and the sash of the fume hood in its down or closed position in such cases the fume hood involved will normally be provided with suitable by pass vents or openings for admitting the requisite amount of minimum air into the fume hood to assure adequate exhaust operations when the fume hood is in use with the sash member raised out of its closed position the limit switch 46 will be in its rest position with the contacts of the upper pole in open condition the relay 108 is therefore de energized and power is thereby supplied to the high speed terminal f of the two speed blower 106 and the auxiliary air blower 107 is operating in the event the motor 37 has an external capacitor 49 associated therewith this may be mounted on either the base member 31 of the construction illustrated in fig1 or the member 82 of the construction illustrated in fig3 while the actuation of the limit switch 46 may take place when the sash member reaches its fully closed position in some instances it may be desirable to so adjust the limit switch 46 that it will be actuated shortly prior to the sash member reaching its fully closed position to allow a small opening to remain for supplying.