the current path comprising isolation diode 45 and line 46 to control circuit 47 then through resistor 48 and diodes 49 and 50 to negative power line 40 the combined forward voltage drop across diodes 49 and 50 is applied through resistor 51 to gate 52 of silicon controlled rectifier scr 53 to gate it into conduction current now flows through indicator light 54 to provide a warning signal to the operator that a brush needs replacement flickering of the light 54 is prevented since the scr 53 will remain in conduction when once gated on even though the probe may momentarily disengage from the brush and interrupt the gating signal similarly light 54 will remain lit if contacts 37 are opened and current ceases to flow through the above current path scr 53 thus functions as a normally open switch which when gated into conduction will close and remain closed even through the gating condition ceases light 54 may be turned off by removing the power therefrom as by opening the normally closed switch 55 if any of the negative polarity brushes 13 of motor 35 should wear to the extent that the probe 27 associated therewith engages the terminating end of the groove therein the probe will be connected through the brush in question and field winding 35f to negative line 40 current can now flow from positive line 38 through the current path comprising resistor 56 resistor 57 and line 58 from the control circuit 47 to and through isolation diode 59 to the probe associated with the.