitude of said input signal; and said phototransistor is coupled to said comparator for providing said output signal having an amplitude linearly proportional to the amplitude of said optical signal.